Class 11 Biology

Toxicity Of Micronutrients

The overall growth and development of the plant are affected by higher concentrations (more than normal) of mineral nutrients especially micronutrients. If animals consume this plant with mineral toxicity, it will also affect the animal body.

Toxicity Of Micronutrients Definition: Mineral toxicity is the phenomenon in which a higher concentration of any mineral nutrient (especially micronutrients) causes a 10% reduction of the total dry weight of a plant along with different structural and functional abnormalities.

The concentration of a micronutrient that reduces dry matter of tissue of a specific plant by 10% is called the critical toxicity level or toxic concentration of that micronutrient for that plant.

Toxicity Of Micronutrient Characteristics:

1. Toxicity level varies from plant to plant. For example, above 600//g/g concentration of manganese (Mn) is toxic for soybean plants, whereas in sunflower, the toxic concentration of manganese is 5300/rg/g.
2. On the other hand, the uptake of other minerals may be reduced due to the toxic concentration of one mineral. For example, Mn toxicity leads to decreased absorption of Fe. Thus, Mn toxicity causes deficiency symptoms of Fe in plants.
3. Heavy metals, in excess concentrations, are the most toxic elements for all types of plants.

Toxicity Of Micronutrients Class 11

Nutrient toxicity and interactions

1. High levels of soil nitrogen can assist phosphorus, calcium, boron, iron, and zinc but an excess can dilute these elements.
2. Low nitrogen levels in soil can reduce phosphorus, calcium, boron, iron, and zinc uptake.
3. Ammonium nitrogen can make molybdenum deficiency appear less obvious.
4. High levels of phosphorus reduce zinc and, to a lesser degree, calcium uptake. It is antagonistic to boron in low-pH soils.
5. High levels of potassium reduce magnesium and to a lesser extent calcium, iron, copper, manganese, and zinc uptake.
6. Boron levels can either be low or toxic under the influence of high potassium. Low levels can accentuate iron deficiency.
7. High levels of copper, iron, or zinc can accentuate manganese deficiency, especially repeated applications of iron in soil.
8. Uptake can be decreased by liming (adding Ca and Mg-rich materials like limestone) or increased by application of sulfur.

Toxicity Of Micronutrients Source:

1. Chemical fertilizers used in agriculture.
2. Industrial chemical wastes.
3. Sail of mine regions contain higher amounts of a particular mineral, that may cause toxicity in plants of that area.

Mechanism Of Absorption And Translocation Of Mineral Nutrients

Mineral nutrients are absorbed by plants from the soil and then they are transported to the leaves. The mechanism of the whole process is discussed below.

Mechanism of absorption of mineral elements:

Uptake And Transport Of Mineral Nutrients

The soil solution contains mineral salts in dissociated condition. They are absorbed in forms of cations and anions, in different amounts by the roots. They are then translocated to different parts of the plant body through the xylem. The movement of ions into or out of the cells is called flux.

The inward movement is called influx and the outward movement is called efflux. Previously it was believed that inorganic salts were absorbed passively along with the water. But, newer hypotheses show that mineral uptake takes place by two processes—

1. First, there is a rapid uptake of ions into the apoplast i.e., intercellular spaces, without any expenditure of metabolic energy (ATP). It is called passive uptake
2. Second, the ions move into the symplast i.e., plasma membrane, cytoplasm, vacuolar membrane, of cells. Such movement of ions requires the expense of energy (ATP). It is called active uptake. The mechanism of absorption of ions has already been described in Chapter 11 (Transport in Plants).

Toxicity Of Micronutrients Class 11

Translocation of mineral elements:

The mineral elements are translocated or transported to different parts of the plant body through the tracheary elements of the xylem. The mineral salts move along the ascending stream of water through the xylem under the influence of transpiration pull. The detailed mechanism of translocation of mineral ions has already been discussed in Chapter 11.

Soil as a reservoir of essential elements

Minerals are present in soil and are absorbed by the plants. So, those can be used for metabolic purposes. Minerals are absorbed by root hairs, mycorrhizae, and the root epiblema (in very small amounts). These minerals occur in the soil due to —

1. Weathering and breaking down of rocks, and
2. Decomposition of organic matter of plant origin mainly leaves. Soil also contains colloid particles. The soil solution is found in small spaces among these particles. The easily available form of soil water is capillary water. It is found in the capillary spaces between soil particles.
   Soil not only supplies minerals and water but also harbors nitrogen-fixing bacteria and other microbes which help the plant to gather necessary nutrients from the environment. Soil also supplies air to the roots and serves as a matrix for them.

The nature of soil and the amount of minerals in it differs with geographical locations. Plants often suffer from different mineral deficiency diseases.

It especially affects the yield of crop plants. To avoid this, agricultural fields are often supplied with the necessary amount of fertilizers containing both micronutrients and macronutrients.

Nitrogen Metabolism

Nitrogen is the most abundant natural element found in the environment. It is the fourth most abundant element found in plant bodies. Nitrogen is found in many essential biomolecules such as nucleic acids, protein chlorophyll, some of the phytohormones, and many of the vitamins. So nitrogen as a component of these biomolecules is involved in most of the biochemical reactions contributing to various life processes.

Nitrogen is easily available in the atmosphere. The main source of nitrogen for the synthesis of nitrogenous organic compounds is the atmosphere. Nearly 80% of the earth’s atmosphere is composed of nitrogen, but most of the plants cannot utilize it in its elementary form.

Nitrogen Metabolism In Plants

So, nitrogen is known as the most critical element. Plants acquire nitrogen in the inorganic form either as ammonium (NH⁴⁺) or as nitrate compounds (NO3^3-) from the soil. Only certain microorganisms can use atmospheric nitrogen directly.

Plants synthesize different organic compounds such as protein, amino acids, etc., from the absorbed nitrogen. Animals get nitrogen by consuming other animals and plants. The nitrogen again returns to the atmosphere through plant and animal wastes.

Nitrogen from these wastes is released into the air by the action of certain decomposers present in the soil. This whole process is known as the nitrogen cycle. This cycle maintains a continuous supply of nitrogen to organisms. Nitrogen metabolism helps to synthesize different organic compounds in the body.

Toxicity Of Micronutrients Class 11

Nitrogen Metabolism Definition: The process by which plants absorb nitrogen from the atmosphere and use the same for their different metabolic activities, is known as nitrogen metabolism.

Nitrogen metabolism plays an important role in various physiological processes of the plants.

Nitrogen cycle

Nitrogen cycle Definition: The continuous cyclic biogeochemical pathway through which circulation of nitrogen occurs among all living organisms, the cyclic pool of lithosphere and reservoir pool of atmosphere, to maintain the balance of nitrogen in the environment, is known as the nitrogen cycle.

Nitrogen Cycle Explanation or Nitrogen Cycle Importance:

1. Nitrogen is present in different components of protoplasm such as protein, DNA, RNA, etc. Hence, nitrogen is essential for the growth and development of organisms.
2. The nitrogen cycle represents one of the most important nutrient cycles and it also maintains the nitrogen balance in the atmosphere.
3. It also maintains the continuous supply of nitrogen to the organisms.

Phases of the nitrogen cycle: The nitrogen cycle comprises of mainly five phases—

1. Nitrogen fixation,
2. Nitrogen assimilation,
3. Ammonification,
4. Nitrification,
5. Denitrification. These phases have been discussed under separate heads.

Toxicity Of Micronutrients Class 11

Nitrogen Fixation

Nitrogen fixation Definition: The process of conversion of elementary, inactive atmospheric nitrogen (N2) into organic form, to make it available for plants, is known as nitrogen fixation.

Method of nitrogen fixation: There are three methods of nitrogen fixation

Nitrogen Fixation And Nitrogen Metabolism

Natural nitrogen fixation: 10% of the total nitrogen fixation occurs through this process. Due to lightning, water vapor and oxygen dissociate into hydroxyl ions, hydrogen ions, and then atomic oxygen respectively. The atomic oxygen and the hydroxyl ion being highly reactive combine readily with atmospheric nitrogen to form nitric acid.

⇒ \(\mathrm{H}_2 \mathrm{O} \stackrel{\text { Lightning }}{\longrightarrow} \mathrm{OH}^{-}+\mathrm{H}^{+}, \mathrm{O}_2 \stackrel{\text { Lightning }}{\longrightarrow} 2[\mathrm{O}]\)

⇒ \(\mathrm{N}_2+2 \mathrm{OH}^{-}+4[\mathrm{O}] \stackrel{\text { Lightning }}{\longrightarrow} 2 \mathrm{HNO}_3 \text { (Nitric acid) }\)

In another way, nitrogen directly reacts with oxygen j to form different oxides in the presence of ultraviolet rays or lightning.

⇒ \(\mathrm{N}_2+\mathrm{O}_2 \underset{\text { UV Rays }}{\stackrel{\text { Lightning }}{\longrightarrow}} 2 \mathrm{NO} \text { (Nitric oxide) }\)

⇒ \(2 \mathrm{NO}+\mathrm{O}_2 \underset{\text { UV Rays }}{\stackrel{\text { Lightning }}{\longrightarrow}} 2 \mathrm{NO}_2 \text { (Nitrogen dioxide) }\)

Some of the oxides of nitrogen are also generated by forest fires. These oxides get dissolved in rainwater or water vapor and get converted to nitric acid and nitrous acid.

These two acids reach the soil through rainwater. In the soil, they react with other inorganic compounds such as salts of potassium, calcium, or magnesium, and form nitrite or nitrate salts of respective metals.

⇒ \(\mathrm{CaCl} 2+2 \mathrm{HNO}_3 \longrightarrow \mathrm{Ca}\left(\mathrm{NO}_3\right)_2+2 \mathrm{HCl}\)

Hence, the free atmospheric nitrogen is naturally fixed in the soil in the form of nitrite or nitrate.

industrial Rxathn: About 30% °f total nitrogen fixation is done industrially to produce fertilizers. Different nitrogen-containing fertilizers are prepared in industries.

For example, ammonia is prepared by combining hydrogen and nitrogen using Haber Bosch method at 300-400°C temperature and 35-100 MPa pressure. Fossil fuel is used as a source of both hydrogen and energy.

Nitrogen Fixation And Nitrogen Metabolism

⇒ \(\mathrm{N}_2+3 \mathrm{H}_2 \frac{300-400^{\circ} \mathrm{C}}{35-100 \mathrm{MPa}} 2 \mathrm{NH}_3\)

Nitrogen assimilation occurs as a result of using ammonia and other nitrogen-containing fertilizers in the Ammonification Nitrification soil.

Toxicity Of Micronutrients Class 11

Biological nitrogen fixation: Some species of symbiotic, free-living bacteria and cyanobacteria can utilize free atmospheric nitrogen directly. These species are termed as ‘nitrogen-fixing’ organisms. These organisms convert this nitrogen into ammonia and other nitrogen-containing compounds with the help of enzymes, present in their cells.

These nitrogen-containing compounds are then assimilated by the microorganisms in their protoplasm. These microorganisms supply nitrogen to the plant body or plants get nitrogen from their dead remains. It is described later in this chapter.

⇒ \(\mathrm{N}_2+3 \mathrm{H}_2 \stackrel{\text { Nitrogenase }}{\longrightarrow} 2 \mathrm{NH}_3\)

Nitrogen Assimilation

Green plants absorb nitrogen in the form of nitrates from the soil. Some leguminous plants uptake ammonium compounds directly from root nodules. The nitrates absorbed by roots are then transported to leaves where nitrates are converted to amino acids through different biochemical reactions that take place inside the plant cells.

These amino acids then polymerize and produce proteins according to the plant’s needs. Animals consume these plants directly or indirectly to fulfill their nitrogen requirement. Thus nitrogen assimilation occurs in three steps

1. Nitrate assimilation,
2. Synthesis of amino acids and
3. Synthesis of proteins. (These steps are discussed later in this chapter).

Ammonification

The organic nitrogen in the soil comes from nitrogen-containing organic compounds such as animal excreta as well as from decaying plant and animal remains. The process of production of ammonia from amino acids and other simpler nitrogenous substances and organic compounds with the help of different bacteria, is called ammonification.

These bacteria (Bacillus, Clostridium, Proteus, Pseudomonas, and Streptomyces) are called ammonifying bacteria. Ammonification of organic compounds is a very important step in the recycling of nitrogen in soils since most autotrophs are unable to absorb amino acids, nucleic acids, urea, and uric acid.

Urea is a major component of animal waste, it is hydrolyzed by urease enzyme into carbon dioxide and ammonia.

⇒ \(\text { Urea } \underset{\mathrm{H}_2 \mathrm{O}}{\stackrel{\text { Urease }}{\longrightarrow}} \mathrm{CO}_2 \mathrm{NH}_3\)

Ammonia immediately combines with hydrogen ions (H+) of water to form ammonium ions (NH4+).

⇒ \(\mathrm{NH}_3+\mathrm{H}^{+} \longrightarrow \mathrm{NH}_4^{+}\)

Toxicity Of Micronutrients Class 11

Nitrification

The process by which certain microorganisms convert ammonia (NH₃) or ammonium ion (NH⁴⁺) present in the soil into nitrite (NO₂^–) and nitrate (N03~), is known as nitrification.

Nitrosomonas and Nitrococcus bacteria first convert NH⁴⁺to NO₂^–ions. Nitrobacter bacteria convert NO₂-to NO_3^– These bacteria are known as nitrifying bacteria. In this reaction, some energy is also produced. This energy is used by the nitrifying bacteria for their carbon assimilation.

Nitrification requires oxygen as a reactant and occurs in aerobic soils. So, these bacteria are also known as chemoautotrophic bacteria.

⇒ \(\mathrm{NH}_4^{+}+2 \mathrm{O}_2 \stackrel{\text { Nitrosomonas }}{\longrightarrow} \mathrm{NO}_2^{-}+2 \mathrm{H}_2 \mathrm{O}\)

⇒ \(2 \mathrm{NO}_2^{-}+\mathrm{O}_2 \stackrel{\text { Nitrobacter }}{\longrightarrow} 2 \mathrm{NO}_3^{-}\)

Denitrification

The biological reduction of nitrogen-containing compounds such as nitrate (NO₃^–) to nitrogen gas (N₂) by bacteria is called denitrification. The bacteria that take part in denitrification are known as denitrifying bacteria. Some examples of denitrifying bacteria are Thiobacillus denitrificans, Pseudomonas aeruginosa, etc. Denitrification occurs when oxygen levels are depleted and nitrate becomes the primary oxygen source for microorganisms. When nitrate (NO₃^–) is dissociated by the bacteria to get the oxygen (O₂), the nitrate is reduced to nitrous oxide (N₂O), and, in turn, nitrogen gas (N₂). Since nitrogen gas has low water solubility, it escapes into the atmosphere as gas bubbles. Denitrification occurs only under anaerobic conditions.

⇒ \(\underset{\text { (Nitrate) }}{\mathrm{NO}_3^{-} \stackrel{\text { Reduction }}{\longrightarrow}} \mathrm{NO}_2^{-} \stackrel{\text { (Nitrite) }}{\underset{\text { (Nitrous oxide) }}{\longrightarrow}} \mathrm{N}_2 \mathrm{O} \text { (Nitrogen) }\)

Biological Nitrogen Fixation

Definition: The physical process that involves the conversion of atmospheric nitrogen into ammonia through reduction by certain microorganisms in symbiotic or free-living conditions for the assimilation of nitrogen into protoplasm, is known as biological nitrogen fixation.

Nitrogen-fixing organisms: During the process of evolution, some bacterial species and cyanobacteria have acquired the capacity to fix atmospheric nitrogen in their body. These organisms are known as nitrogen-fixing organisms. They can be symbiotic or free-living.

Importance of biological nitrogen fixation:

1. Molecular nitrogen becomes a part of protoplasm by the process of biological nitrogen fixation.
2. Free-living nitrogen-fixing organisms can fix 5 kg of nitrogen per j hectare of land in one year. Hence, many nitrogen-fixing organisms such as Anabaena and Azotobacter are used as j biofertilisers.
3. The rate of nitrogen fixation by symbiotic Rhizobium is higher than other bacteria. It is about 25-60 kg/year/hectare. Hence, the cultivation of legumes helps increase the nitrogen content of the soil naturally.

Types of biological nitrogen fixation:

Biological nitrogen fixation may be of two types—

1. Symbiotic nitrogen fixation and
2. Non-symbiotic nitrogen fixation. These are discussed under separate heads.

Special facts about nitrogen-fixing bacteria

The nitrogen-fixing bacteria remain more active under the soil in an alkaline or neutral medium. In the acidic soil, the nitrogen fixation process stops. The nitrogen-fixing bacteria, synthesize nitrogenase enzymes by the action of if genes present in them. A symbiotic bacteria, Rhizobium, is found in the roots of 90% of the leguminous plants. They live by forming nodules. The most common symbiotic bacteria is Rhizobium leguminosarum.

