The Calvin Cycle

Calvin and his co-workers worked out the whole pathway and showed that it operates in a cyclic manner, with RuBP being regenerated at the end so the cycle can continue. It is important to understand clearly that the Calvin pathway occurs in all photosynthetic plants; it does not matter whether they have the C3 or the C4 (or any other) pathway. For ease of understanding, the Calvin cycle can be described under three stages: carboxylation, reduction and regeneration.

1. Carboxylation — Carboxylation is the fixation of CO2 into a stable organic intermediate. It is the most crucial step of the Calvin cycle, in which CO2 is used for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase, and results in the formation of two molecules of 3-PGA. Because this enzyme also has an oxygenation activity, it is more correctly called RuBP carboxylase-oxygenase, or RuBisCO.

2. Reduction — These are a series of reactions that lead to the formation of glucose. The steps involve:

• Utilisation of 2 molecules of ATP for phosphorylation, and two of NADPH for reduction, per CO2 molecule fixed.
• The fixation of six molecules of CO2 and 6 turns of the cycle are required for the formation of one molecule of glucose.

3. Regeneration — Regeneration of the CO2 acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted. The regeneration steps require one ATP for phosphorylation to form RuBP. …