The Calvin cycle, limiting factors and rate investigations

Last time

Before we start: last time, in Chloroplasts, pigments and the light-dependent stage, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Building sugar from air

A tree is built mostly from carbon dioxide taken out of the air. The Calvin cycle is the pathway that fixes it into sugar, using the A T P and reduced N A D P from the light stage. How fast it runs depends on light, carbon dioxide and temperature, and one of them is always the factor in short supply.

Carbon fixation

The Calvin cycle takes place in the stroma. Carbon dioxide combines with a five carbon sugar, ribulose bisphosphate, called R u B P. The reaction is catalysed by the enzyme rubisco. The product is an unstable six carbon compound which splits at once into two molecules of glycerate 3 phosphate, G P, each with three carbons. This is called carbon fixation, because inorganic carbon dioxide is turned into organic carbon compounds.

Reduction and regeneration

Glycerate 3 phosphate is reduced to triose phosphate, T P. This uses A T P for energy and reduced N A D P as the source of hydrogen. Most triose phosphate is used to regenerate R u B P, which needs A T P. Without it the cycle would stop. The remaining triose phosphate leaves the cycle. It is used to make glucose, starch, cellulose, lipids and amino acids.

The bookkeeping

Six carbon dioxide molecules combine with six R u B P. That is thirty plus six, thirty-six carbons, forming twelve G P molecules. The twelve G P become twelve T P. Two T P, six carbons, leave to make one hexose. The other ten T P, thirty carbons, regenerate six R u B P. The cost is eighteen A T P and twelve reduced N A D P for one hexose, three A T P and two reduced N A D P for each carbon dioxide fixed.

A past-paper example

October November 2025, Paper 44, question 10. Outline the differences between cyclic and non cyclic photophosphorylation, then complete a passage on the Calvin cycle. Non cyclic makes A T P, reduced N A D P and oxygen. Cyclic makes only A T P. In the Calvin cycle, carbon dioxide combines with R u B P using rubisco. Glycerate 3 phosphate is reduced to triose phosphate, using A T P and reduced N A D P.