Symbiotic nitrogen fixation

Definition: The biological nitrogen fixation carried out by the microorganisms associated with other plants in symbiotic relationships is known as symbiotic nitrogen fixation.

Symbiotic nitrogen-fixing organisms:

Many species of bacteria and cyanobacteria fix atmospheric nitrogen in symbiotic association with other plants. These are known as symbiotic nitrogen-fixing organisms. Some common examples are given in Table 12.8 along with their host plants.

Process: Symbiotic N₂ -fixation involving both legumes and Rhizobium is unique and ecologically very important for biological N₂ -fixation. The physiological and biochemical process of nitrogen fixation by Rhizobium is discussed below—

Physiological process of symbiotic nitrogen fixation

1. Pre-fixation stage: Rhizobium are gram-negative and rod-shaped bacteria. These bacteria cannot fix nitrogen when they are in free-living form. They can fix nitrogen only when they come in contact with their host plants.

The following series of events occur during this stage—

1. Generation of chemical signals in the pre-infection phase: The bacteria are attracted by certain chemical components secreted from the roots of the host plants.
   The secreted chemical compounds are—
   • Legume lectin, a carbohydrate-binding protein secreted by root hairs of the legumes creates a signal to attract bacteria toward the host plant.
   • Homoserine, an amino acid secreted by root hair cells,
   • Flavonoid, a secondary metabolite (chemoattractant) secreted from the root hairs.
2. Bacterial association on plant root: Bacteria are attracted towards the root by the signal-producing chemicals secreted from the root. Then, certain reactions occur between the bacterial cell wall and the root hairs, that lead to the attachment of bacteria to the root hairs. The number of bacterial cells increases at this time due to the effect of chemoattractants.
3. Nod gene activation: Flavonoid activates the nod gene present in bacteria. Activation of the nod gene results in the synthesis of the nod factor. This nod factor is a type of growth hormone (fi -indole acetic acid) that plays an important role during the infection stage.

2. Infection stage: Root hairs are curled due to the effect of RHCF (Root Hair Curling Factor) and nod factor. The nod factor causes the formation of pores at the tip of the root hairs, through which bacteria enter into the root hairs.

3. Formation of infection thread: After entering into the root hairs, the bacteria move deeper into the cortex of the root through an infection thread. This infection thread, made with the plasma membrane, grows inward from one host cell to another. The effect of the nod factor increases the number of bacteria in the infection thread.

4. Nodule and bacteroid formation: During the development of the infection thread, the individual cells near the vascular bundle form nodule primordia (singular: nodule primordium).

These nodule primordia are produced by the individual cells which divide rapidly by the effect of cytokinin hormone present in the root with the combined effect of nod factor of bacteria. The infection thread slowly combines with the nodule primordia.

The bacteria start dividing faster after entering into the nodule primordia. After some time, the bacteria stop dividing, become polyhedral in shape, and reside in the nodule primordia in immobile condition. This immobile state of bacteria is known as bacteroid and in this state, they are prepared for nitrogen fixation.

This bacteroid is morphologically different from the bacteria that entered the root hair. The membrane of individual cells that surrounds the bacteroid is known as the peribacteroid membrane.

The cells of nodule primordia also start dividing and’ form root nodules. Inside the nodules, a pink color is seen. This coloration is developed by a pigment, named leghaemoglobin, produced by the joint activity of bacteria and the host cell cytoplasm.

Different nod factors are responsible for the formation of bacteroid and root nodules. The nodule thus formed, establishes a direct vascular connection with the host for exchange of nutrients.

Leghaemoglobin

Leghaemoglobin is a dark pink-colored pigment, present in the root nodules of leguminous plants, which is closely related to hemoglobin, the red pigment found in human red blood cells.

This pigment is an oxygen acceptor, which accepts free oxygen from the environment. During nitrogen fixation, leghaemoglobin absorbs the oxygen produced during the reaction and provides an anaerobic condition required for the process of nitrogen fixation.

Biochemical process of symbiotic nitrogen fixation:

Atmospheric nitrogen gas is reduced to ammonia by the enzyme nitrogenase present in the bacteroid. This enzyme remains active in the absence of oxygen. Leghaemoglobin traps the atmospheric oxygen and provides an anaerobic condition for the activity of nitrogenase. The steps of biochemical reactions are

1. Glucose and fructose, produced from the sucrose in the nodules, are converted to glucose-6-phosphate by the action of carbohydrate dissociating enzyme present in the bacteroid.

⇒ \(\text { Glucose } \longrightarrow \text { Glucose-6-phosphate }\)

2. Glucose-6-phosphate is converted to 6-phosphogluconic acid due to the presence of NADP⁺ (nicotinamide adenine dinucleotide phosphate, a co-enzyme, that acts as an electron acceptor) in the bacteroid cells. NADP⁺ is reduced to form NADPH⁺ H⁺

⇒ \(Glucose-6-phosphate +\mathrm{NADP}^{+}+\mathrm{H}_2 \mathrm{O} \longrightarrow 6-phosphoglucose acid + \mathrm{NADPH}+\mathrm{H}^{+}\)

3. This reduced NADPH⁺ H⁺ gets oxidized by donating an electron to ferredoxin which acts as an electron carrier.

4. The reduced ferredoxin gets oxidized by reducing the Fe protein of the nitrogenase enzyme.

5. Now, reduced Fe protein gets oxidized by reducing the Mo-Fe protein of the nitrogenase enzyme.

6. Atmospheric nitrogen is reduced by this Mo-Fe protein. Hydrazine (N₂H₄) and ammonia (NH₃) are produced in this process.

7. The energy required for the electron transport chain is supplied by ATP. 8 electrons are required for the fixation of 1 molecule of nitrogen.

⇒ \(\begin{array}{r} \mathrm{N}_2+8 \mathrm{e}^{-}+8 \mathrm{H}^{+}+16 \mathrm{ATP} \stackrel{\text { Nitrogenase }}{\longrightarrow} \\ 2 \mathrm{NH}_3+\mathrm{H}_2+16 \mathrm{ADP}+16 \mathrm{Pi} \end{array}\)

8. Ammonia is not released after completion of the reaction. A small amount of ammonia is very toxic for the microbes. Nitrogen-fixing bacteria protect themselves from this toxic effect of ammonia by synthesizing organic acids. The reaction between ammonia and organic acids produces amino acids. These amino acids are moved to all parts of the plant through the phloem.

Non-symbiotic Nitrogen Fixation

Definition: The process of biological nitrogen fixation, that is carried out by free-living bacteria in the soil, is known as non-symbiotic nitrogen fixation.

Non-symbiotic microorganisms: Some species of cyanobacteria and bacteria that fix nitrogen nonsymbiotically are known as non-symbiotic microbes or free-living microbes.

Process: Like symbiotic nitrogen fixation, nitrogenase enzyme is also involved in non-symbiotic nitrogen fixation. Besides nitrogenase enzyme hydrogenase, ferredoxin, Tpp (Thiamine pyrophosphate), pyruvate, Mg²⁺, inorganic phosphate, and Co-enzyme A (CoA) are also required for non-symbiotic nitrogen fixation.

Nitrogenase

Nitrogenase is a nitrogen-fixing enzyme, which plays a key role in the conversion of atmospheric nitrogen into ammonia under anaerobic conditions. Nitrogenase becomes inactive in the presence of oxygen. Nitrogen-fixing aerobe, Azotobacter, or symbiotic bacteria Rhizobium, helps the enzyme to remain active, by some special process. This enzyme is formed of two subunits—Fe protein and Mo-Fe protein. These two subunits transfer electrons separately.

According to Burris (1965), pyruvate provides energy and ferredoxin carries electrons during this process.

Hydrogen may also serve as the electron donor. Other sources of electrons are the photosynthetic water splitting, the assimilated carbon, and reduced sulfur compounds.

The end product, ammonia, reacts with organic acids to form amino acids (same as symbiotic nitrogen fixation).

Nitrate Assimilation

The nitrate absorbed from the soil is oxidized in two steps to form ammonia within the plants. The steps are—

Reduction of nitrate into nitrite: This occurs in the presence of the enzyme nitrate reductase. The nitrate reductase is a molybdenum-containing flavoprotein. FMN (flavin mononucleotide) and FAD (flavin adenine dinucleotide) are present as hydrogen acceptors. Molybdenum helps in electron transport.

⇒ \(\begin{aligned}
\mathrm{NO}_3^{-}+\mathrm{NAD}(\mathrm{P}) \mathrm{H}+\mathrm{H}^{+} & \frac{\text { Nitrate reductase }}{\mathrm{FMN} \text { or FAD }} \\
& \mathrm{NO}_2^{-}+\mathrm{NAD}(\mathrm{P})^{+}+\mathrm{H}_2 \mathrm{O}
\end{aligned}\)

Reduction of nitrite into ammonia: This reaction occurs in the presence of nitrite reductase. Two prosthetic groups—Fe-Cu complex and haem, are required for the activation of the enzyme. Ferredoxin is required for carrying electrons and co-enzyme NAD(P)H transports hydrogen.

The evolved NH₃ converts to ammonium ion (NH₄⁺) by accepting a proton. As minute amounts of these components are harmful to plants, they are converted to amino acids by reacting with organic acids, secreted by the nodules.

Nif gene

The gene that maintains the structure and controls the activity of the nitrogenase enzyme is known as the nif gene. This gene is present in the body of Rhizobium. The main genes are—

Synthesis Of Amino Acid

Amino acid is the structural unit of proteins. Amino acids are the primary compounds for nitrogen assimilation. The synthesis of amino acids involves two steps.

Reductive amination: Ammonia comes in contact with the organic acids by reductive amination. In this process, ammonia forms glutamic acid by reacting with ar-ketoglutarate. This process is controlled by the enzyme glutamate dehydrogenase.

Transamination: In this process, one amino group from one amino acid is transferred to a keto group of the keto acids. Glutamate can produce about 17 amino acids by the process of transamination. This reaction occurs in the presence of the enzyme transaminase.

Protein Synthesis

We know that amino acids are the structural units of proteins. Amino acids are the amino compounds of keto acids such as pyruvic acid, etc.

Amino acids contain three functional groups, they are—

1. Amino group (— NH₂),
2. Carboxylic group (— COOH),
3. Alkyl group (-R).

A single protein contains one or more polypeptide molecules. Polypeptides are combinations of some amino acids arranged in a long chain. The amino group of one amino acid remains attached to the carboxyl group of the adjacent amino acid by a peptide bond.

This peptide bond (— NH—CO—) is formed by the release of one molecule of water by the chemical bonding of the amino group and the carboxyl group. Chains of several amino acids form a polypeptide molecule. Living cells contain 20 different kinds of amino acids.

In the polypeptide molecule, amino acids are arranged according to the sequence of coded information present in mRNA. Protein synthesis takes place by the activity of both tRNA in cytosol and rRNA in the ribosome. The sequence of nitrogenous bases in mRNA is different in different species of animals.

Notes

Airstone: A piece of limestone or porous stone, used as aquarium furniture, which gradually diffuses air into the tank, eliminating noise and large bubbles of conventional air filtration system.

Antioxidative: A property or a molecule that inhibits the oxidation of other molecules

Buffer: A solution that resists changes in pH when acid or alkali is added to it.

Calmodulin: A multifunctional intermediate Ca2+ binding messenger protein expressed in all eukaryotic cells.

Chelating agent: A substance whose molecules can form several bonds to a single metal ion. example ethylenediamine.

Chemoattractant: A chemical substance that induces chemotactic movement of an organism towards a certain direction in which its concentration is increasing.

Ligand: A substance that forms a complex with the central metal atom of a biomolecule to serve a specific biological purpose.

Mottle: Mark with spots or smears of color. ‘

Necrosis: The word has originated from the Greek ‘nekros’ (dead body); death of tissue due to injury, radiation, infection, toxicity, or deficiency.

Rosette: A circular arrangement of leaves where all leaves are at a similar height.

Elementary Idea Of Hydroponics And Other Plant Culture Methods

Plants are cultured in artificial culture media, to study mineral requirement and their essentiality. These culture media contain mineral nutrients that are essential for plant growth and development. Three culture media are used to study mineral requirements. These are—

1. Solution culture media or hydroponics,
2. Sand culture media, and
3. Aeroponics.

Concept Of Solution Culture Or Hydroponics

Hydroponics Definition: Hydroponics is the method or technique of growing plants in a nutrient solution without using soil (soilless culture).

The word hydroponics technically means working water in which plants can grow. It has been derived from the Latin words hydro meaning ‘water’, and ponos meaning ‘labor’. Recently hydroponically growing plants have been gaining popularity and are being sold across many different markets.

This process of soilless culture or hydroponics was first described by Francis Bacon (1627) in his book Sylva sylvarum. Later it was described by John Woodward (1699). He experimented on the mint plant.

Hydroponics Explanation: During the nineteenth century, there was a practice of growing plants in soil-less conditions. This method was modernized by Sachs (1860) and Knop (1865). They prepared a balanced nutrient solution to grow plants in soil-less conditions.

Sachs’ and Knop’s solutions are mostly used for the process of hydroponics. They included only macronutrients and iron as essential minerals in their solution media. Iron was used as ferrous chloride by Sachs and as ferrous phosphate by Knop.

Later Arnon and Hoagland (1940) included micronutrients in this solution. These scientists found that iron remained soluble as salt of citric acid and tartaric acid. The chemical agent that keeps a metal in the soluble state is called a chelating agent.

The most commonly used chelating agents are EDTA (Ethylenediaminetetraacetic acid) and DTPA (Diethylenetriaminepentaacetic acid). They used Fe and Na-EDTA in this solution.

Types of hydroponics systems or techniques

There are two types of hydroponics techniques—passive and active techniques.

The application of passive or active hydroponics depends on the requirement. Passive techniques are more suitable for growing on a small scale, for example, indoor plants in the living room. However, the active techniques are more suitable for large-scale cultivation. These hydroponics techniques are described below in separate heads.

Passive technique: In this technique, the nutrient solution is stationary and the plant takes what it needs from the solution. Different types of passive techniques are discussed below.

Stagnant nutrient solution without substrate: This is a very simple type of hydroponic system. The system shows the following features—

1. A small reservoir of glass or metal is filled with distilled water containing appropriate amounts of essential nutrients.
2. The reservoir is covered with a wooden plank with holes or wire gauge, as a plant support system. The plants are kept on the plant support system in such a manner, that their roots directly touch the nutrient solution present in the reservoir.
3. This reservoir also has an aeration system through which oxygen is added to water from the environment.

Stagnant nutrient solution with substrate: This technique is suitable for the plants grown inside the rooms.

The system shows the following features—

1. One plant or many plants can be kept per pot.
2. The roots of the plant are impregnated with an inert and porous substrate. The nutrient solution is brought to the roots of the plant by the capillary activity of the substrate.
3. The nutrient solution takes up 1/3 of the height of the pot and the root system occupies the upper 2/3 part (height) of the pot.

Semi-hydroponics: This technique is a combination of hydroponics and irrigation system for growing plants in a soil-free mineral solution.

It shows the following features—

1. The plant itself is in a pot with soil and this pot is hung over a nutrient solution.
2. The nutrient solution reaches the roots through the capillary activity of clay granules.

Active technique: In this dynamic technique, the nutrient solution is allowed to flow around the roots. The nutrient solution is brought to the roots by the use of a pump of electric vaporizer.

This technique is more complex than the passive technique but it allows better management of nutrient solution as this is stored in a separate container. This technique may be accomplished in the following ways—

Nutrient-Film hydroponics Technique (NFT) or Continuous Flow System: It is the most commonly used hydroponics system. The system shows the following features—

1. A container which is actually a tube is without substrate. The tube has several holes through which plants are attached in such a way that some branches of the root or the main root touch the base of the container. This is slanted over the water reservoir containing an appropriate amount of mineral nutrients.
2. Here, the nutrient solution from the water reservoir is pumped into the slanted vessel by using a pump. So, the roots remain in contact with a thin film of nutrient solution. In this system, roots are not submerged in the solution.

Ebb and Flow hydroponics system: Ebb and flow are the two types of movements of water observed in this system. Ebb is the outgoing and the flow is the incoming phase.

