The Calvin Cycle and Limiting Factors

The Calvin cycle

In the stroma, carbon dioxide is joined to the $5$-carbon acceptor ribulose bisphosphate by the enzyme rubisco. The resulting $6$-carbon compound is unstable and splits at once into two molecules of the $3$-carbon glycerate 3-phosphate. GP is then reduced to triose phosphate, using both products of the light-dependent stage: reduced NADP supplies the hydrogen and ATP supplies the energy. Most of the triose phosphate is used, with more ATP, to regenerate ribulose bisphosphate; the rest leaves the cycle and is built into hexose sugars, starch, cellulose, lipids and, with added nitrogen, amino acids. Because each turn fixes only one carbon, six turns are needed to make one $6$-carbon sugar.

What happens when you change one thing

Questions almost always test the same skill: change one input and predict which intermediate accumulates. Put a plant in the dark and no ATP or reduced NADP is made, so GP cannot be converted to TP: GP rises and, because TP is no longer available to regenerate the acceptor, RuBP falls. Cut the carbon dioxide supply and fixation slows, so less GP is made while regeneration continues: GP falls and RuBP rises. Reasoning from the diagram is far safer than memorising the four outcomes.

Limiting factors

The rate of photosynthesis at any moment is set by whichever factor is in shortest supply — light intensity, carbon dioxide concentration or temperature. On a graph of rate against light intensity, the rising part shows light limiting, and the plateau shows that something else has become limiting. Raising carbon dioxide concentration or temperature then lifts the plateau, which identifies the second factor. Temperature behaves differently from the other two: above the optimum the rate falls, because the enzymes of the Calvin cycle denature. At the compensation point the rate of photosynthesis exactly equals the rate of respiration, so there is no net exchange of gases.