The Calvin cycleEdexcel A-Level Biology A: Revision notes

The light-independent reactions make carbohydrate from carbon dioxide. They take place in the stroma of the chloroplast, in a cycle of reactions called the Calvin cycle.

They do not use light directly, but they depend on the products of the light-dependent reactions: ATP supplies the energy and reduced NADP supplies the hydrogen (electrons and protons) to reduce carbon dioxide. Because the supply of ATP and reduced NADP stops soon after the light goes off, the Calvin cycle also stops soon after the light is switched off.

The cycle involves three stages: carbon fixation, reduction of GP to GALP, and regeneration of RuBP.

In carbon fixation, carbon dioxide diffuses into the stroma and combines with the 5-carbon acceptor molecule ribulose bisphosphate (RuBP). The reaction is catalysed by the enzyme rubisco (ribulose bisphosphate carboxylase), which is the most abundant enzyme in leaves.

The product is an unstable 6-carbon compound that immediately splits into two molecules of glycerate 3-phosphate (GP), each with 3 carbons:

CO₂ + RuBP (5C) → 2 GP (3C)

Carbon dioxide has been "fixed", converted from an inorganic gas into an organic compound.

GP is converted to glyceraldehyde 3-phosphate (GALP), also called triose phosphate, in a reduction reaction:

• Reduced NADP donates the hydrogen (and is oxidised back to NADP).
• ATP supplies energy (and is hydrolysed to ADP + Pi).

GALP is a 3-carbon sugar and is the first carbohydrate made. The NADP, ADP and Pi return to the thylakoid membranes, where they are reused in the light-dependent reactions.

For the cycle to continue, RuBP has to be regenerated. Five out of every six GALP molecules are used to remake RuBP, in a series of reactions that also uses ATP. The remaining GALP (one in six) leaves the cycle and is used to make glucose and other organic molecules.

Worked example: fixing six CO₂ gives 12 GP, then 12 GALP. Ten GALP (30 carbons) regenerate six RuBP (30 carbons); two GALP (6 carbons) form one hexose. If each GP reduced uses 1 ATP and 1 reduced NADP and each RuBP regenerated uses 1 ATP, the total is 12 + 6 = 18 ATP and 12 reduced NADP for one hexose.

GALP and the sugars made from it are used by plants, and by the animals and other organisms that feed on them, in:

• Respiration: glucose is oxidised to release energy for making ATP.
• Polysaccharides: glucose is joined by condensation reactions to make starch (storage), cellulose (cell walls) and, in animals, glycogen.
• Lipids: glycerol is made from GALP and fatty acids from intermediates of respiration.
• Amino acids: sugars provide the carbon skeletons; the nitrogen comes from nitrate in plants and from proteins in the diet of animals.
• Nucleic acids: sugars provide the pentose sugars (ribose, deoxyribose); nitrogen for the bases and phosphate are also needed.

Animals cannot fix carbon dioxide, so they get all of these from the food they eat.

Using radioactive ¹⁴CO₂ and the "lollipop" apparatus, Calvin showed that GP was the first compound to be labelled, followed by GALP and then sugars.

The cycle can be investigated by changing one factor:

• Light off: no ATP or reduced NADP, so GP rises and RuBP falls.
• Carbon dioxide removed: RuBP is not used, so RuBP rises and GP falls.

Raising the carbon dioxide concentration increases the rate of fixation until another factor, such as light, temperature or the amount of rubisco, becomes limiting.