The system shows the following features—

1. In this system, a small reservoir is kept over the large reservoir. The large reservoir contains the nutrient solution.
2. Plants are kept on the wooden plank at the top of the small reservoir.
3. The nutrient solution is moved to the small reservoir through a pump.
4. After reaching a certain height, the nutrient solution comes back to the large reservoir through an overflow pipe. This helps maintain the level of solution in the smaller reservoir.
5. The nutrient solution in the upper reservoir is aerated by using an electrical air pump. In this way, the roots get an optimum proportion of nutrients, water, and air supply.

Bubbleponics: The system shows the following features—

1. In this system, plants are arranged in the same way as in the stagnant nutrient solution without substrate.
2. The nutrient solution is pumped up to the roots to water them from above at regular intervals.
3. The advantage is that the plant grows quickly and does not need to develop roots first to reach the nutrient solution.

Drip System:

The system shows the following features—

1. It is the most widely used type of hydroponic system. In a drip system, the nutrient solution is pumped to the roots, from the reservoir, with individual drippers at each plant.
2. The plants are kept in a nutrient medium. Here a timer controls the pump.

Wick System: A wick is a gauze strip that is made up of porous material. This is used to draw the liquid by its capillary action.

The system shows the following features—

1. It is the simplest of all hydroponics systems. Here, the nutrient solution is provided to the plants with the help of a wick.
2. A variety of growing mediums, such as perlite, vermiculite coconut fiber, etc., can be used in this system.

Study Of The Mineral Requirements Of Plants Through Hydroponics System

Plants require different minerals for their growth and development.

An experiment for studying the essentiality of different minerals is given as follows—

1. During this study, a normal balanced nutrient solution is prepared which contains all the essential elements in an appropriate amount. This is taken as control.
2. Then a series of nutrient solutions are also prepared. Each of these nutrient solutions has any one of the nutrient elements missing.
3. Now, the seedlings of the same size are reared in the nutrient solutions.
4. The glass bottles are covered with black paper so that direct sunlight can not reach the roots. It will also prevent the growth of algae.
5. The bottles are kept in a place where they can get the appropriate amount of sunlight and air.
6. The growth of all the seedlings is observed carefully and regularly.
7. The growth of each seedling is compared with that of the seedling in the normal balanced nutrient solution. From the comparison, it has been found that all the seedlings planted in deficient nutrient solutions have stunted growth.
8. The deficiency symptoms for each element can be observed clearly. The observations are given below in a tabular manner.

From this experiment, it can be concluded that all the nutrients in their appropriate proportions are important and essential for the proper growth of a plant.

Advantages and disadvantages

The advantages and disadvantages of hydroponics techniques are discussed separately below.

Hydroponics Technique Advantages:

1. Hydroponics helps to study the nutrient requirements of the plants. We can find out—
   • The essentiality of different nutrients for the growth and development of a plant,
   • Deficiency symptoms of any element,
   • Harmful effects caused by toxic concentration of any element,
   • Role of essential elements in. metabolism of plants.
2. Hydroponics is used for growing off-season commercial flowers and vegetables such as lettuce, seedless cucumber, and tomatoes.
3. As this method does not require soil, plants be grown irrespective of soil type.
4. Plants can be grown without any contamination by soil pathogens and pollutants.
5. It is environment-friendly as it does not require any chemical fertilizer.
6. Generally, the plants grown by this method give better yields.

Hydroponics Technique Disadvantages:

1. The infrastructural cost for this technique is very high.
2. The chances of water-borne plant diseases are high, so, maintaining a sterile environment is necessary.
3. Chemicals used in the preparation of nutrient solutions may contain toxic impurities. Toxins may affect the normal plant growth.
4. Technical expertise is required.

Concept Of Sand Culture

The cultivation of plants in the sand medium is called sand culture. Here acid acid-washed quartz is used as a substrate. Perlite or vermiculite is used along with quartz to enhance root growth. Cultivation is done in a clay vessel.

It is divided into two tiers. The upper tier is filled with sand and the lower tier with nutrient solution. This solution is pumped to the sand layer, continuously or at regular intervals. It is an easy method. It provides a natural environment for plants. However, in this method, it is difficult to maintain proper aeration and pH.

Concept Of Aeroponics

Aeroponics is a type of soil-less plant culture. In this process, the plants are kept suspended in a growth chamber. The nutrient solution is sprayed on the roots of the plants. This spraying may be done continuously or at regular intervals. This method was first discovered by Weathers and Zobel (1992).

The nutrient solution used in this method is more concentrated than that used in hydroponics. In this system, the roots have an excellent aeration. For long-term aeroponic cultivation, the root systems must be free from surrounding constraints. Generally, citrus fruits and olives are grown by this method.

Class 11 Biology WBCHSE Mineral Nutrition Questions And Answers

Question 1. How are the essential minerals classified according to their functions?
Answer: According to functions, the essential minerals are divided into four groups—

1. Constituents of essential structural components, such as C, H, 0, N.
2. Constituents of energy-providing chemical components, such as Mg, P.
3. Enzyme activators and inhibitors such as Mg, and Zn.
4. Osmotic potential regulators such as K.

Question 2. What are catalytic elements?
Answer:  The elements, which act as co-enzymes are known as catalytic elements. These elements, regulate the rate of biochemical reaction in which they are involved. For example, manganese acts as a coenzyme in the enzyme mangano-protein of PS II.

Question 3. What are critical elements?
Answer: Nitrogen, phosphorus, and potassium are critical elements for plants as they are required in large amounts. In soil, these elements are often found in very small quantities but they are very essential for plant growth. So, these elements are provided to plants through fertilizers.

Read and Learn More WBCHSE Solutions For Class 11 Biology

Question 4. What is leghaemoglobin?
Answer: A dark pink-colored pigment, present in the root nodules of leguminous plants, is called leghaemoglobin. It is produced by the joint activity of bacteria and host cell cytoplasm. Due to its similarity with hemoglobin, it is named leghaemoglobin. It plays a key role in nitrogen fixation.

Question 5. Write the similarities and differences between hemoglobin and leghaemoglobin.
Answer:

Similarities:

1. Leghaemoglobin and hemoglobin both act as oxygen carriers.
2. Both are chemically and structurally similar. Hence, the color of the two pigments is almost the same.

Differences:

1. Leghaemoglobin accepts and stores oxygen. This way it makes a suitable anaerobic environment for nitrogen fixation. However, hemoglobin transports oxygen during cellular respiration.
2. Leghaemoglobin is formed due to the infection caused by the nitrogen-fixing bacteria, in the root nodules of leguminous plants. Haemoglobin is found in the red blood cells of the vertebrates naturally.

Class 11 Biology WBCHSE

Question 6. What is solution culture?
Answer: The method of growing plants in a nutrient solution, in the absence of soil is known as solution culture. This is also known as hydroponics.

Question 7. What are the sources of mineral toxicity?
Answer:

The different sources of mineral toxicity are—

1. Use of excess fertilizers in agriculture.
2. The mineral components from factories and industries, mix with the soil and water.
3. High concentration of any mineral in the soil.

Question  8. What is dieback disease?
Answer: This disease of plants is caused by the deficiency of Cu2+ and K+. In this disease, the apical bud is destroyed first followed by other parts of the plants.

Question 9. What are balancing elements?
Answer: The toxic effect of any mineral can be decreased by the application of certain elements. These elements are known as balancing elements. For example, K and Ca balance the toxic effect of heavy metals.

Question 10. What are diazotrophs?
Answer: The plants, that can bind atmospheric nitrogen in their body, are known as diazotrophs.

Question 11. What do you mean by biological nitrogen fixation?
Answer: The blue-green algae or cyanobacteria and some species of bacteria reduce atmospheric nitrogen to form ammonia by the enzyme nitrogenase. This process is known as biological nitrogen fixation.

Question 12. What is nodule meristem?
Answer: The meristematic tissues, involved in the nodule formation of leguminous plants, form the nodule meristem.

Class 11 Biology WBCHSE Mineral Nutrition Multiple-Choice Question And Answers

Question 1. Select the mismatch

1. Rhodospirillum — Mycorrhiza
2. Anabaena — Nitrogen fixer
3. Rhizobium — Alfalfa
4. Frankia- Alnus

Answer: 1. Rhodospirillum — Mycorrhiza

Question 2. In which of the following, all three are macronutrients?

1. Iron, copper, molybdenum
2. Molybdenum, magnesium, manganese
3. Nitrogen, nickel, phosphorus
4. Boron, zinc, manganese

Answer: 3. Nitrogen, nickel, phosphorus

Question 3. During biological nitrogen fixation, the inactivation of nitrogenase by oxygen poisoning is prevented by

1. Cytochrome
2. Leghaemoglobin
3. Xanthophyll
4. Carotene

Answer: 2. Leghaemoglobin

Question 4. Deficiency symptoms of nitrogen and potassium visible first in

1. Senescent leaves
2. Roots
3. Young leaves
4. Buds

Answer: 1. Senescent leaves

Class 11 Biology WBCHSE

Question 5. Excess of manganese inhibits the translocation to the shoot apex—

1. Calcium
2. Potassium
3. Iron
4. Magnesium

Answer: 1. Calcium

Question 6. One element is involved in the opening and closing of stomata, the other helps to maintain the ribosome structure. They are

1. Potassium and calcium
2. Phosphorus and sulfur
3. Potassium and magnesium
4. Iron and magnesium
5. Calcium and sulphur

Answer: 3. Iron and magnesium

Question 7. Which of the following groups of minerals are micronutrients?

1. Magnesium, manganese, copper, boron and phosphorus
2. Manganese, copper, molybdenum, zinc and boron
3. Nitrogen, potassium, molybdenum, and zinc
4. Carbon, potassium, phosphorus, nitrogen and oxygen

Answer: 2. Manganese, copper, molybdenum, zinc, and boron

Question 8. Match the minerals in column 1 with the enzymes activated in column 2 and choose the correct option

1-2,2-3,3-1

1-1,2-2,3,3

1-2,2-1,3-3

1-3,2-1,3-2

1-3,2-2,3-1

Answer: 1. 1-2,2-3,3-1

Class 11 Biology WBCHSE

Question 9. Which element plays a vital in the splitting of water to liberate oxygen during photosynthesis?

1. Copper
2. Boron
3. Chlorine
4. Manganese

Answer: 4. Manganese

Question 10. Which one is the cofactor of carbonic anhydrase?

1. Iron (Fe)
2. Copper (Cu)
3. Zinc (Zn)
4. Magnesium (Mg)

Answer: 2. Copper (Cu)

Question 11. Nickel contributes to the formation of one of the following

1. Urease
2. Dehydrogenase
3. Rubisco protein
4. Nitrate reductase

Answer: 1. Urease

Question 12. The plant ash indicates

1. Organic matter of plants
2. Mineral salts absorbed by plants
3. Both mineral salts and organic matter
4. Silica absorbed by plants

Answer: 2. Mineral salts absorbed by plants

Class 11 Biology WBCHSE

Question 13. Minerals are absorbed by plants in

1. Colloidal Form
2. Ionic Form
3. Precipitated Form
4. None Of The above

Answer: 2. Ionic Form

Question 14. The first stable product of fixation of atmospheric nitrogen in leguminous plants is—

1. NO_2^–
2. Ammonia
3. NO₃^–
4. Glutamate

Answer: 2. Ammonia

Biology Class 11 WBCHSE Mineral Nutrition Very Short Answer Type Question And Answers

Question 1. What is mineral nutrition?
Answer: The term mineral nutrition refers to the process, by which a living plant absorbs and assimilates essential mineral elements from the soil for its proper growth and development

Question 2. Name the part of the plant that absorbs mineral nutrients.
Answer: Plants absorb mineral nutrients from the soil by means of their root hairs.

Question 3. Name the mineral element, that is essential for the splitting of water in the process of photosynthesis.
Answer: Manganese (Mn) is essential for the splitting of water (photolysis) during the light phase of photosynthesis.

Question 4. Name the enzyme involved in biological nitrogen fixation. What are the two mineral nutrients needed for the activity of this enzyme?
Answer: The enzyme involved in biological nitrogen fixation is nitrogenase, produced from the ni gene cluster of certain bacteria and blue-green algae. Iron (Fe) and molybdenum (Mo) are two mineral nutrients essential for its activity.

Question 5. Which part of the urease enzyme catalyzes the hydrolysis of urea to CO₂ and then NH₄?
Answer: Nickel-containing metallic parts of urease catalyzes the hydrolysis of urea.

Question 6. Name one best-known symbiotic nitrogen-fixing bacteria.
Answer: The well-known symbiotic nitrogen-fixing bacteria is Rhizobium.

Question 7. Name the pigment that protects the enzyme nitrogenase from oxygen.
Answer: Leghaemoglobin

Question 8. Write the exact location of leghaemoglobin in the root nodules of leguminous plants.
Answer: Cytosol or the cytoplasm of the root nodules

Question 9. What are hunger signs?
Answer: The deficiency7 symptoms of different mineral nutrients, in plants, are called the hunger signs.

Question 10. Deficiency of mineral nutrition is not responsible for etiolation Yes or no? If no, explain the reason.
Answer: No, mineral deficiency is not responsible for etiolation. It is related to the absence of light.

Question 11. Name the enzyme responsible for nitrate reduction.
Answer: Nitrate reductase is the enzyme responsible for nitrate reduction.

Question 12. Where do the plants get hydrogen?
Answer: Plants get hydrogen from water.

Biology Class 11 WBCHSE

Question 13. Name two macronutrients that play important roles in limiting plant growth globally.
Answer: Calcium and nitrogen.

Question 14. Name the principal mineral anion present in extracellular fluid.
Answer: Calcium

Question 15. From the following list identify the two minerals, that are not needed by a majority of plants but are important for almost all animals: Calcium, Sodium, Potassium, Iron, and Iodine.
Answer: Sodium and iodine are not needed by the majority of plants but are essential for almost all animals.

Question 16. Name two insectivorous plants.
Answer: Nepenthes adnata and Dionaea muscipula

Question 17. Name the aquatic fern and a gymnosperm, with which cyanobacteria have a symbiotic association.
Answer: Azollafiliculoides is an aquatic fern and Cycas is a gymnosperm, with which cyanobacteria remain in symbiotic association.

Question 18. Name two immobile elements in plants.
Answer: Calcium and sulfur.

Question 19. Yellowish edges appear in leaves deficient in.
Answer: Magnesium

Question 20. Name the macronutrient, which is a component of all organic compounds but is not obtained from soil.
Answer: Nitrogen.

Biology Class 11 WBCHSE

Question 21. How are organisms like Pseudomonas and Thiobacillus of great significance in the nitrogen cycle?
Answer: These are denitrifying bacteria. They maintain constant nitrogen levels in the atmosphere.

Question 22. A farmer adds Azotobacter culture to the soil before sowing maize. Which mineral element is being replenished?
Answer: Nitrogen (fixed into ammonia)

Question 23. Name a plant that accumulates silicon.
Answer: Equisetum.

Transportation In Plants Introduction-Absorption Of Water, Gas, And Nutrients

We need oxygen, water, organic substances (food), and minerals for our survival. To meet that requirement, we breathe in, eat a regular healthy diet, and drink sufficient water.

There are various organ systems in our body, which carry these substances to different body parts. There they are utilized and absorbed for the proper functioning of the body.

Even plants need these essential nutrients for their growth and survival. A question that comes to our mind is how they acquire these or how are these substances transported throughout their body. Is there any such system that carries them? We shall learn all about these facts in this chapter.

Water, gas, and nutrients are the essential components for a plant to grow and develop. Without these components, plants are unable to prepare food, grow, or maintain their physiological balance. Plants acquire these components from the environment using certain organs and organ systems like root hairs and root systems.

Following this, the necessary components are transported to the specific organs for utilization. In this chapter, we shall learn about all those processes by which a plant initiates the uptake of water, gases, and nutrients and also transports them throughout its body.

Transportation In Plants Absorption Of Water By Plants

Water is the most essential abiotic (non-living) component for plant growth and development. Water acts as a medium of transport in plants. It also plays a central role in photosynthesis. Despite this dependence on water for various life processes, plants retain less than 5% of the total water absorbed by roots.

This water is used for cell expansion and plant growth. The remaining water absorbed by roots is given out by the plants directly into the atmosphere by a process known as transpiration. The interrelationship between transpiration and photosynthesis forms the basis of the existence of plants.

Water is very important in a plant’s life for the following reasons:

1. Water forms 80-90% of the total weight of a plant.
2. The translocation of minerals and gases occurs within plants only in dissolved forms.
3. Water helps maintain the turgidity of the plant cells.
4. Water is essential for the germination of seeds.
5. Major biochemical reactions take place in the presence of water.
6. Water imparts a cooling effect in plants through the process of transpiration.
7. It forms a medium that helps in the transportation of minerals, glucose, and other metabolites in the plant.

