Instant Notes: Plant Biology

The Calvin cycle

TheCalvin cycle has three stages: carboxylation, the incorporation of CO 2 ;

reduction, utilizing ATP and NADPH; and regeneration where the CO 2

acceptor is formed again. The entire cycle, together with the number of moles of

each molecule produced and the number of moles of ATP and NADPH used is

shown in Fig. 4.

Stage 1: Carboxylation

Ribulose bisphosphate (5C) + CO 2 → 2 ×3-phosphoglycerate (3C)

In this stage, a carbon atom from CO 2 is added to one molecule of 5-C ribulose

bisphosphateby the enzyme ribulose bisphosphate carboxylase/oxygenase

(Rubisco) to yield two molecules of 3-C 3-phosphoglycerate. The enzyme is the

world’s most abundant protein, often constituting around 40% of the soluble

protein in a leaf. Rubisco is a CO 2 acceptor, which binds with sufficient affinity

to ensure carboxylation of ribulose 1,5-bisphosphate. The reaction is

energetically favourable, so the cycle runs in favor of 3-phosphoglycerate

without additional energy input.

Stage 2: Reduction

3-phosphoglycerate (3C) →1,3-bisphosphoglycerate (3C) →glyceraldehyde 3-phosphate (3C)

The process involves two enzymes, 3-phosphoglycerate kinaseandNADP

glyceraldehyde-3-phosphate dehydrogenase. In the first enzymatic reaction,

one mole of ATP is used and in the second, one mole of NADPH is used for

each mole of 3-phosphoglycerate. Glyceraldehyde-3 phosphateis a 3-carbon

sugar, some of which is used in the next stages of the cycle and some removed

as the product of the cycle (Fig. 4).

Stage 3: Regeneration

Regenerationinvolves the steps from glyceraldehyde-3-phosphate to ribulose

1,5-bisphosphate. Most of the stages are energetically favourable and do not

J2 – Major reactions of photosynthesis 141

Stroma pH 8

ATP synthase

complex

Thylakoid lumen

pH 5

AT P

ADP + Pi H+

H+

H+

H+

H+

H+ H+H

+

H+

Thylakoid

membrane

Fig. 3. ATP synthase.