3 Biology -- Photosynthesis

Decribe C4 cycle.

Decribe C4 cycle.

It is also a pathway for fixation and reduction of carbondioxide. It was discovered by Hatch and Slack in 1977. The first acceptor of CO2 is phosphoenol pyruvic acid and the first stable compound is oxalo acetic acid which is 4-carbon compound. So, this cycle is called C4 cycle. The anatomy of leaf of C4 plants is called krantz anatomy. 

   It is completed in the following ways:- 

1. Fixation of CO₂ takes place in mesophyll cell. In mesophyll cell, the affinity of PEP carboxylase (Phosphoenol pyruvic acid carboxylase) to wards CO₂ is higher and than RuBISCO. So, Calvin cycle is absent in mesophyll cell. Phosphoenol pyruvic acid reacts with CO₂ in presence of an enzyme PEP carboxylase to form first stable product, oxaloacetic acid (C4 compound) and phosphoric acid.

PEP + CO2 + H₂O→oxaloacetic acid

2. Oxaloacetic acid is reduced to malic acid in presence of malic dehydrogenase.

Oxaloacetic acid + NADPH₂→ malic acid + NADP˖

3. Now, malic acid is translocated to bundle sheath cell through plasmodesmata where decarboxylation takes place.

Malic acid + NADP˖ →CO₂ + pyruvic acid+ NADH + H˖

Thus, formed pyruvic acid is transported into mesophyll cell where it is changed into PEP.

Pyruvic acid+ ATP + H3PO4 → PEP +AMP+ pyrophosphate 

4. Cells of bundle sheath contain RuBP Carboxylase so CO₂ is again fixed inside the bundle sheath cells through Calvin cycle. Carbohydrates are synthesized in C, cycle. C₁ cycle is expensive from energy point of view. In C, cycle, ATP molecules are utilized. The overall reaction is given below.

6PEP+6RuBP + 6CO₂ + 30ATP + 12 NADPH₂ → 6PEP+ 6 RuBP + C₆H₁₂O₆ + 30ADP+30H₃PO₄ + 12NADP 


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