Means Of Transportation In Plants

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.

Xylem and phloem function in plant transportation

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.

active transport in plants

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.

Difference between xylem and phloem in plants

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.

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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.

Examples of transportation in plants with explanation

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.

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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.

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