The Calvin Cycle and Limiting Factors

Carbon dioxide combines with the five-carbon molecule ribulose bisphosphate, RuBP, in a reaction catalysed by rubisco. The unstable six-carbon product gives two three-carbon molecules of glycerate 3-phosphate, GP. The carbon dioxide carbon is now incorporated into organic molecules.

ATP and reduced NADP from the light-dependent reactions are used to convert GP to triose phosphate, TP. ATP supplies coupled energy transfers and reduced NADP supplies reducing power. A decrease in light can therefore affect this stage even though carbon fixation itself does not directly absorb photons.

Most TP is used, with ATP, to regenerate RuBP, allowing further carbon dioxide fixation. Some TP leaves the cycle to form other organic compounds including sugars and molecules contributing to lipids and amino acids. Calling the pathway a cycle does not mean that no material enters or leaves.

For three carbon dioxide molecules fixed, three RuBP molecules provide fifteen existing carbon atoms. Six GP molecules contain eighteen carbons; after conversion to TP, five TP molecules provide the fifteen carbons needed to regenerate three RuBP, leaving one net three-carbon TP. This balance explains why a small net output can require substantial recycling.

Light intensity, carbon dioxide supply and temperature can limit photosynthesis through different mechanisms. At low light, increasing carbon dioxide may have little effect if ATP and reduced NADP production are limiting. At low carbon dioxide, more light may give little further increase. Temperature affects enzyme-controlled reactions and can become damaging if sufficiently high.

In controlled agriculture, lighting, carbon dioxide enrichment or heating may improve yield only while the changed factor is limiting and other requirements remain adequate. Evaluate the additional output against energy use, financial and environmental costs using the information supplied. A plateau in a graph identifies a constraint under those conditions, not a universal maximum for every plant.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.