To carry out these activities, water must be absorbed by the plants through various means. Depending on the category of plants, absorption of water takes place by different parts of the body.

Absorption by roots: In advanced terrestrial plants, capillary water (a thin film of water present around the soil particles) is absorbed from the soil by the root system. Within the root system, only unicellular root hairs absorb water by endosmosis.

The absorbed water is then transported from the cortex to xylem vessels by cell-to-cell osmosis. Water is transported throughout the plant body by the xylem vessels. This has been explained later in this chapter.

Absorption by body surface: Aquatic plants which are either submerged (such as Hydrilla, and Chara) or partially submerged (such as lotus), absorb water by their submerged body surfaces.

Absorption by velamen: Velamen is a special, sponge-like tissue that is found in epiphytic orchid plants. As these epiphytes lack root caps and root hair in their aerial roots, water gets absorbed in the form of water vapor by the velamen.

Transportation In Plants Absorption Of Gases By Plants

Plants absorb gases mainly from the atmosphere. However, the process of absorption of gases is carried out by various body parts.

Some of these include:

• Autotrophic plants directly absorb carbon dioxide (CO₂) and oxygen (O₂) from the atmosphere. O₂, present in soil, is required for gaseous exchange in roots.
• Gaseous exchange between plant body and atmosphere occurs through open stomata. This gaseous exchange takes place during the day as in most plants stomata open during the day.
• In xerophytes, stomata remain closed during the day, but open at night. So, CO₂ is absorbed at night, which is temporarily stored in a cell vacuole, as organic acids.
• Many halophytes absorb gases by the fine pores on their respiratory roots (pneumatophores). These pores are called pneumatophores.
• Some blue-green algae and free-living bacteria absorb atmospheric nitrogen. Generally, plants take nitrogen from soil in the form of nitrate and nitrite compounds. Leguminous plants absorb atmospheric nitrogen with the help of symbiotic nitrogen-fixing bacteria present in their roots.

Transportation In Plants Absorption Of Nutrients By Plants

The important substances, both organic and inorganic, that are required by plants for growth, development, and metabolism are called nutrients. The nutrients required by plants are also called minerals. These minerals are mostly water soluble. These remain in the soil in a dissolved state,- which facilitates plants in their better absorption.

Transportation In Plants Means Of Transport Diffusion Facilitated Diffusion And Active Transport

Water, water-soluble substances, and gaseous components are transported across the plant body by various physical processes, such as— diffusion, facilitated diffusion, active transport, etc. All the physical processes that are involved in cellular transport are described below under separate heads.

Transportation In Plants Simple Diffusion

The movement of molecules or ions of any substance, from a region of their higher concentration to the region of lower concentration, until both regions become equal in concentration is called simple diffusion.

Simple Diffusion Explanation: The method of diffusion is completely physical in nature. The molecules of any substance are always in motion due to Brownian motion. This is the random movement of microscopic molecules in liquid or gas, caused by collisions with molecules of the surrounding medium.

Because of this movement, molecules possess kinetic energy which allows the molecules to move from their higher to lower concentration.

Diffusion is the movement of molecules or ions of various substances like gases, liquids, and solids from their higher concentration to lower concentration.

Molecules of gases have higher kinetic energy than those of the liquid. This diffusion continues till the equilibrium is established between two regions. This finally causes the diffusion process to stop.

Simple Diffusion Characteristics feature:

1. Substances that undergo diffusion, maybe in any state—solid, liquid, or gas.
2. Diffusion is a physical and passive (does not require expenditure of energy) transport process.
3. Diffusion of molecules can take place in the presence or absence of a permeable or semi-permeable membrane. example, O₂ diffuses from the blood vessel into the cell through the plasma membrane of the cell.
4. In diffusion, molecules or ions always move from a region of higher concentration to a lower concentration. This movement continues until the concentration of both regions becomes equal.
5. Diffusion depends on pressure gradient (for gases), concentration gradient (for liquid or solution), and electrical or potential gradient (in the case of electrolytes)

The different gradients related to diffusion are:

Partial pressure gradient: The partial pressure difference of gases, between two regions or on both sides of a semipermeable membrane is called the partial pressure gradient of gas.

Concentration gradient: The difference in the number of molecules per unit volume of a substance between two adjacent regions is called concentration gradient.

Electrical gradient: The difference in the concentration of ions, carrying similar charge, on both sides of a semipermeable membrane is called electrical or potential gradient.

Diffusion Pressure or DP:

Diffusion Pressure or DP Definition: The pressure exerted by the tendency of a molecule or ion of liquid gas or solid to diffuse from the region of higher concentration to a region of lower concentration, is called diffusion pressure or DP.

Diffusion Pressure or DP Explanation: During diffusion, ions or molecules move from a region of high DP to a region of low DP. The magnitude of diffusion pressure is inversely proportional to the average distance between the molecules/ ions or directly proportional to the concentration, i.e., the higher the concentration of the molecules/ions, the greater their diffusion pressure.

It is directly proportional to the temperature, i.e., the average energy of a molecule/ion in a homogeneous substance rises as the temperature increases. However, it is constant for various substances at a given temperature.

Diffusion Pressure or DP Example: The DP and concentration of gaseous molecules in an inflated balloon is more than the atmospheric air. When DP becomes excessively high, the balloon bursts. At that time, the gas moves out from the region of higher DP to lower DP, by diffusion.

Diffusion Pressure Deficit or DPD:

Diffusion Pressure Deficit or DPD Definition:

• The difference between the diffusion pressure of pure solvent and the diffusion pressure of the same solvent in a solution is called diffusion pressure deficit or DPD.
• DPD= DP of pure solvent – DP of solvent in a solution
• The term ‘diffusion pressure deficit’ was coined by Meyer (1938).

Diffusion Pressure Deficit or DPD Explanation: Pure solvent shows diffusion pressure. If the solute is added to a solvent, the chemical potential of the solvent decreases, and a diffusion pressure deficit (DPD) is developed. So DPD of the solvent is proportional to the amount of the solute added to it.

When sucrose is dissolved in water, the diffusion pressure (DP) of water decreases. Thus, DPD develops between a sugar solution and pure water. If solution and pure water are separated by a permeable membrane, then molecules of pure water will diffuse into the sugar solution due to its high DP. This proves that the DPD of a solvent is proportional to the concentration of solute added to it.

Diffusion Pressure Deficit or DPD Example: Higher plants absorb water from the soil through their root due to lower diffusion pressure deficit of the cell sap in comparison to water in the soil.

Different types of diffusion medium:

Diffusion requires a medium. However, diffusion of gases can take place in a vacuum as well. Suppose, H₂s are kept in a jar, and a vacuum chamber is connected to it. After some time, H₂s gas will fill up this chamber. This happens due to the diffusion of gas molecules, from the lower jar into the connected vacuum chamber.

Thus, diffusion can occur through any medium, depending upon the diffusing substance.

Diffusion of solid in liquid: A glass jar is filled 2/3rd with water. A CuSO₄ crystal is dropped into it. After some time, it is observed that the crystal begins to dissolve in water, changing its color to blue. Finally, it is found that the crystal has dissolved completely and the water has attained a uniform blue color.

Diffusion of liquid in liquid: If a few drops of eosin (red-colored dye) or blue-colored ink is added to water. The drop dissolves and colors the water red or blue, respectively. This happens due to the diffusion of the dye in water.

Diffusion of solid in gas: Naphthalene balls are used in wardrobes to keep away insects. These solid balls disappear after some time. This is because naphthalene balls are volatile in nature. The molecules diffuse into the air and spread throughout the wardrobe.

Diffusion of liquid in gas: When a bottle of perfume is opened, its smell spreads throughout the room due to diffusion of the liquid in the air.

Diffusion of gas in gas: The scented fumes of an incense stick spread in the air due to diffusion.

Factors influencing the rate of diffusion:

The rate of diffusion is influenced by the following factors.

Gradient: The rate of diffusion of liquids and solids is proportional to their respective concentration gradient. Similarly, the rate of diffusion of gases and electrolytes is proportional to the partial pressure gradient and electrical or potential gradient respectively. This law is also known as Fick’s law of diffusion.

Rate of diffusion (r) α Concentration gradient of the liquid or the solution

Temperature: The rate of diffusion is directly proportional to the temperature of the medium. An increase in temperature increases the kinetic energy of the constituent molecules, thereby increasing their activity. As a result, the rate of diffusion increases.

Rate of diffusion (r) α Temperature of a medium (T)

The density of gaseous substance: The rate of diffusion of gaseous substance is inversely proportional to the square root of its density at constant temperature. This law is also known as Graham’s law of diffusion of gas.

The molecular weight of molecules undergoing diffusion: The rate of diffusion of molecules is inversely proportional to the square root of their molecular weight.

Diffusion medium: The rate of diffusion of any substance depends on the medium of diffusion. The rate of diffusion increases with the decrease in density (concentration of ions or molecules) of the medium.

Therefore, the lesser the concentration of ions or molecules in the medium, more will be their rate of diffusion. Due to this reason, diffusion of gaseous particles occurs faster in a vacuum than in any other medium.

Size of pores of the membrane (permeability): The rate of diffusion decreases with the increase in the size of the molecules. The size of the diffusing particles should always be smaller than the intermolecular space between the molecules of the diffusion medium.

Solubility: Solubility of a substance in the medium is also responsible for the change in the rate of diffusion of that substance. The more the solubility of the substance in the medium the greater its rate of diffusion.

Diffusion pressure gradient or DPG: Differences between the diffusion pressure of two regions are known as diffusion pressure gradient or DPG. The rate of diffusion of any substance is directly proportional to the difference in diffusion pressure gradient of two regions of a system and inversely proportional to the distance between two regions of the system.

These two factors can be written together as:

Viscosity: The rate of diffusion of any gas or liquid depends on its viscosity. The more viscous the fluid, the more its rate of diffusion.

Significance of diffusion:

Diffusion plays an important physiological role in plants.

Some of them are as follows:

1. During transpiration, excess water is removed from the mesophyll cells in leaves, into the atmosphere by diffusion.
2. Plants living in aquatic habitats take in O₂(during respiration) and CO₂ (during photosynthesis), through their body surface, by diffusion. They also take in the different minerals dissolved in water, by the process of diffusion.
3. Aquatic organisms absorb gases and essential minerals dissolved in water by diffusion.
4. Terrestrial plants take in different gases, i.e., CO_2, and O₂ from the atmosphere by diffusion. The gaseous exchange through intercellular spaces also takes place by this process.
5. Secretory substances like hormones, enzymes, etc., are released from cells by diffusion.
6. The growth of weeds is stopped by adding salt to its roots. Salt causes plasmolysis of the root cells, which results in the death of those weeds.
7. The passive upward movement of cell sap through the cell wall of plants is called apoplast.
8. The pressure developed due to the kinetic energy of the diffusing molecules is called diffusion pressure.
9. The pathway that forms through interconnected protoplast or plasmodesmata of adjacent cells in plants is called symplast.
10. The process by which hydrophilic colloids increase in volume by absorbing water is called imbibition.
11. Rubber does not show imbibition.
12. Wooden doors and windowpanes increase in volume in the rainy season due to imbibition and so, cannot be opened and closed easily.

Substances that undergo imbibition are:

1. raisins, dry seeds, velamen roots of orchids, and dry lichen.
2. Osmotic pressure is measured by an osmometer or osmotic pump.
3. When the concentration of the extracellular solution is more than the concentration of the intracellular solution, then such an extracellular solution is called a hypertonic solution.
4. When the concentration of extracellular solution is less than the concentration of the intracellular solution then such extracellular solution is called hypotonic solution.
5. When concentrations of both extracellular and intracellular solutions are the same, then both solutions are called isotonic solutions.
6. In a flaccid cell, suction pressure is equal to osmotic pressure.
7. Water potential is measured in terms of ‘bar’ or ‘pascal’. 1 bar = 0.098 atm or 106 dyne/cm2
8. Underground water is of four types—gravitational water, hygroscopic water, chemically combined water, and capillary water.
9. In some plants, some special structures containing an opening at the tip of veins (leaf margins) are called hydathodes. They remove excess water with dissolved enzymes, minerals, amino acids, etc. This process is called guttation.
10. The thin-walled cells of the endodermis of plant roots through which water enters the xylem are called passage cells.
11. In plants, food is translocated in the form of sucrose.
12. Dixon and Jolly (1894) gave the cohesion-adhesion theory regarding the ascent of sap in plants.
13. Surface tension does not participate in the transport of ions.

The pathway for the conduction of water from the soil to the xylem:

1. Pericycle is soil —>protoxylemroot hairs—>cortexmetaxylem.—> metaxylem.
2. Guard cells help in transpiration. When guard cells become turgid, stomata open, which causes the removal of excess water from the plant body.
3. Ganong’s photometer is an instrument that is used to measure the rate of transpiration.
4. Transpiration is responsible for the mass flow of ions.
5. Transpiration removes excess water from the plant body in the form of vapor. It keeps the plant body cool.
6. If the rate of transpiration exceeds the rate of absorption of water, then plants droop down. This is called wilting.
7. A pyrometer is used to measure the rate of opening of stomata.
8. A psychrometer is used to measure the rate of both
   relative humidity and transpiration.
9. Transpiration through lenticels occurs throughout the day and night.
10. Some minerals like P, N, Mg, Ca, etc., control the opening and closing of stomata.
11. Photoactive stoma remains open during the day and closes at night.

Scotoactive stoma remains open at night and closes during the day:

1. Red and blue light stimulates the opening of the stomata. However, blue light is more effective than red light.
2. Humidity in the air influences transpiration. If the humidity in the air is low, the rate of transpiration increases. Again when humidity in the air is high, the rate of transpiration decreases.
3. Transpiration helps in the passive absorption of water.
4. The chemicals which reduce the rate of transpiration, when applied to leaves, without interfering with other metabolic activities of plants, are called antitranspirants.
5. Plants remove 80-90% of total water intake by root in the atmosphere as vapor, by transpiration.
6. Food is synthesized in leaves translocated, to other parts of plants, and utilized in metabolism. The excess food remains stored in specialized storage organs.
7. Active transport is an energy-dependent process, in which ions or molecules are transported against the concentration or electrical gradient, by a carrier across the cell membrane.
8. The molecules that are transported through diffusion and osmosis, do not require energy.
9. The Munch hypothesis of the translocation of food is based on the movement of organic substances according to the gradient of turgor pressure.
10. According to protein-lecithin carrier theory by Bennet and Clark (1965), active transport occurs by carrier system. In this case, the phospholipid lecithin is the protein carrier. The choline group of this carrier acts like anion and the phosphatidyl group acts like a cation. Enzyme lecithinase facilitates the movement of substances.

 

Facilitated Diffusion

Facilitated Diffusion Definition: The process of passive transport (no energy required) by which molecules or ions of living cells move from a region of higher concentration to a region of lower concentration, through the integral protein present in the cell membrane is called facilitated diffusion.

Facilitated Diffusion Explanation: Diffusion of molecules across the membrane depends on their solubility in lipids. Polar ions are soluble in water but insoluble in lipids. They are large in size and so cannot cross the cell membrane. Moreover, the lipids present in the cell membrane have hydrophobic tails.

Due to this, polar ions cannot enter the cell by simple diffusion. In order to transport such ions, carrier proteins and channel proteins are present within the cell membrane. These proteins are called transmembrane proteins.

They have specific configurations that help them to transport specific molecules. Glucose, amino acids, and ions (Na⁺, Cl^–) are transported across the cell membrane by this mechanism.

Facilitated Diffusion Characteristic features:

1. Facilitated diffusion in living cells is a passive process that does not require any energy.
2. Facilitated diffusion occurs along the concentration gradient.
3. Certain large proteins called ionosphere or porin, span across the cell membrane. They have specific binding sites for particular molecules. These proteins change their configuration to form huge pores or channels when they bind with specific molecules. This allows the entry of large peptides, small proteins, and molecules of different substances inside the cells.
4. Smaller molecules show a higher rate of facilitated diffusion than larger molecules.
5. Channel proteins allow the transport of some specific substances across the cell membrane.
6. Channel proteins are usually gated i.e., they may be open or closed. When a gate is open, solutes of appropriate size may diffuse.

Types based on the direction of transport: Facilitated diffusion allows the transportation of one or more substances across the membrane. Based on the number of transported substances and the direction of their movement, facilitated diffusion has been classified into the following three types.

Facilitated Diffusion Significance:

1. Carbohydrates, amino acids, phosphate, CO⁺, Na⁺, K⁺, etc., are transported into the cell by this process.
2. Polar molecules and ions are also transported across membranes of cell organelles like mitochondria, through this process.

