C4-Dicarboxylic Acid Pathway (With Diagram) | Photosynthesis

The below mentioned article provides a study note on the C₄-Dicarboxylic Acid Pathway.

C₄-Dicarboxylic Acid Pathway (Hatch Slack Pathway, C₄ Pathway):

It was worked out by Hatch and Slack (1965, 1967). However, initial discovery was made by Kortschak (1965) who found that labelled carbon dioxide (¹⁴CO₂) assimilated by Sugarcane leaves first appeared in a 4-carbon compound oxalo-acetic acid (OAA or oxaloacetate).

Because of the initial discovery by Kortschak, this pathway of carbon assimi­lation is also called HSK (Hatch Slack Kortschak) pathway. Hatch and Slack found it a regular mode of CO₂-fixation in a number of tropical plants, both monocots and dicots, e.g., Maize, Sugarcane, Sorghum, Panicum, Pennisetum (Pearl Millet), Atriplex, Amaranthus, Salsola, etc.

These plants are called C₄ plants because of the first stable photosynthetic product being a 4-carbon compound. Other plants are C₃ plants. C₄ plants live in hot moist or arid and non-saline or saline habitats. They have Kranz anatomy (Fig. 13.24).

In Kranz anatomy, the mesophyll is undifferentiated and its cells occur in concentric layers around vascular bundles. Vascular bundles are surrounded by large sized bundle sheath cells which are arranged in a wreath-like manner (kranz— wreath) in one to several layers.

The mesophyll and bundle sheath cells are connected by plasmodesmata or cytoplasmic bridges. The chloroplasts of the mesophyll cells are smaller. They have well developed grana and a peripheral reticulum but no starch. Mesophyll cells are specialised to perform light reaction, evolve O₂ and produce assimilatory power (ATP and NADPH).

They also possess enzyme PEP Case for initial fixation of CO₂. The chloroplasts of the bundle sheath cells are agranal. They possess a peripheral reticulum and starch grains.

Thylakoids occur as stroma lamellae. ATP can be synthesised through cyclic photophospho­rylation. However, photolysis and O₂ evolution are absent. Rather, bundle sheath cells are well protected from O₂ being released from mesophyll cells. Bundle sheath cells possess Rubisco.

Initial Fixation:

In C₄ plants, initial fixation of carbon dioxide occurs in mesophyll cells. The primary acceptor of CO₂ is phosphoenol pyruvate or PEP. It combines with carbon dioxide in the presence of PEP carboxylase or PEPcase to form oxalo-acetic acid or oxaloacetate.

Transport:

Malic acid or aspartic acid is trans-located to bundle sheath cells through plasmodesmata. Inside the bundle sheath cells they are decarboxylated (and deaminated in case of aspartic acid) to form pyruvate and CO₂. Since a number of mesophyll cells are feeding bundle sheath cells, the latter come to have a carbon dioxide concentrations several times that of atmosphere.

Final Fixation:

CO₂ released in bundle sheath cells is fixed through Calvin cycle. RuBP of Calvin cycle is called secondary or final acceptor of CO₂ in C₄ plants.

Regeneration of PEP:

Pyruvate and PEP formed in bundle sheath cells are sent back to mesophyll cells. Here, pyruvate is changed to phosphoenol pyruvate. Energy is required for this. The same is provided by ATP. The latter is changed into AMP (adenosine monophos­phate).

Conversion of AMP to ATP requires double the energy than energisation of ADP to ATP. Therefore, actual requirement of energy is equal to two molecules of ATP.

This energy is in addition to 3 ATP required for fixation of one molecule of CO₂ through Calvin cycle. Therefore, C₄ plants consume 5 ATP molecules per molecule of CO₂ fixed instead of 3 ATP molecules for C₃ plants. For the formation of a glucose molecule, C₄ plants require 30 ATP while C₃ plants utilize only 18 ATP.

Importance:

(i) C₄ plants have a disadvantage. They consume more energy (2 more ATP molecules per molecule of CO₂ fixed). However, sufficient energy is available in the tropics where the plants grow. Further, C₄ plants have little photorespiration while in C₃ plants, more than half of photosynthetic carbon may be lost in photorespiration. C₄ pathway, is therefore, of adaptive advantage,

(ii) C₄ plants are more efficient in picking up CO₂ even when it is found in low concentration because of the high affinity of PEP.

(iii) Concentric arrangement of mesophyll cells produces a smaller area in relation to volume for better utilization of available water and reduce the intensity of solar radiations,

(iv) They can tolerate excess salts because of the presence of organic acids,

(v) Normal oxygen concentration is not inhibitory for the growth in contrast to C₃ plants,

(vi) They are adapted to high temperature and intense radiation of tropics.

Crassulacean Add Metabolism:

It is a mechanism of photosynthesis involving double fixation of CO₂ which occurs in succulents belonging to crassulaceae, cacti, euphorbias and some other plants of dry habitats where the stomata remain closed during the daytime and open only at night.

The process of photosynthesis is similar to that of C₄ plants but instead of spatial separation of initial PEP case fixation and final Rubisco fixation of CO₂, the two steps occur in the same cells but at different times, night and day, e.g., Sedum, Kalanchoe, Opuntia, Pineapple, Agave, Vanilla. The initial fixation of CO₂ occurs at night and final fixation occurs during day time. This results in conserving water.

Cardinal Points (Fig. 13.27):

Sachs (1860) found that a factor influencing a physiological process has three principal values called cardinal points— minimum, optimum and maximum.

The minimum of a factor is that value below which the physiological process cannot continue. Maximum of a factor is that value beyond which the process comes to stop. Optimum value of the factor is that point where the physi­ological process can continue indefinitely at its highest velocity.