Effect of Limiting Factors on the Calvin Cycle

The levels of the three Calvin cycle intermediates, RuBP, GP and TP, change in predictable ways when a limiting factor changes. Use the cycle to work them out:RuBP + CO₂ → 2 GP → (ATP, reduced NADP) → TP → (ATP) → RuBPLight intensity falls. Less ATP and reduced NADP are made. GP is not reduced to TP, so GP rises. TP falls, so less RuBP is regenerated, and RuBP is still being used to fix carbon dioxide, so RuBP falls.Carbon dioxide concentration falls. Less RuBP is converted to GP. RuBP rises (still regenerated from TP), GP falls and then TP falls.Temperature falls. All the enzyme-controlled reactions slow down, including RuBisCO. Levels of GP, TP and RuBP all fall, and the whole cycle turns more slowly.The classic evidence comes from Calvin's experiments with Chlorella. When the light was switched off, the level of GP rose and RuBP fell. When the carbon dioxide supply was cut, RuBP rose and GP fell. These results confirmed that GP is made from RuBP and carbon dioxide, and that ATP and reduced NADP are needed to turn GP into TP and regenerate RuBP.Tip: the intermediate just before the blocked step builds up, and the one just after it falls.