Similarities between simple diffusion and facilitated diffusion

1. Both are passive transport processes, hence both do not require energy.
2. In both cases, ions or molecules move from a region of higher concentration to a region of lower concentration

Active Transport

Active Transport Definition: The process of transport in which ions or molecules of a substance are transported, across the cell membrane with the expenditure of metabolic energy, using carrier protein or enzymes, against the concentration gradient is called active transport.

Active Transport Characteristics features:

1. Metabolic energy is utilized in active transport. This energy is obtained by the breakdown of ATP (ATP —> ADP + Pi) during metabolic reactions.
2. Active transport is carried out by carrier proteins which are present in the plasma membrane. Carrier proteins are substance-specific.
3. Active transport occurs against the concentration gradient, i.e., movement of molecules or ions occurs from lower concentration to higher concentration. So, this type of transport is also called uphill transport.
4. In the absence of oxygen, respiration slows down. It lowers ATP production. As the availability of energy decreases, the rate of active transport declines.
5. In most cases, active transport is accompanied by passive transport.
6. Substances called Inhibitors inhibit the process by reacting with the membrane proteins.
7. The rate of transport reaches its maximum level when all protein carriers are already bound with ions or molecules.

Active Transport Significance:

1. Absorption of water by roots j takes place through an active transport mechanism. Although soil water is absorbed by endosmosis, but absorption of minerals occurs by an active transport process, called an ion exchange mechanism.
2. During photosynthesis and respiration, ATP molecules are synthesized through a mechanism that is dependent upon the active transport mechanism.

Different theories or models have been proposed by various scientists regarding the active transport of ions or molecules across the plasma membrane.

Heinz and Walsh’s model for active transport:

1. In 1958, Heinz and Walsh first proposed a model for the active transport of ions or molecules across plasma membranes. The model is described as follows
2. Carrier proteins bind with ions or molecules which are to be transported. This complex thus formed is called carrier complex. It is formed by using kinetic or metabolic energy, at the external face of the plasma membrane. The carrier complex moves across the plasma membrane and reaches the internal face.
3. On the internal face, carrier protein and ions or molecules separate and ions or molecules enter the cytoplasm.
4. Carrier proteins become inactive due to the action of enzymes and return to the external face of the cell membrane,
5. The cycle repeats when the carrier proteins regain their activity by the action of metabolic energy.

Singer’s model of solute transport: There are two types of proteins in the cell membrane. Proteins present on the outer surface of the membrane are extrinsic or peripheral proteins. Proteins embedded in the lipid are intrinsic or integral proteins. Singer (1974-75) has described the role of extrinsic and intrinsic proteins in the active transport of solutes.

1. Integral proteins (1) in the plasma membrane are extended across the membrane and form a protein channel within it. Water molecules pass through this channel but solute molecules cannot.
2. Extrinsic or peripheral proteins (P) bind with ions and molecules of substances and fuse with integral proteins (1).
3. Conformational change of integral protein occurs due to the breakdown of ATP. This creates a passage through the channel protein and allows the entry of ions or molecules into the cell.

Plant Water Relations Water Potential, Osmosis, Plasmolysis And Imbibition

Water is an important component of plant cells. Mature plants have 5-10% of total water in the cytoplasm. The remaining 90-95% water is present within the large, centrally placed vacuole in the cell. The content of the vacuole is known as the cell sap. In actively dividing plant cells, water constitutes 80-95% of total cell mass. Water is a polar molecule.

Constituent molecules of water remain attached by forming hydrogen bonds. Due to the presence of hydrogen bonds, water molecules have high cohesive and adhesive forces. Cohesive force imparts high tensile strength to water.

All these characteristics of water molecules enable them to pass through the cell membrane easily. The factors that are related to the movement of water molecules across the cell membrane are—imbibition, water potential, plasmolysis, and osmosis. But, before discussing these factors, we must learn about the permeability of the membrane.

Permeability

Definition: The property of the cell wall and cell membrane, or any other membrane that allows different ions and molecules of elements and compounds to pass through them is called permeability.

Types of membranes on the basis of permeability

Based on the permeability, membranes are classified into the following types.

Permeable membrane: The membranes that allow movement of the molecules of both solutes and solvents through them are called permeable membranes. example, thin and cellulosic cell walls of parenchyma and meristematic tissue of plants.

Impermeable membrane: The membranes which do not allow the movement of either solute or solvent molecules through them, are called impermeable membranes. example, lignified or cutinized cell wall, a thin artificial membrane made of rubber or plastic.

Semipermeable membrane: The membranes which allow the passage of solvent molecules but do not allow the passage of solute molecules through them, are called semipermeable membranes. example parchment paper, cellophane paper, swim bladder of bony fishes, and the thin membrane under the hard shell of an egg.

Selectively permeable membrane or differentially permeable membrane: The membranes that allow only specific solute and solvent molecules to pass through them, according to the cell requirement, are called selectively permeable membranes or differentially permeable membranes. the example plasma membrane, nuclear membrane, and tonoplast.

Water Potential

The free energy per mole, i.e., per gram molecular weight of any substance is called its chemical potential. This free energy is required by all living organisms in order to maintain physiological activities. Under constant temperature and pressure, if the density of water increases in a system, then its kinetic energy also increases. This causes an increase in the chemical potential of water. On the basis of these observations, the concept of water potential was conceived by scientist Slayer (1967).

Definition: Under constant temperature and pressure, RU the difference between the free energy or chemical potential of a given water sample and the chemical potential of pure water is called water potential.

Water potential is denoted by the Greek letter psi with ‘w’ (to denote water) subscript position.

Water potential (ψ_w) is measured in the following ways—

1. Water potential can be obtained by dividing the chemical potential of water, of a given sample by the volume of 1 mole of water.
2. Water potential is also expressed as free energy per unit volume of water. Its unit is Joule per cubic meter or Jm-3. The measuring unit of water potential is Pascal (Pa) and Megapascal (MPa).

Major factors controlling water potential in plants

Three factors that control water potential in plants are—

Solute potential or osmotic potential (ψ)

Pressure potential (ψ_p)

gravity potential (ψ).

Water potential is expressed by these three terms as—

Ψ_w=ψ_s+Ψ_p+ψ_g

Osmotic potential (ψ_s): The potential of a solvent molecule to move from a lower concentration to a higher concentration across a semipermeable membrane, is called osmotic potential. The osmotic potential of pure water is zero. When a solute is dissolved in pure water, the osmotic potential of the solution becomes less than zero. Osmotic potential is also known as solute potential. It is denoted by ψ_s. It is measured in Bars. [1 Bar = 0.987 atmospheric pressure]. It is always negative for a solution.

Pressure potential (Ψ_p): When a plant cell is immersed in pure water, water enters the cell. This increases the turgidity. The pressure created on the cell membrane due to this turgid condition is called turgor pressure. This turgor pressure with respect to the water potential is called pressure potential. It is denoted by Ψ_p. Its magnitude is always positive. This pressure potential generates wall pressure in plant cells.Ψ_p of pure water is 0 MPa, although standard wall pressure is 0.1 MPa.

Gravity or Gravitational pressure potential (ψ_g): The effect of gravitational force on water potential is called gravitational pressure potential. It is denoted by ψ_g. It depends on the height of the water column (h), the density of water (pw), and acceleration due to gravity (g), i.e., ψ_g = yowgh. Water potential for a water column of 10m height will be, 0.1 MPa. So, for a height of 5m, ψ_g is negligible. In such cases, Ψ_w=ψ_s+Ψ_p

Plasmolysis

Plasmolysis Definition: The phenomenon in which the protoplasm of the plant cell separates from the cell wall and starts shrinking at the center due to exosmosis, when the cell is placed in a hypertonic solution, is called plasmolysis.

Plasmolysis Explanation: Mature plant cells have centrally located large vacuoles. When kept in hypertonic solution (OP_e>OP_i), water moves out by the process of exosmosis, into an external solution, from cytoplasm and cell sap. This causes a reduction in turgor pressure and volume of cell vacuole.

This situation is called the first stage of plasmolysis or limiting plasmolysis. At this stage cell starts to shrink but protoplasm remains attached to the cell -wall. The cell reduces slightly from its original size.

If the diffusion of water to the external solution continues due to exosmosis, then the vacuole contracts further, and protoplasm along with the cell membrane shrinks, but the cell wall remains intact. It appears that the protoplasm separates from the corners of the cell wall. This condition is known as the second phase of plasmolysis or incipient plasmolysis.

If the shrinking of protoplasm still continues due to continuous exosmosis; it will separate completely from the cell wall and assume a spherical shape. This phase is known as evident plasmolysis or complete plasmolysis.

According to the shrunken shape of the protoplast, plasmolysis is of two types

Convex plasmolysis: The protoplast appears completely contracted and becomes convex-shaped in this stage.

Concave plasmolysis: The protoplast does not appear contracted completely and remains attached to the cell wall at some points, through the protoplasmic fibers.

Test for plasmolysis

Materials required: Leaves of Rhoeo discolor plant, sugar or sucrose solution, some Petri dishes, forceps, glass slide, coverslip, distilled water, microscope.

Plasmolysis Procedure:

1. A series of sugar solutions, of different concentrations (0.1M, 0.2 M, 0.3 M, 0.4M) are prepared.
2. These are kept in different Petri dishes and are marked accordingly.
3. A Rhoeo leaf is taken. Its lower surface is peeled off and cut into small pieces. A few of these pieces are placed in each of the sucrose solutions made and a few are immersed in distilled water as control.
4. The Petri dishes are kept undisturbed for about 30-40 minutes.

Plasmolysis Observation and inference: Anthocyanin is a water-soluble pigment present in the cell sap of Rhoeo leaves. This pigment gives a purplish color to the cytoplasm. This helps in the visual identification of the changes in the cytoplasm.

After 30-40 minutes, the peels are taken out from the Petri dishes. They are then placed on glass slides and are covered with coverslips. Now they are observed under a microscope.

In distilled water and sucrose solutions of low concentration, no changes are observed in the cell cytoplasm or the vacuole. Thus we may infer that cell sap concentration remained unchanged.

Hence, plasmolysis has not occurred in these cells. However, leaves kept in a sucrose solution of high concentration show shrunken protoplasm which is detached from the wall. This means incipient plasmolysis has occurred.

This plasmolyzed protoplasm shows a darker purple color because of the low water content in the cell sap.

Plasmolysis Significance:

1. A living cell can be distinguished from a non-living cell through plasmolysis, as it does not occur in dead cells.
2. The osmotic pressure and osmotic concentration of any cell can be measured by limiting plasmolysis.
3. If the plasmolyzed condition prevails for a longer duration in a cell, it dies.
4. The occurrence of plasmolysis signifies the permeability and elasticity of the cell wall.

Deplasmolysis

If a plasmolyzed cell is kept in a hypotonic solution or in pure water, then the protoplast expands and restores its normal condition due to endosmosis (OP_e < OP_i). This phenomenon is called ‘deplasmolysis’.

What will happen when a filament of Spirogyra is kept in a 2% sucrose solution?

In the cells of a filament of Spirogyra cytoplasm is present as primordial utricle. Its central vacuole contains cell sap. But cell sap is less concentrated than the external 2% sucrose solution. So, water will move out of the cell to external sucrose solution, through the cell membrane.

This will cause a reduction in turgor pressure in the cell. Cytoplasm, with cell membrane, will shrink and become, almost spherical. Hence, plasmolysis takes place.

However, if these plasmolysed filaments of Spirogyra are kept in pure water, then OP_e< OP_i. Thus, water will enter the cell or endosmosis will occur. The cell will become turgid again. Hence, we can say deplasmolysis has occurred.

Imbibition

Imbibition Definition: The physical process by which water or any other liquid is absorbed by hydrophilic colloids like protein, polysaccharides, etc., is called imbibition.

Imbibition Explanation: Dry seeds swell up when they are soaked in water for some time. Wooden doors absorb water in the form of water vapor present in the atmosphere and swell during the rainy seasons. As a result, we face difficulty in opening doors. These are all consequences of imbibition.

This is a physical process by which hydrophilic colloid materials swell due to the absorption of water or other fluids. Components of cell walls such as cellulose, pectin, etc., are hydrophilic in nature. Therefore, they absorb water and swell up.

Different types of organic materials have imbibing properties. Those substances which have this property are called imbibing. The liquids which can be imbibed, are called imbibate. Protein, agar agar, starch, gelatin, etc., are some imbibing. Imbibants in the living organisms are mainly hydrophilic colloids.

Imbibition is absent in hydrophobic substances like lignin, cutin, etc. Imbibition is mainly the diffusion of an imbibe (for example water) within an imbibing. When the diffusion pressure of the imbibing is equal to the diffusion pressure of the imbibing, then the equilibrium is established. This leads to the cessation of the imbibition process. Gaseous substances also can be imbibed.

Effects of imbibition: Imbibition has several effects on the imbibing, such as—

Imbibition Swelling: The volume of the imbibant increases during the process of imbibition. This is commonly known as swelling.

Liberation of heat: Heat is released during imbibition

Imbibition pressure: Due to imbibition, huge pressure develops within the imbibant. This may cause the implant to burst or crack. This pressure is called imbibition pressure.

Imbibition Characteristic features:

1. Imbibition is a physical process.
2. The more hydrophilic the colloidal substances are, the faster will be the rate of imbibition.
3. During imbibition, imbibate molecules are attracted by adhesive forces, present on the surface of the imbibant. These molecules move fast towards the imbibant and their movement generates kinetic energy. As soon as they bind to the surface of the imbibing, the kinetic energy gets converted to heat energy.
4. The affinity between imbibing and imbibing facilitates the process of imbibition.
5. Imbibition pressure is generated in the implants due to an increase in volume property. Those substances which have thiafter the absorption of the imbibate.
6. Temperature, nature of solution, pH, presence of ions, etc., regulate the rate of imbibition.
7. This process does not require any semipermeable membrane.
8. Transport of imbibate molecules does not occur during imbibition. In fact, they remain bound to the imbibant molecules but do not get transported.
9. It is a passive process, that takes place along a diffusion gradient.
10. Imbibition requires direct contact between imbibant and imbibate molecules.

Factors affecting the rate of imbibition: The rate of imbibition is influenced by the following factors.

Imbibition Temperature: With the increase of temperature, I rate of imbibition increases.

The texture of Imbibing material: Imbibition occurs slowly in compact materials, like wood, and fast in lighter or soft materials, like gelatin.

Nature of imbibition: The nature of organic substances influences the rate of imbibition. For example, proteins have more imbibition capacity than starch. Cellulose has even less imbibition capacity than starch.

Imbibition Pressure: The rate of imbibition decreases with increasing pressure on imbibing.

Imbibition pressure or matric potential: Any colloidal substance that has imbibition property, is called matric. These substances are hydrophilic in nature and so, absorb water upon contact. The pressure that develops in the imbibant by imbibing water or other fluids, is called imbibition pressure or matric potential.

Imbibition Significance:

1. Absorption of water during germination of seeds takes place only through imbibition. Breaking of seed coat during germination occurs due to imbibition of water.
2. Many dry fruits (cotton balls, pods of mung bean, etc.) undergo dehiscence after absorbing water.
3. In roots, the cellulose molecules of the cell wall, absorb water by this process.

Long-Distance Transport Of Water

Water acts as the transporting medium in plants. Plants with well-developed vascular systems and root systems conduct water from the soil to the aerial parts (mainly leaves) of plants. Water is absorbed from soil by root hairs. The absorbed water is then transported upward through the xylem vessel to the leaves and other parts of plants.

Movement of water and minerals occurs over several meters, which cannot be achieved by diffusion. Long-distance transport takes place by means of mass flow or bulk flow. This process involves the movement of substances in bulk from one region to another due to differences in pressure between the two regions.

This pressure difference can be achieved either by a positive hydrostatic pressure gradient (for example a garden hose) or by a negative hydrostatic pressure gradient (for example suction by a straw). The bulk movement of substances in plants through conducting or vascular tissue is called translocation.

Long-distance transport of water in advanced plants, mainly vascular plants includes—

1. Absorption of water and water-soluble minerals by root hairs and transport of water and water-soluble minerals from root hairs to xylem vessels.
2. Upward movement of water and water-soluble minerals through xylem vessels by the process of the ascent of sap.

Water Absorption By Plants

Definition: The process by which plants obtain water and water-soluble minerals by root hairs from the soil for the proper functioning of their various physiological processes like nutrition, growth, etc., is called absorption.

The main source of soil water is rain. Some percentage of water received on earth during rainfall evaporates and most of the water flows away to other regions known as run-way water. The rest of the water is absorbed by soil particles under the action of gravitational force. The water which remains within the soil is divided into four categories.

