Crassulacean Acid Metabolism Notes

Crassulacean Acid Metabolism

Crassulacean Acid Metabolism

This metabolic pathway was first discovered in plants of the family Crassulaceae.

Crassulacean Acid Metabolism Definition: The metabolic pathway by which mainly the members of the family Crassulaceae assimilate carbon while preventing water loss by evaporation is called Crassulacean add metabolism (CAM)

The pathway is also common in Cactaceae, Euphorbiaceae, Rhizoaceae, Liliaceae, Bromeliaceae, Orchidaceae, etc., families. The plants which show CAM pathways are called CAM plants. E.g., A few economically important plants, including pineapple, Bryophyllum sp, Sedum sp., etc., are CAM plants.

Characteristics of CAM plants:

1. These plants are generally found in dry and desert regions.
2. In these plants, stomata remain open at night and closed during the day. These are called photoactive stomata.
3. These plants have low compensation points.
4. Decarboxylation of malate during the day yields CO₂ inside the photosynthetic tissues. This CO₂ is fixed normally by RuBisCO in C₃ cycle. This permits CO₂ assimilation without letting in CO₂ inside the cell directly from the air.
5. Transpiration and photorespiration are greatly reduced in CAM plants.

Pathway of CAM

Cam Plants Stomata

The CAM pathway, consisting of dark acidification and light deacidification reactions, is divided into the following phases

Phase-1 (Dark acidification): The main aim of this phase is to produce malic acid in the dark. As the stomata remain open at night, CO₂ diffuses in. This CO₂ requires temporary storage as an intermediate carbon compound. CAM pathway requires the breakdown of starch in the dark to produce phosphoenol pyruvate (PEP) through glycolysis. PEP is the substrate for carboxylation by PEP-carboxylase. The product generated is oxaloacetic acid (OAA).

⇒ \(\text { PEP } \underset{\text { dehydrogenase }}{\stackrel{\text { Malate }}{\longrightarrow}} \mathrm{OAA}+\mathrm{Pi}\)

During this period, PEP-carboxylase is active and fixation of C02 takes place irrespective of the presence or absence of oxygen. OAA is then reduced to malate by NAD-dependent malate dehydrogenase.

⇒ \(\text { OAA } \underset{\text { dehydrogenase }}{\stackrel{\text { Malate }}{\longrightarrow}} \text { Malate + NADP }{ }^{+}\)

Malate accumulates as malic acid in the vacuole towards the end of the dark period, when the rate of CO₂ fixation declines. This is due to the inhibition of PEP-case by accumulated malate and a decrease in the cytosolic pH.

Phase-2 (Light de-acidification): The intensity of light is high during the day. Malic acid gets released from the vacuoles as malate. Malate gets converted to pyruvate by malate decarboxylase. Decarboxylation of malate occurs with a low rate of CO₂ fixation. CO₂ enters C₃ cycle during the synthesis of PGA.

Pyruvate resulting from malate decarboxylation is converted to PEP by pyruvate orthophosphate dikinase, present in the chloroplast.

⇒ \(\text { Malate }+\mathrm{NADP}^{+} \stackrel{\text { Malic enzyme }}{\rightleftharpoons} \text { Pyruvate }+\mathrm{CO}_2+\mathrm{NADPH}+\mathrm{H}^{+}\)

CO₂ produced by any of the decarboxylation reactions gets fixed by RuBisCO through C₃ cycle. The pyruvate or PEP resulting from malate decarboxylation may be oxidised to C02 by the mitochondrial TCA cycle. This CO₂ will get fixed by C₃ cycle.

Significance of CAM:

CAM is an adaptation to an extremely xerophytic environment. Even under conditions of severe water stress, carbon dioxide uptake continues in CAM plants.

