The Calvin cycle has three main steps: RuBP combines with CO2 to form GP (catalysed by rubisco); GP is reduced to triose phosphate (TP) using ATP and reduced NADP; most TP is used to regenerate RuBP, which also uses ATP. Changing a limiting factor changes the levels of these intermediates in a predictable way.
Low light intensity: less ATP and reduced NADP come from the light-dependent reaction. GP cannot be reduced to TP as quickly, so GP accumulates. Less TP is made, so less RuBP is regenerated and RuBP falls. TP also falls.Low carbon dioxide concentration: RuBP cannot be converted to GP as quickly, so RuBP accumulates and GP falls. Less GP means less TP is made.Low temperature: all the enzyme-controlled reactions of the cycle, including the one catalysed by rubisco, slow down. Less GP, TP and RuBP are produced each minute, so less glucose is made. Temperatures well above the optimum denature these enzymes and also reduce the rate.Worked example: In an experiment, the light was switched off. Within a minute, the concentration of GP rose and the concentration of RuBP fell. Explanation: without light there is no ATP or reduced NADP, so GP is not reduced to TP and builds up, while RuBP is still used by rubisco but is no longer regenerated.Remember the pattern: the molecule just before the slowed step builds up, and the molecule after it runs down.
Low light intensity: less ATP and reduced NADP come from the light-dependent reaction. GP cannot be reduced to TP as quickly, so GP accumulates. Less TP is made, so less RuBP is regenerated and RuBP falls. TP also falls.Low carbon dioxide concentration: RuBP cannot be converted to GP as quickly, so RuBP accumulates and GP falls. Less GP means less TP is made.Low temperature: all the enzyme-controlled reactions of the cycle, including the one catalysed by rubisco, slow down. Less GP, TP and RuBP are produced each minute, so less glucose is made. Temperatures well above the optimum denature these enzymes and also reduce the rate.Worked example: In an experiment, the light was switched off. Within a minute, the concentration of GP rose and the concentration of RuBP fell. Explanation: without light there is no ATP or reduced NADP, so GP is not reduced to TP and builds up, while RuBP is still used by rubisco but is no longer regenerated.Remember the pattern: the molecule just before the slowed step builds up, and the molecule after it runs down.