These are as follows

Gravitational water: Most of the soil water percolates under the ground through the spaces between soil particles, under the action of gravitation force. This is called gravitational water. Gravitational water is beyond the reach of plants. So, it has no importance in their life.

Capillary water: Water molecules downwards, under the ground, by the action of gravitation force. During this descent, a few of them get trapped in spaces between and around soil particles due to the surface tension property of the soil particles. This water is known as capillary water. Most plants absorb this capillary water by endosmosis. Capillary water is essential for many physiological functions of plants.

Hygroscopic water: Capillary water attached to superficial soil particles evaporates. The remaining water persists as a thin film, bound with soil particles. This thin layer of water is known as hygroscopic water. Except for some grass and herbs, this water is not usable for most plants.

Chemically combined water: A small portion of water remains bound to soil particles through chemical reactions. This water is called chemically combined water. Roots of plants cannot absorb this water. Hence, it is not important to them.

Water-absorbing parts of plants

In plants, the absorption of water and minerals is accomplished by different organs. However, root is the main organ for absorption in most of the plants. Terrestrial plants mostly absorb water from the soil through root hairs. Root hairs are thin-walled slender extensions of root epidermal cells that greatly increase the surface area for absorption.

Water absorbing system

The root is the major point of entrance of water and minerals in higher plants, except in submerged plants. The area of young roots, where most absorption takes place, is the root hair zone. The root hairs lack cuticle and provide a large surface area. They are extensions of the epidermal cells.

They have sticky walls by which they adhere tightly to soil particles. Root hairs remain in contact with soil water. Water enters into the root hairs of the epiblema. From there, water reaches up to the endodermis through the cortex by cell-to-cell osmosis. The path followed is

Soil —> Root Hair —> Epiblema of root —> Pericycle —> Endodermis of root —>Xylem

They take water from the intervening spaces mainly by osmosis. Water movement across the regions of roots takes place in three probable systems or pathways

1. Apoplastic pathway,
2. Symplastic pathway,
3. Transmembrane pathway.

Munch (1930) first gave the idea about apoplastic and symplastic pathways.

Apoplastic pathway: In this pathway, the movement of water occurs exclusively through the dead cell wall, forming a continuous channel, without the involvement of any living membrane. The apoplast does not provide any barrier to water molecules, the movement depends purely on gradient and occurs through mass flow. In the plant body, a major portion of water is transported through the apoplastic pathway.

Symplastic pathway: In this pathway, the movement of water molecules takes place from one cell to another, through plasmodesmata.

The walls of some endodermal cells contain depositions of lignin and suberin. These depositions are known as Casparian strips. The Casparian strips, separate the cortex and the endodermis.

Suberin, present in these strips, blocks water and solute molecules from passing through the cell wall of the endodermis. In this case, water movement takes place through the endodermal cells.

In some plants, some specialized endodermal cells take part in the transport of water. These endodermal cells are located at the opposite of xylem vessels. They are called passage cells. These cells are poorly supervised.

This forces the water to go through the cell membranes of different cells

Transmembrane pathway: In this pathway, water and water-soluble minerals from root hair cells move to the protoplast of living parenchymatous cells of the epidermis and cortex through the plasma membrane. The water molecules cross the vacuolar membrane, i.e.tonoplast and then move via symplast.

Mycorrhizal Absorption

Mycorrhiza is a symbiotic association between a fungus and a seeded plant. The fungus is known as mycorrhizal fungus. The fungal mycelia form a network around the young root or they penetrate the cortical cells of the root.

Hyphal growth increases the root surface area which assures increased absorption of water and minerals from soil. The fungus provides minerals and water to the roots. The roots, in turn, provide sugars and N-containing compounds to the fungus.

Some plants have an obligate association with the mycorrhizae. For example, association with mycorrhizal fungi is essential for the germination of Pinus, seeds as well as the establishment of young Pinus seedlings in soil.

There are two mechanisms regarding the absorption of water by root hairs

1. Passive absorption, and
2. Active absorption.

Passive absorption: In this process, the absorption of water does not require any expenditure of energy (breakdown of ATP). About 98% of total water uptake by a plant occurs by this process. In this process, water is absorbed through roots. The root cells act as a physical absorbing system.

In this type of absorption, the force originates due to transpiration. Transpiration creates a lower water potential (or tension) in the mesophyll cells of leaves, thus water is drawn upward from the xylem.

As the water column is continuous from leaves to roots through the xylem, the tension is transmitted to the xylem in roots and ultimately in the root hairs. This develops a water potential gradient between root hair and soil solution and helps in the movement of water across the root.

During passive absorption, the transport of water mainly occurs through the apoplast, but it endodermis through the symplast.

Active absorption: This is absorbed due to the osmotic pressure of the roots with the help of energy. A root hair cell functions as an osmotic system. Water is absorbed by root hair due to the difference in water potential between soil water and cell sap.

Thus, water moves from the region of high water potential towards the lower water potential. Water continues to enter root hair cells as long as its water potential remains lower. This type of absorption involves symplast.

Theories regarding the absorption of water by plants

Regarding the mechanism of water absorption, there are two theories—osmotic and non-osmotic theory.

Osmotic Theory: According to this theory, the transportation of water takes place by osmosis along a gradient of decreasing water potential (Ψ_w) or increasing solute potential ( ψ_s), from the soil solution to the root xylem. The cell sap of root hair possesses a higher osmotic potential (ψ_s) than that of the soil solution.

The difference in the osmotic potential of the two systems causes the water to enter the cells of root hair. Entry of water decreases the protoplasmic concentration of the cell which causes water to move to the next cellular layer and ultimately to endodermis.

So, in this way, water moves from the soil to the root xylem and finally to the endodermis, by cell-to-cell osmosis. Exudation of sap from the cut end of a plant and guttation are some important examples of the osmotic absorption of water from the soil.

Non-osmotic theory: According to this theory, absorption of water can also occur by expending energy (provided by respiration). Active water absorption needs metabolic energy, hence it is restricted to living cells.

Water Movement Up A Plantascent Of Sap

Plantascent Of Sap Definition: The process by which the xylem vessel of the root and shoot of terrestrial plants transport water and water-soluble minerals or xylem sap, against gravity, to leaves and other aerial parts of plants, is known as the ascent of sap.

The ascent of sap in plants occurs in xylem tissue. Tracheids and vessels or tracheae of xylem tissue are the cells involved in the ascent of sap.

Tracheids are long, dead cells, with tapered ends. The end walls of matured tracheal cells are perforated. These are known as perforation plates. Tracheids and tracheae have thick and lignified cell walls.

Ornamentation like pits or secondary thickenings are also seen on the walls. Minerals are transported upward mainly by xylem vessels. But solutes are transported laterally to the phloem by cell-to-cell diffusion. Hoagland and Stout (1939), used radioactive potassium (K²⁴) to prove this.

Experiments to show the path of water transportation

The path of water transportation can be shown through some simple experiments.

Eosin Dye Test

Eosin Dye Test Ingredients: Large glass jar, Peperomia sp. plant with intact root, eosin dye, cork with a central hole, mustard oil, glass marker.

Eosin Dye Test Procedure:

1. A Peperomia sp. plant with intact root is collected.
2. 2/3^rdparts of a clean glass jar is filled with water and a few drops of eosin are added to it. The water turns red.
3. The level of water is marked by a glass marker.
4. The root portion of the plant is inserted through the central hole of the cork. It is placed in such a way that the root must submerge in water.
5. A few drops of mustard oil are added to the surface of the water to prevent evaporation.
6. The mouth of the glass jar is sealed by placing the plant-containing cork.
7. The arrangement is kept under direct sunlight for some hours.

Eosin Dye Test Observation: The stem of Peperomia sp. is translucent. After a few hours, it is observed that the stem and leaves become reddish and the level of water in the jar has gone down. If transverse sections are cut from the shoot and are observed under a microscope, only the wall of the xylem vessels will show red coloration.

Inference and explanation: The above experiment proves that the roots of plants absorb water and transport it in an upward direction. The absorbed water reaches the leaves through the xylem.

Eosin Dye Test Ringing experiment

Ingredients: A freshly collected Impatiens balsamina plant with intact root system, knife, beaker, water, and clamp.

Eosin Dye Test Procedure:

1. A section of the stem of the Impatiens balsamina plant is girdled, i.e., of a strip of bark (consisting of phloem and other tissues but not xylem)
2. The roots of this plant are submerged in water taken in a beaker. The plant is kept erect with a damp.
3. This arrangement is kept under light for 2-3 days.

Eosin Dye Test Observation: After 2-3 days, it is observed that the j leaves of the plant are still fresh and turgid. Leaves are F not showing signs of wilting.

Inference and explanation: It happens due to the upward movement of water and water-soluble minerals from roots to leaves, through xylem vessels. Lateral tissues (tissue present in the removed part) are not involved in the upward transport of water.

Theories On The Ascent Of sap

The water column in the stem of plants can rise up to a height of j 10m (34 ft), under standard atmospheric pressure j (1 atm). However, in some plant groups, most of the I members are more than 10m in height.

For example, the height of Eucalyptus and Sequoia sp., are more than 90m. Sometimes the height can be up to 120m. In such plants, the ascent of sap cannot be explained by root pressure only.

Various theories have been proposed from time to time to explain the mechanism of the ascent of sap in plants.

The two main theories are

1. Root pressure theory and
2. Cohesion-tension or Transpiration pull theory.

Root Pressure Theory

This theory was proposed by Priestley (1916). According to this theory, the absorbed water in roots generates hydrostatic pressure which extends up to passage cells of the endodermis.

Root pressure: When water is transported through the various cellular layers, from root hair to epiblema of roots, the cells become turgid and flaccid, alternatively. The hydrostatic pressure that develops in those cells, forces the sap into the lumen of the xylem. This pressure is known as root pressure. Stephan Hales (1727) first used the term ‘root pressure’.

Experiment to demonstrate root pressure

Root pressure Materials required: A potted tomato plant, knife, rubber band, a glass tube, one manometer, clamp, and standing water.

Root pressure Procedure:

1. A potted tomato plant is taken. It is watered properly,
2. The shoot of the plant is cut transversely at a height of 20 cm from the ground.
3. A glass tube and a manometer are tied up at the cut end with the rubber band.
4. The manometer is kept upright by means of a clamp and stand.

Root pressure Observation: After a few hours, it is observed that the glass tube has been filled with water, and the mercury in the manometer has risen.

Root pressure Inference and explanation: Due to root pressure, water conducted from the root through the shoot, enters the glass tube through the cut shoot end. This pressure, in turn, causes the mercury in the manometer to rise.

Role of root pressure in water transport: Water and minerals absorbed by root hair reach the endodermis. However, this sap cannot penetrate the wall of the xylem vessel by simple endosmosis. This is because most of the cells of the xylem are dead. The sap is actually pumped into the xylem. This happens with the help of the root pressure.

Absorption of water by root hairs causes a decrease in the concentration of cell sap. However, the concentration of cell sap is greater in adjacent parenchyma cells of the living cortex. So, water is transported by cell-to-cell osmosis to parenchyma cells. This increases turgidity in the cells.

Again, when water leaves those turgid cells and moves to adjacent cells, the former cell becomes flaccid. This alternate turgid and flaccid condition the of cells generates a hydrostatic pressure that originates in root hair and extends up to endodermal cells.

However, this hydrostatic pressure is not enough for the movement of water across endodermal cells having Casparian strips. Passage cells in endodermis, remain associated with the xylem vessels. These cells exert a huge pressure (root pressure) on the water stream to push water into the xylem lumen.

Root pressure conducts water and minerals to the tracheids and tracheae through these passage cells. Endodermis acts as a water dam. As a result, water cannot revert back once it crosses the endodermal layer. The upward movement of water, through the xylem, is made possible by root pressure.

Guttation: In herbaceous plants (like grasses) and plants with serrated leaves of warmer climates droplets of water have been observed on the tip of the leaves. These small droplets can be seen at regular intervals, late at night, and during early morning in the spring season. Earlier these droplets were well known as condensed atmospheric moisture or dew drops.

However, after experiments, it has been found that these water droplets contain dissolved enzymes, proteins, organic acid minerals, and waste products of plants. Moreover, these droplets can be distinguished by the fact that guttation occurs only at the tips and margins of the leaf lamina but not over the whole surface of a leaf.

Through vigorous further research, it has been found that the margin of the leaves of these plants contains stomata-like openings. These are called hydathodes through which waste substances of plants come out.

This process of eliminating waste as fluid is called exudation or guttation. However, this process of exudation occurs only in herbs, it has never been observed in trees.

Guttation Definition: The process by which herbaceous plants of tropical areas expel water, containing dissolved substances such as enzymes, proteins, organic acids, carbohydrates, minerals, nitrogenous substances etc., through openings at the leaf margins called hydathodes, at night and early morning mainly in the spring season, is called guttation.

Structure of hydathode: Hydathodes are special structures present on leaf margins where veins and venules get terminated. The terminal pore of each hydathode is surrounded by two guard cells. Unlike stomata, guard cells always remain open.

A water cavity is found within the opening of the hydathode. Each hydathode contains a tissue of small, thin-walled clusters of parenchyma cells with extensive intercellular spaces, called epithem. Beneath the epithem, vascular bundle, and specially terminated ends of tracheids are present. On both of the lateral sides of tracheids chloroplast containing parenchyma, i.e., chlorenchyma tissues are present.

2. Conditions and reasons for guttation:

• It generally occurs in the spring season, either at night or in the early morning, in herbs,
• In these plants, the rate of water absorption is more than the rate of transpiration. Thus, excess root pressure is the main cause of exudation,
• Guttation is prevented by water deficiency, water logging conditions, and excessive mineral deficiency in the soil.

3. Process of guttation: Due to excessive root pressure in a vascular bundle, excess water, and minerals are transported through tracheids to the water cavity of hydathodes. At last, these water and minerals ooze out on the leaf surface in the form of droplets.

However, the eliminated water is not pure. It contains dissolved enzymes, proteins, organic acids, minerals, excretory substances, etc. A spot or impression is left by the salt on the leaf surface after the evaporation of these water droplets.

4. Importance of guttation:

1. Some herbs absorb more water than that being lost by of transpiration. So, guttation helps to remove the excess water from the plant body,
2. Guttation also removes excretory substances from the plant body.

Bleeding: In plants growing in well-watered soil, the injured parts exude water with dissolved organic and inorganic substances i.e., sap. This is known as bleeding, Exudation of latex from laticiferous ducts in Euphorbia sp., Morns sp., etc., are examples of bleeding.

Contribution of root pressure: Root pressure provides a gentle push in the process of water transport in plants this helps to re-establish the continuous water column in xylems which often break under the tension created by transpiration.

Contribution of root pressure: Root pressure provides a gentle push in the process of water transport in plants this helps to re-establish the continuous water column in xylems which often break under the tension created by transpiration.

Drawbacks of root pressure theory: Kramer, Unger, Renner, Dixon, and Jolly strongly opposed the root pressure theory. They pointed out some drawbacks in this theory.

Those are—

1. The magnitude of root pressure is very low which may not be applicable for tall trees. Root pressures developed in herbaceous plants are sufficient to move water to the top but not in gymnosperms with lofty habits. So, this theory is not applicable to tall trees.
2. Root pressure is negligible during summer when transpiration is very high.
3. It is very less or absent in gymnosperms.
4. It is absent in plants growing in cold or drought conditions and in poorly aerated soil. But the ascent of sap is still normal in those plants. Hence root pressure cannot explain the ascent of sap.

Cohesion-tension theory or Transpiration pull theory

According to many scientists, living cells have no role in ascent of sap. Rather, some physical processes are responsible for the ascent of sap.

In 1894 Henry H. Dixon, an Irish botanist, and John Jolly, a physicist developed the idea of this specific theory of ascent of sap. This has been the most accepted theory until today.

It states that these two physical forces are responsible for the ascent of sap.

1. Cohesion and adhesion forces, and
2. Transpiration pull

Cohesion and adhesion forces: The force of attraction between two similar molecules is called cohesion. The force of attraction between two different molecules is called adhesion. The water molecules remain bound to each other by force of cohesion.

The water molecules remain attached to the cellulose molecules of the inner surface of the xylem vessel, by force of adhesion. The xylem vessels are tubular structures, which extend from roots to the top of plants.

They are placed above one another to form a column. They are connected through their perforated end wall. This helps to create a continuous water column within the vessel.

Transpiration puli: During the daytime, water in the mesophyll cells of leaves is released into the atmosphere through stomata, by the process of transpiration. This creates a water deficit in these cells. As a result, water potential declines, and diffusion pressure increases in cells of that region.

