What are the Four Phases of Basic Calvin Cycle?

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(a) Carboxylation Phase

In the mesophyll cells, the fixation of CO, (as IICO,_t ) takes place by the carboxylation of phosphoenolpyruvate (PEP) forming oxaloacetic acid (OAA), a 4-carbon compound. This carboxylation reaction is catalyzed by the enzyme PEP carboxylase.

(b) Transport Phase

OAA is converted into another 4-carbon compound (malic acid and/or aspartic acid, depending! upon plant species) which is transported to bundle sheath cells.

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(c) Decarboxylation Phase the 4-carbon compound in the bundle sheath cells is decarboxylated to generate CO, and a 3-1 carbon compound. The released CO_o is then reduced to carbohydrate via the Calvin cycle which is operative in the bundle sheath cells but not in the mesophyll cells in C₄ plants.

(d) Transport and Regeneration Phase

The 3-carbon compound formed by the decarboxylation reaction in bundle sheath cells is transported to the mesophyll cells where it is converted back to PEP, the acceptor molecule of HC0₃ ^_, utilizing photosynthetically generated ATP.

The transport of metabolites between mesophyll and bundle sheath cells in stages (b) and (d) above is driven by diffusion gradient along the number of plasmodesmata present between these cells.