CAM plants can eliminate the loss of CO₂ as they can retain and fix the released CO₂

Chemosynthesis

1. Some bacteria do not use light energy to carry out the synthesis of food.
2. Instead, they oxidise biochemical compounds to release energy that is utilised during food synthesis. This energy, along with CO₂, is used to produce food for the plants. This process by which food is synthesised, using the energy released by oxidation of chemical compounds is known as chemosynthesis. Different types of bacteria, that carry out chemosynthesis, are known as chemosynthetic bacteria.
3. Iron bacteria like Leptothrix spv Ferrobacillus sp., etc., oxidise ferrous salts to ferric salts. The energy released in the process is used up for chemosynthesis.
4. On the other hand, sulphur bacteria like Thiothrix sp., etc., oxidise HZS to S and the energy released is used for chemosynthesis.
5. Similarly, nitrifying bacteria like Nitrosomonas sp. convert ammonia to nitrate, while Nitrobacter sp. converts nitrites to nitrates, to carry out chemosynthesis.

Factors Affecting Photosynthesis

The external and internal factors that affect photosynthesis are discussed under the following heads.

The external factors influencing photosynthesis are—

Light: Both quality and intensity of light influence the rate of photosynthesis.

intensity:

1. The rate of photosynthesis is directly proportional to the rate of photosynthesis.
2. However, very high intensity of light oxidises chlorophylls (photooxidation of chlorophyll) which photosynthesis. This phenomenon is called solarisation.
3. The amount of light intensity at which the rate of respiration is equal to that of photosynthesis is called the light compensation point.

Quality:

1. A wavelength of light between 400 nm and 700 nm is most effective for photosynthesis. This light is called photosynthetically active radiation (PAR).
2. Comparatively more photosynthesis occurs in red and blue regions of PAR though others show significant photosynthesis.

Co₂ concentration:

1. It is found that if the atmospheric CO₂ concentration (0.03-0.04%) increases by 0.01%, the rate of photosynthesis increases significantly.
2. This is achieved in the greenhouses under controlled conditions.
3. If the CO₂ concentration increases further, the rate of photosynthesis decreases. The following graph shows how different CO₂ concentrations affect the rate of photosynthesis.

Water: A deficiency of water causes the stomata to close, thereby reducing the C02 availability. Besides, water deficit stress also causes wilting of leaves, thus reducing the surface area of the leaves and their metabolic activity as well.

Temperature: The optimum temperature for photosynthesis is generally 20-30°C. Generally, the rate of photosynthesis increases with the rise in temperature. This effect is seen within the temperature range of 6 to 37 c by this, the rate of photosynthesis decreases and ceases at 43c photosynthesis of different plants also depends on their habitat. Some bacteria can carry out photosynthesis at 70°C while some at -35 °C.

The following graph shows changes in the rate of photosynthesis with temperature.

Temperature coefficient or Q₁₀

Scientist Vant Hoff postulated this law. According to this law, the rate of a biochemical reaction doubles with every 10°C rise, within a specific temperature range (0-30°C). It is known as the temperature coefficient. This coefficient may be written in the following way, in the case of photosynthesis—

⇒ \(\mathrm{Q}_{10}=\frac{\text { Rate of photosynthesis at } \mathrm{T}^{\circ} \mathrm{C}+10^{\circ} \mathrm{C}}{\text { Rate of photosynthesis at } \mathrm{T}^{\circ} \mathrm{C}}\)

In this case, T°C is a specific temperature.

Oxygen concentration: Increased O₂ concen¬ traction is known to inhibit the activity of the photosynthesis enzymes. The rate of photosynthesis declines when atmospheric oxygen concentration rises above its normal value i.e. 21%. This effect is known as the Warburg effect. However, some plants do not show this effect.

Presence of chemical substances: Chemical substances like gaseous and metallic pollutants decrease the rate of photosynthesis. For example, ozone, sulphur dioxide, fluorides, hydrogen sulphide, chloroform, etc., have an inhibitory effect on photosynthesis.

Low Limiting Factor

Blackman’s law of limiting factor is a modification of Leibig’s law of minimum. It states that if a biochemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value. This factor is called the limiting factor. A limiting factor is a factor, whose change may directly affect the process.

For example, despite optimal light and CO₂ conditions, a green leaf may not photosynthesise, if the temperature is very low. This leaf, if given the optimal temperature, will carry on the process of photosynthesis.