Which establishes a negative gradient of pressure or tension along the path of the ascent of sap. mesophyll cells of leaves and cells of xylem vessels experience a suction pressure that is transmitted all the way down to the unbroken column of water through the xylem vessel up to the roots.

The molecules of water show cohesion and the vessels show adhesion to the water molecules. due to these forces water column does not break and is pulled upwards by the transpiration pull.

Evidence in support of the theory:

The facts which support this theory are—

1. Pressure in the mesophyll cells of leaves is 20 atm. At 1 atm pressure, water can rise in a column of 10m. So in tall plants, the ascent of sap can occur without any difficulty up to a height of 200 meters.
2. The ascent of sap is a physical process. For this, metabolic energy is not required. Even if energy is required, it is almost negligible. 0.5 cal energy is spent to pull 1ml of water to a height of 100m.
3. The magnitude of the force of transpiration pull in the xylem vessel is 25-300 atm; so a continuous column of water forms.
4. This is supported by the fact that, on a warm day, the diameter of the xylem vessel gets reduced. This indicates a presence of strong transpiration pull.

Demonstration of transpiration pull in plants:

The process of transpiration pull occurring in plants can be explained through a simple experiment.

Demonstration Of Transpiration Pull In Plants Ingredients: Two beakers, two glass tubes, one cork with a central hole, porous container, mercury, pure water, sealing wax, glass marker, and a freshly cut, young, leafy twig.

Demonstration Of Transpiration Pull In Plants Procedure:

1. One end of a glass tube is fixed with a holed cork. A freshly cut, young, leafy twig is fixed through the hole in the cork.
2. The gap between the glass tube and the cork and the gap at the hole of the cork is sealed with sealing wax.
3. Another glass tube is fitted with a porous container.
4. Both the tubes are now filled with water.
5. The open end is placed carefully in the beakers containing mercury, by covering the open end with a finger.
6. The initial level of mercury in the glass tube is marked with a glass marker.
7. The setups are kept in direct sunlight for a few hours.

Demonstration Of Transpiration Pull In Plants Observation: After a few hours it will be observed that the levels of mercury in both the glass tubes have risen. The levels are again marked.

Demonstration Of Transpiration Pull In Plants Inference and explanation: In the setup with the plant no evaporation has occurred as it was airtight. But the leafy shoot attached to the glass tube has transpired. Due to transpiration water is lost from the plant body. This has created a tension or transpiration pull in the plant. This has caused the rise of mercury levels through glass tubes.

Similarly, evaporation of water has occurred in the setup without a plant, through the porous container, it has generated a vacuum in the tube. To fill this vacuum mercury has risen through the tube.

Postethwalt and Rogers (1958) used radioactive phosphorus (P32) to prove that even in the presence of gas bubbles in the water column of xylem vessels, the ascent of sap occurs normally. As xylem vessels are lined laterally by other cells, water may get transported through those supporting cells. When transpiration pulls decrease at night, the air bubbles dissolve and the continuous water column is again restored.

Drawbacks of the cohesion tension and transpiration pull theory:

1. In the root pressure theory, it has been already demonstrated that water exudes from a cut stem that has no leaf. This observation contradicts the transpiration pull concept.
2. A fact against this theory is that a temperature variation in day and night can cause the formation of bubbles in the water column in the xylem vessels with larger diameters. In such a case, a continuous water column may not form.

Translocation Of Mineral Ions

Most mineral ions are absorbed by roots through active or passive processes, or a combination of the two. They are further transported up to the xylem, and from there they reach all over the plant body. Mineral ions are frequently remobilized, particularly from older, senescing parts to the young growing tissues.

Common minerals that are remobilized in plants are P, S, N, and K. Minerals such as Ca are not remobilized. This translocation of minerals takes place by two paths. They are

Radial translocation in root: Solutes absorbed by root hair pass inwardly through the epidermis, cortex, endodermis, pericycle, xylem parenchyma, and xylem channels. Translocation of mineral ions takes place through apoplasts, symplast, or both.

Upward translocation in shoot: This takes place through xylem channels along with the ascent of sap. Hoagland and Stout (1939) confirmed that the xylem takes most of the absorbed minerals along with the water current to leaves.

Minerals are carried by the phloem if they are needed to be distributed from leaves to other parts of the plant body. Minerals convert to organic form and can diffuse into the xylem and are carried upward.

The chief storage regions or sinks for the mineral elements are meristems, young leaves, developing flowers, fruits, seeds, and storage organs. Minerals are unloaded into the living cells from the fine vein endings through diffusion and active uptake.

Phloem Transport – Flow From Source To Sonk

The carbohydrates synthesized through photosynthesis, are transported from the leaves to different parts of the plant by the phloem tissue. This food is utilized by the plant for cellular respiration and for storing. The leaf acts as the source and the utilizing organs and the storage organs are the sinks.

Plant hormones or phytohormones are synthesized by different parts of the plant such as roots, stems, leaves, etc. These hormones are also transported by phloem tissue.

Translocation Of Organic Solutes

Definition: The process by which organic food, other organic compounds, and hormones are transported through phloem tissue to the main storage organs or different sites of action of plants, is called translocation of organic solutes.

Green plants containing chlorophyll in their leaves synthesize food in the presence of sunlight. Glyceraldehyde-3-phosphate (GAP) is the primary product of the Calvin Cycle. It is used in a variety of biosynthetic pathways, both inside and outside the chloroplast.

For the first time, it is transported out of the chloroplast of the source cells (mesophylls) in the form of dihydroxyacetone phosphate (DHAP). The triose-phosphates combine in the cytosol to form fructose 1,6-bisphosphate (F6P) by the enzyme aldolase.

F6P is then isomerized to glucose 6-phosphate by the enzyme hexose phosphate isomerase. The glucose and fructose molecules are ultimately converted to sucrose for transport.

Sucrose is then loaded in the phloem sieve tube through apoplastic (cell wall) or symplastic (through plasmodesmata) pathways for long-distance transport. Sucrose is transported to the site of utilization or storage or sink.

Plant hormones or phytohormones are synthesized by different parts of the plant such as roots, stems, leaves, etc. These hormones are also transported by phloem tissue.

Plant tissues involved in the translocation of solutes The Translocation of solutes in plants occurs through phloem tissue.

Plant tissues involved in the translocation of solutes

Translocation of solute in plants occurs through phloem tissue. Sieve tubes of phloem are the main components that translocate solute. Companion cells remain associated with each sieve tube and accompany sieve tubes in this process. However, according to modern scientists (Cutter, 1978), companion cells too translocate solute.

Cells of sieve tubes (sieve cells) are without a nucleus and stop;asm is present in the form of a primordial utricle. These cells are elongated and cylindrical in shape. Sieve cells are arranged consecutively along their length to form sieve tubes.

The sieve tubes are so named because their end walls are perforated. These perforations allow cytoplasmic connections (plasmodesmata) between the cells. This helps in the transport of sugars and amino acids from the leaves, to the rest of the plant. Companion cell provides energy to the sieve cells.

Companion cells transport products of photosynthesis from cells of leaves to the sieve tube elements through plasmodesmata. They synthesize the various proteins used in the phloem. They also contain numerous mitochondria that provide the energy required for active transport.

Solute transport pathway: Translocation of solute occurs mainly in a descending manner. However, other modes of translocation like, ascending and lateral translocation also occur to some extent.

Descending translocation: The food produced in the leaves and other photosynthetic parts of plants, is translocated downward in a dissolved state through phloem tissue to roots, underground stem, and other storage parts of the plant. Some hormones are produced at the tip of leaves, stems, and branches which are transported downward to their target organs.

Ascending translocation: The organic food, solvent, and different phytohormones are also translocated upward in plants. During winter, many deciduous plants shed their leaves. In such plants, the stored food is dissociated into simple sugar which is transported through phloem to different parts of plants.

Lateral translocation: According to modern concepts, translocation of solute also occurs laterally. Trees show lateral growth of branches and stems. These lateral branches of roots and stems carry out lateral transport of solute.

Translocation of starch through the phloem

There is experimental proof that the transport of organic food occurs through phloem tissue.

Ringing or girdling experiment:

This experiment was done by T. G. Mason and E. J. Maskell (1928). They took a potted woody plant and carefully cut out a ring in the woody stem of the plant. The ring is cut outside the vascular cambium in such a way that only the bark, cortex, and phloem are removed.

Thus, the upper region of the plant remains connected with the lower region, through the xylem and pith only. After a few days, they observed that the stem of the upper part at the cut region had swelled up and adventitious roots had also developed. It has also been observed that the underground roots degenerate after some days.

This experiment proved that the translocation of food through phloem is descending. When the phloem is cut out, food accumulates at the cut region and cannot reach up to the roots. Due to the absence of food, roots ultimately die.

Isotopic study: Burr et al (1945) used radioactive carbon to trace descending translocation of food through the phloem. They supplied CO₂ for photosynthesis in the bean plant, in which the carbon used was C₁₄. As a result, the carbohydrate food produced by bean plants was also radioactive.

By autoradiography, they traced the translocation path of this food through phloem. This experiment clearly proved that food is translocated downward through sieve tubes of phloem tissue.

Composition of phloem sap

1. The main component of phloem sap is sucrose. Besides, in some plants carbohydrates like raffinose, stachyose, oligosaccharide, mannitol, sorbitol, etc., are also found. Apart from these, glucose and fructose are commonly found in the phloem sap of all plants.
2. Phloem sap contains amino acids like glutamic acid, aspartic acid, leucin, valine, phenylalanine, threonine, etc.
3. Phloem sap also contains enzymes required in various processes of respiration.
4. It also contains ions like K⁺, Mg²⁺, Cl^2- PO_4^3–, etc.
5. It is composed of hormones (IAA, GA, ABA), various organic acids (mainly malic acid), etc.

Process of translocation of solute through phloem

There are various theories regarding the translocation of phloem sap through sieve tubes. Some of the theories are described below.

Munch’s mass flow or pressure flow hypothesis: This theory was first proposed by Hartig (1860). Later Munch (1930) gave its proper scientific explanation.

Munch carried out a simple experiment. He took two membranous spheres, both connected by a glass tube. The two spheres (A and B) are kept in two beakers filled with water. A contains a sugar solution and B contains water.

It would be observed that due to high osmotic pressure in A, endosmosis of water will take place. This will increase the turgor pressure in A. This will cause sugar from A to move into B. This process will continue till both the solutions attain the same osmotic pressure. If sugar is removed from B, then the flow of sugar from A will continue.

According to Munch, during the day, food is synthesized which increases the concentration of starch in the mesophyll cells of the leaf. This in turn causes an influx of water from surrounding cells to mesophyll cells and increases their turgor pressure.

The parts or organs where food is prepared in plants, i.e., the leaves are called the source, and the other nongreen portions of the plants that utilize or store the food are called the sink.

The two are connected by sieve tubes of phloem tissue through which mass flow of phloem sap occurs. In the experiment, A is the source and B is the sink and the glass tube is the comparable to sieve tube.

Evidence in support of this theory:

1. Munch (1930), Dixon (1933), Huber (1941), and Crafts (1939) observed the exudation of sap by cutting phloem or through any wounds of woody stem and herbs.
2. Benette (1937) and Rohrbaugh and Rice (1949) have applied C14 lebelled 2,4-D (2,4-dichlorophenoxyacetic acid, also known as chemically prepared auxin) on plants to observe the translocation of food.

Criticism against this theory:

1. According to the mass flow hypothesis, the turgor pressure of the source should always be greater. But, Curtis and Schofield (1933) have experimentally proved that in most plants, a source has low turgor pressure.
2. Munch stated that sieve tubes are dead cells so mass flow of sap is a passive transport. However, recent experiments proved that mass flow of sap is an active process that requires ATP.
3. Munch’s hypothesis cannot explain the bidirectional translocation of sap.

Phloem loading and unloading

The translocation of food from leaves to sieve tubes is called phloem loading. Translocation of food from sieve tubes to non-photosynthetic parts of plants such as roots, stems, etc., is called phloem unloading.

Protoplasmic streaming theory:

de Vries (1855) stated that solute in phloem can move upward, due to cyclosis in protoplasm. Later, this hypothesis was explained by Curtis (1935).

Factors controlling the rate of translocation

Translocation is controlled by two groups of factors.

External factors: The external factors controlling translocation are—

Temperature: An increase in the temperature of soil increases the rate of translocation of solute towards roots. This results in a fall in the rate of movement of solutes inside the leaves as well. Generally, 25°C to 30°C is the optimum temperature range in which plants show the highest rate of translocation.

Light: A decrease in light intensity causes a decrease in the rate of translocation of solute.

Oxygen: As translocation of food is an active mechanism, 02 is required. In higher concentrations of oxygen, the rate of translocation increases.

Moisture stress: Moisture stress in leaves affects the rate of translocation.

Infernal factors: The internal factors controlling translocation are—

Solute potential: During the day, water in leaves decreases. This increases solute concentration which in turn increases the rate of translocation. The reverse occurs at night.

Translocation of sucrose through the apoplast pathway: When the concentration of sucrose increases in the apoplast pathway, the rate of translocation of solute also increases.

Number of P-proteins: P-protein is an important factor in the translocation of solutes. When P-protein gets stored in sieve tubes, it blocks the sieve plate and prevents translocation. Thus, an increase in P-protein results in a decrease in the rate of translocation.

Metabolic energy: ATP is required for translocation of solute which is essential for phloem loading and unloading. So, the rate of translocation increases with the increased rate of metabolic energy production through respiration (increased ATP production).

Turgor pressure: Recently it has been observed that water from the xylem vessel enters cells of sieve tubes and increases its turgor pressure. In comparison to the upper parts, turgor pressure in the lower parts of plants is less. This causes a downward translocation of sap. With the increase of difference of turgor pressure between the source and sink, the rate of translocation also increases.

Carrier protein: The plasma membrane of cells of sieve tubes and accessory cells contain carrier proteins. Carrier proteins like SUT1, SUT2, and SUT4 are present in the plasma membrane of cells of sieve tubes. They play an important role in the translocation of solutes.

Notes

Adhesion: The attraction between the molecules of dissimilar substances.

Apoplast: The space outside the plasma membrane within which materials can diffuse freely. It consists of intercellular spaces and xylem channels.

Bidirectional translocation: Two streams of sugars moving in opposite directions through phloem in plants.

Capillary water: The water held between the capillary spaces between the soil particles against the force of gravity.

Casparian strips: These are bands of cell wall materials mainly suberin and sometimes lignin, present in the radial and transverse walls of the endodermis.

Cohesion: The attraction between the molecules of the same substances.

Cyclosis: Cytoplasmic streaming in living cell.

Density: Mass of a substance per unit volume of it.

Eosin: A fluorescent red dye with chemical formula C2oH8Br405. Its sodium or potassium salt is used as a biological stain for cytoplasmic structures.

Gradient: An increase or decrease in the magnitude of a property, such as—temperature, pressure, concentration, etc.

Manometer: An instrument for measuring the pressure acting on a column of fluid.

Microfibril: A small fibril in the cytoplasm or cell wall, consisting of glycoproteins and cellulose.

P-protein: A phloem-specific protein found in sieve tubes in large amounts.

Plasmodesmata: A narrow thread of cytoplasm that passes through the cell walls of adjacent plant cells and allows communication between them.

Primordial utricle: The cytoplasmic lining formed on the. the inner side of the cell wall in a fully developed vacuolated cell.

Symplast: A continuous network of interconnected plant cell protoplasts in which water and low molecular weight solutes can freely diffuse.

Tensile strength: Resistance of a substance or material to breaking down under tension.

Tonoplast: A membrane that bounds the chief vacuole of a plant cell.

Transport In Plants Multiple Choice Questions

Question 1. Which of the following facilitates the opening of the stomatal aperture?

1. Decrease in turgidity of guard cells
2. Radial orientation of cellulose microfibrils in the cell wall of guard cells
3. Longitudinal orientation of cellulose microfibrils in the cell wall of guard cells
4. Contraction of an outer wall of guard cells

Answer: 2. Radial orientation of cellulose microfibrils in the cell wall of guard cells

Question 2. The water potential of pure water is

1. Less than zero
2. More than zero but less than one
3. More than one
4. Zero

Answer: 4. Zero

Read and Learn More WBCHSE Multiple Choice Question and Answers for Class 11 Biology

Question 3. Water vapor comes out from the plant leaf through the stomatal opening. Through the same stomatal opening, carbon dioxide diffuses into the plant during photosynthesis. Reason out the above statements using the following option.