Cam Plant

The theory of three cardinal points, which is related to the law of limiting factors, was given by Sachs in 1860. According to this concept, there is minimum, optimum and maximum value for each factor.

For every factor, there is a minimum value when no photosynthesis occurs, an optimum value showing the highest rate of photosynthesis and a maximum value, above which photosynthesis fails to take place. The law can be explained best by an illustration.

1. Light intensity provided to a leaf is sufficient to allow it to utilise a fixed concentration of CO₂. Initially, at level A, no photosynthesis occurs due to non-availability of CO₂.
2. If the concentration of CO₂ is increased further, the rate of photosynthesis will increase up to a maximum value (from level A to level D).
3. If the CO₂ concentration is further increased, the rate of photosynthesis will remain constant. Further increase in the rate of photosynthesis beyond level D is possible only when light intensity is increased, which at this point, is working as a limiting factor.

The factor which is quantitatively the least, may not be the limiting one. Instead, the factor which is relatively less than the amount actually required will act as the limiting factor. This law may also be named as ‘Law of relatively limiting factor’ or ‘Law of most significant factor’.

Internal Factors

The internal factors regulating photosynthesis are as follows—

Chlorophyll concentration: The concentration of chlorophyll affects the rate of reaction as it is the pigment that absorbs the light energy. Chlorophyll deficiency results in chlorosis. It can occur due to disease, mineral deficiency or the natural process of ageing (senescence). Lack of iron, magnesium, nitrogen and light affects the formation of chlorophyll and thereby causes chlorosis.

Internal structure of the leaves: The efficiency of mesophyll tissues, stomata, guard cells, etc., determines the rate of photosynthesis. Any change in the diameter of the stomata influences the rate of photosynthesis.

Accumulation of photosynthetic products: Accumulation of photosynthetic products, such as starch, within the mesophyll tissues, decreases the rate of photosynthesis. Hence, the products of photosynthesis need to be removed from the leaves, regularly.

Ageing of the leaves: As the leaves begin ageing, the number of chloroplasts within the mesophyll tissues also decreases. This, in turn, decreases the rate of photosynthesis.

Enzymes: Photosynthesis is an enzyme-catalysed process. Hence, the presence and activity of enzymes affect the rate of photosynthesis.

Hormones: Auxins, gibberelins, cytokinin, etc., increase the rate of photosynthesis. Abscisic acid, on the other hand, decreases the rate of photosynthesis.

Relation Between Photosynthesis And Respiration

During the process of respiration, food is oxidised and energy is released in the utilisable form which remains stored as ATP. Using this ATP, all the cellular activities are performed. CO₂ and water are also released during respiration. The overall equation is—

⇒ \(\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{O}_2 \stackrel{\text { Aerobic cell }}{\longrightarrow} 6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O}+673 \mathrm{kcal}\)

The equation for respiration is the reverse of that of photosynthesis. In photosynthesis, CO₂ is consumed from the atmosphere, while in respiration CO₂ is released.

Crassulacean Acid

These two processes exist in precise balance in nature. By these opposing processes, the concentration of carbon dioxide in the atmosphere is maintained at a nearly constant amount (0.03%). Approximately sixteen billion tonnes of carbon is processed in this way every year.

Note:

Carbon scavenging: Process by which the element carbon (C) is used up or removed.

Chanosis: Loss of green colouration of leaves in a plant.

Chromatic adaptation: Modification of photosynthetic apparatus according to different wavelengths of incident light, resulting in absorption of energy.

Facultative: Capable of switching to any other pathway other than its usual if the need arises.

Feedback control: Process by which the concentration of the product generated during a reaction controls the above-mentioned reaction.

Free energy: Energy available in a system that can be converted to work.

Proton pump: An integral membrane protein that allows protons to pass across a cell membrane.

Spectrophotometer: An instrument used to measure the absorption spectrum of different pigments with respect to different wavelengths of light.

Stratosphere: The second layer of the atmosphere.