1. Both processes can happen together because the diffusion coefficient of water and C02 different
2. The above processes happen only during nighttime
3. One process occurs during day time and the other at night
4. Both processes cannot happen simultaneously

Answer: 1. Both processes can happen together because the diffusion coefficient of water and CO₂ different

Question 4. Root pressure develops due to—

1. Increase in transpiration
2. Active absorption
3. Low Osmotic potential in soil
4. Passive absorption

Answer: 2. Active absorption

Question 5. A column of water within xylem vessels of tall trees does not break under its weight because of—

1. Positive root pressure
2. Dissolved sugars in water
3. Tensile Strength Of Water
4. Lignification of xylem vessels

Answer: 3. Tensile Strength Of Water

Question 6. The osmotic expansion of a cell kept in water is chiefly regulated by

1. Mitochondria
2. Vacuoles
3. Plastid
4. Ribosomes

Answer: 2. Vacuoles

Question 7. In which of the following, expenditure of energy is required?

1. Osmosis
2. Diffusion
3. Active transport
4. Passive transport

Answer: 3. Active transport

Question 8. In a plant cell, the diffusion pressure deficit is zero when it is

1. Plasmolyzed
2. Turgid
3. Flaccid
4. Incipient

Answer: 2. Turgid

Question 9. Guttation occurs through

1. Roots
2. Hydathode
3. Trichome
4. Stomata

Answer: 2. Hydathode

Question 10. Choose the wrong statement.

1. Cells swell in hypertonic solutions and shrink in hypotonic solutions
2. Water potential is the kinetic energy of water which helps in the movement of water
3. The absorption of water by seeds and dry woods takes place by a special type of diffusion called imbibition
4. Solute potential or ψ_s is always negative
5. Less than 1% of the water reaching the leaves is used in photosynthesis and plant growth

Answer: 1. Cells swell in hypertonic solutions and shrink in hypotonic solutions

Question 11. Which of these is/are not a property of facilitated Which of these is/are not a property of transport?

1. Requires special membrane proteins
2. Highly selective
3. Uphill transport

Choose the correct option.

1. [1] and [2]
2. [3] and [4]
3. [2] and [3]
4. [2] and [4]

Answer: 2. [3] and [4]

Question 12. When water moves out of the plant cell and the cell membrane of a plant shrinks away from its cell, then this condition is known as

1. Plasmolysis
2. Exosmosis
3. Hydrolysis
4. Endosmosis

Answer: 1. Plasmolysis

Question 13. A special type of diffusion when water is absorbed by solids is called

1. Osmosis
2. Plasmolysis
3. Both A And B
4. Imbibition

Answer: 4. Imbibition

Question 14. A layer of cells impervious to water because of a band suberised matrix is called the

1. Endodermis
2. Casparian strip
3. Plasmodesmata
4. None of these

Answer: 2. Casparian strip

Question 15. Study the following table showing the components of water potential of four cells of an actively transpiring plan Identify the four cells as root hair, cortical cell, endodermal cell (lacking Casparian strips), and pericycle respectively in the young root (assuming symplastic water flow through them).

1. B, D, C, A
2. D, A, C, B
3. A, D, C, B
4. A, C, B, D

Answer: 3. A, D, C, B

Question 16. Flaccid cell means

1. Cell turgidity
2. Plasmolysed cell
3. The Cell in which water flows in and out of the cell is in equilibrium
4. The Cell kept in a hypotonic solution

Answer: 2. Plasmolysed cell

Question 17. Cell A and cell B are adjacent plant cells. In cell A, ψ_s = -20 bars and ψ_s = 8 bars. In cell B, ψ_s = -12 bars and = 2 bars. Then.

1. Water moves from cell B to cell A
2. An equal amount of water is simultaneously exchanged between cell A and cell B
3. Water moves from cell A to cell B
4. There is no movement of water between cell A and cell B

Answer: 1. Water moves from cell B to cell A

Question 18. Cell A has an osmotic potential of -18 bars and a pressure potential of 8 bars, whereas, cell B has an osmotic potential of -14 bars and a pressure potential of 2 bars. The direction of flow of water will be—

1. From cell B to cell A
2. From cell A to cell B
3. No flow of water
4. In both directions

Answer: 2. From cell A to cell B

Question 19. The property of semi-permeability belongs to—

1. Cell wall
2. Plasma membrane
3. Mitochondria only
4. None of these

Answer: 2. Mitochondria only

Question 20. Translocation of photosynthetic end products in sieve tubes is

1. 305 mm/h
2. 3-5 cm/h
3. 1-15 cm/h
4. 60-100 cm/h

Answer: 4. 60-100 cm/h

Question 21. Transpiration is measured by—

1. Potometer
2. Parameter
3. Auxanometer
4. Respirometer

Answer: 1. Potometer

Question 22. Which of the following theories gives the latest explanation for the closure of stomata?

1. ABA theory
2. Munch theory
3. Starch glucose theory
4. Active K⁺ transport theory

Answer: 4. Active K⁺ transport theory

Question 23. By which mechanism, the salt-resistant plants can get rid of excess Na+ ions to the outer side, through the roots?

1. H⁺—ATPase uniport system
2. Na⁺—ATPase uniport system
3. H⁺—Cl^–symport system
4. Na⁺— H⁺ antiport system

Answer: 4. Na⁺— H⁺ antiport system

Biology Class 11 WBCHSE Transport In Plants Question and Answers

Question 1. Why is simple diffusion called passive transport?
Answer: In simple diffusion, solid, liquid, and gaseous ions or molecules are transported from their region of higher concentration to their region of lower concentration, without requiring any metabolic energy. So, simple diffusion is a type of passive transport.

Question 2. (1) What is porin? or (2) What is the composition of water channels?
Answer:

1. Porin is a type of protein that makes large pores in the outer membrane of plastids, mitochondria, and some bacteria for the transportation of large molecules across the membrane.
2. Water channels or aquaporins are composed of six transmembrane alpha-helices arranged in a right-handed bundle.

Question 3. In which physiological process of plants, osmotic balance is maintained?
Answer: Osmotic balance in plants is maintained by the process of transpiration.

Read and Learn More WBCHSE Solutions For Class 11 Biology

Question 4. (1) What is tyw’? or (2) What is the value of tj/w of pure water?
Answer:

1. Refers to water potential.
2. At standard temperature and pressure, pure water is zero.

Question 5. (1) Why water potential of any solvent is
negative? or (2) What is the measuring unit of ψ_w?
Answer:

1. The water potential of pure water is highest, i.e., zero. When a solute is added to it, its water potential decreases. So, the magnitude of the water potential of a solution is always negative.
2. The measuring unit of water potential (ÿw) is Pascal (Pa) Megapascal (MPa) or Kilopascal (kPa).

Biology Class 11 WBCHSE

Question 6. Two solutions A and B are separated by a permeable membrane. Ψ_w of A is -2.0 kPa and that of B is -1.0 kPa.

1. Whose Ψ_w is more?
2. Diffusion of solution will occur in which direction?

Answer:

1. Ψ_w of solution B is more.
2. Water diffuses from regions of higher water potential to lower water potential. So, the direction of diffusion should be from B to A.

Question 7. (1) What will be present between the cell wall and the shrunk protoplast of a plasmolysed cell? or (2) What will happen when such a cell is kept in pure water?
Answer:

1. The outer hypertonic solution is present between the cell wall and the shrunk protoplast of a plasmolyzed cell.
2. When a plasmolyzed cell is kept in pure water, deplasmolysis will occur. It means water will diffuse into the cell, protoplast will regain its normal shape. The volume of the vacuole will also increase.

Question 8. (1) What is turgor pressure? or (2) What will be of a fully turgid cell?
Answer:

1. When a plant cell is kept in a hypotonic solution, water enters the cell by endosmosis. This increases the turgidity of the cell. In such a case, the hydrostatic pressure exerted by the protoplast on the cell wall is called turgor pressure.
2. Refers to pressure potential. Water does not enter a fully turgid cell. So, will be zero.

Question 9. Why fishes are salted for preservation?
Answer:

Fishes are salted for preservation because common salt creates a hypertonic solution around the fish. Pathogenic bacteria, fungi that are intolerant to high salt concentrations are killed. They die of exosmosis and plasmolysis.

Question 10. (1) Since ancient times, dry woods have been used to crack rocks by placing them within the rocks. Which property is utilized here to produce the pressure? or (2) Write one application of that property in the human body.
Answer:

1. The property which is utilized to produce the pressure is imbibition.
2. Skin grafting (replacement by new skin) utilizes imbibition property. Newly grafted skin lacks capillaries. Under such conditions, O₂ and other substances are transported to the grafted skin by imbibition, until capillaries form.

Question 11. (1) What is the Casparian strip? or (2) What are passage cells?
Answer:

1. The suberin and lignin deposited between the inner wall and lateral wall of the endodermal parenchyma cells in mature plant roots is called the Casparian strip.
2. Casparian strips prevent the entry of water into the stele from the cortex. However, this strip is absent in some regions through which water is transported to the xylem vessels. The cells present in those regions, which allow water to enter are called passage cells.

Biology Class 11 WBCHSE

Question 12. (1) What is mycorrhiza? (2) How does it help to absorb water and minerals in roots?
Answer:

1. Roots of some phanerogams (like pine trees) are occupied by fungi to form symbiotic associations. These symbiotic associations between fungi and roots of phanerogams are called mycorrhiza.
2. Hypha of the fungi spread along the root. This increases the surface area for the absorption of water and minerals,- which in turn helps the plants.

Question 13. (1) Name the process and the plant part, by which terrestrial plants absorb water from soil. or (b) How do these plants absorb minerals and ions from soil?
Answer:

1. Terrestrial plants absorb water from the soil by the process of endosmosis through root hairs.
2. Terrestrial plants obtain minerals and ions from soil either dissolved in water or freely (when no water is being absorbed) by passive or active absorption methods.

Question 14. (1) When and in which type of plants do guttation occur? Which plant part is involved in guttation? (2) What is the main cause of guttation?
Answer:

1. Guttation mainly occurs at night and in the early morning in the spring season, in some plants of tropical areas. It occurs through the permanent openings called hydathodes, located at the leaf margins.
2. The main cause of guttation is root pressure.

Question 15. Describe the role of microfibrils of guard cells in opening and closing of stomata.
Answer: According to recent studies, it has been stated that the arrangement of microfibrils in guard cells controls the opening and closing of stomata. The cellulosic microfibrils are arranged parallel and radially. This lengthwise increase of microfibrils helps in the opening of stomata.

Biology Class 11 WBCHSE

Question 16. (1) Upto what height can water rise through the xylem vessel, due to transpiration? or (2) Which component of the xylem vessel is like a capillary tube that takes part in the ascent of sap?
Answer:

1. Experimentally it has been proved that, due to transpiration pull, water can rise up to a height of 130m through a xylem vessel.
2. The tracheids and trachea of the xylem vessel act as fine capillary tubes and they help in the ascent of sap.

Question 17. (1) Which tissue is involved in the transport of solute in higher plants? (2) Which components of that tissue help in the transport of solute?
Answer:

1. Transport of solute occurs through phloem tissue in phanerogams.
2. Sieve tubes and companion cells of phloem tissue are involved in the translocation of solute.

Question 18. (1) What is a sieve plate? or ( 2) How do the cells of sieve tubes maintain connections between them?
Answer:

1. The transverse, porous plate present between the consecutively arranged, columnar cells of mature sieve tubes, is called a sieve plate.
2. The cells of sieve tubes maintain connections between them by fine cytoplasmic projections and plasmodesmata that extend through pores in sieve plates.

Question 19. Transport of solute is bidirectional Explain.
Answer:

The facts that support the statement are as follows—

1. The organic food produced in the leaf of photosynthetic plants is transported downward through phloem tissue to roots and stems. There, it is utilized to produce energy and the excess food is stored in the form of starch.
2. Deciduous plants shed their leaves in winter, due to which they cannot photosynthesize. During this condition, the stored food in roots and stem are converted to simple sugar. This simple sugar is then transported upward to different parts of the plant.

Class 11 Biology WBCHSE Transport In Plants Very Short Answer Type Questions

Question 1. What do you understand by antitranspirant?
Answer: The chemicals that reduce the rate of transpiration in plants, when applied to leaf surfaces, without disrupting normal metabolic activity are called antitranspirants. For example, wax

Question 2. What is water potential?
Answer: The difference between the free energy of a molecule of pure water and the free energy of water in any other sample under normal pressure and temperature, is called water potential.

Question 3. What is wall pressure?
Answer: The opposite pressure applied by the cell wall against the turgor pressure in a fully turgid cell, is known as wall pressure.

Question 4. What is the unit of water potential?
Answer: The unit of water potential is bar. 1 bar = 0.987 atmosphere = 106 dyn/cm2.

Question 5. A flowering plant is planted in an earthen pot and watered. A lot of urea is added to make the plant grow faster, but after some time the plant dies. Name the process responsible for this.
Answer: The process is called exosmosis.

Question 6. In a plant cell, the cytoplasm is surrounded by both cell wall and cell membrane. However, it has been observed that the transport of substances across the cell membrane is very specific. Which feature is responsible for this?
Answer: The cell membrane is selectively permeable, so it shows specificity during the transport of substances.

Question 7. What do you understand by pressure potential?
Answer: When a cell is kept immersed in pure water, endosmosis occurs which increases the turgidity that pushes the cell organelles towards the membrane, generating turgor pressure. This is called pressure potential.

Question 8. A plant cell is kept in an aqueous solution and it gets plasmolyzed. What is the nature of the solution?
Answer: The nature of the solution is hypertonic.

Question 9. What is facilitated diffusion?
Answer: The process of diffusion across the membrane with the aid of a carrier is called facilitated diffusion

Question 10. Write three effectors that influence water potential.
Answer: Concentration, pressure, and relative density influence water potential.

Question 11. What is meant by wilting?
Answer: Wilting is the loss of turgidity by the cells at soft aerial parts like leaves and young branches. It occurs when the rate of transpiration is higher than the rate of water absorption.

Question 12. Which instrument is used to measure the magnitude of water absorption in the plant body?
Answer: The absorption of water is measured by a photometer.

Question 13. Name the minerals, that control the opening and closing of stomata.
Answer: Na, K Cl, etc., regulate the opening and closing of stomata.

Class 11 Biology WBCHSE

Question 14. What is passive absorption?
Answer: When water is absorbed by root hairs without utilizing any metabolic energy, it is known as passive absorption.

Question 15. What does the expression mean—Ψ_w=ψ_s+Ψ_p?
Answer: This expression means that water potential is the sum of osmotic potential (or solute potential) and pressure potential

Question 16. What is chemical potential?
Answer: The free energy of 1 mole of any substance is called its chemical potential.

Question 17. How osmotic pressure is controlled by temperature?
Answer: Osmotic pressure is directly proportional to temperature. It means when temperature increases, osmotic pressure too increases.

Question 18. What is known as the epithem?
Answer: The thin-walled parenchyma cells with extensive intercellular spaces which are found in hydathodes, are called epithem.

Question 19. How is transpiration related to the ascent of sap?
Answer: If the rate of transpiration increases, the ascent of sap will also increase. Therefore, transpiration is directly proportional to the ascent of sap.

Question 20. What is the water potential of pure water?
Answer: The water potential of pure water is zero.

Question 21. What is apoplast?
Answer: The pathway by which the passive transport of water and dissolved minerals occur through the cell wall of the plant cell, is called apoplast.

Question 22. What is meant by diffusion pressure?
Answer: The pressure that develops due to the kinetic energy of diffusible ions across the membrane is called diffusion pressure.

Question 23. Which plant hormone regulates the opening of stomata?
Answer: Cytokinin hormone controls the opening of stomata.

Question 24. What is known as capillary water?
Answer: The water that is present as non-colloid within soil particles is called capillary water.

Question 25. What is the approximate water potential of root hair?
Answer: The approximate water potential of root hair is 2 bar.

Question 26. Which enzyme makes transportation through cell membranes easier?
Answer: The transmembrane enzyme protein permease enables membrane transport.

Class 11 Biology WBCHSE

Question 27. Where does root pressure develop?
Answer: Root pressure develops in the xylem cells of the root

Question 28. If a plant cell of advanced species, that is surrounded by deposition of cutin and suberin, is kept in water for 15 minutes, what will happen then?
Answer: The volume of the cell will remain the same because cutin and suberin are impervious to water.

Question 29. Plant cell does not burst if immersed in pure water. Why?
Answer: The elasticity, tough structure, and extensibility of the cell wall of plant cells prevent it from bursting if immersed in pure water.

Question 30. State True or False: Active absorption utilizes ATP
Answer: True