Correction to the essay worked solution (Nov 2004)

We made a mistake in our answer to the essay on Calvin cycle. We described the Krebs cycle instead (apologies!). Below is the correct answer for the particular essay question.

The Calvin cycle comprises of three stages: (i) carbon fixation, (ii) reduction of glyceraldehyde-3-phosphate (G3P) and (iii) regeneration of RuBP.

Initially, a molecule of carbon dioxide combines with the 5C ribulose bisphosphate (RuBP) in a reaction that is catalyzed by ribulose bisphosphate carboxylase. The product is an unstable 6C compound that immediately breaks down into two 3C molecules of phosphoglycerate (PGA).

In the next stage, PGA is reduced to glyceraldehyde-3-phosphate (G3P) that requires ATP and NADPH (from the light-dependent reactions). For every six CO₂ that enter the cycle, 12 G3P molecules are made of which two are used to synthesize carbohydrate (e.g. sucrose, starch or cellulose).

The remaining G3P molecules undergo reactions to form ribulose phosphates (RuP) which are then phosphorylated to regenerate RuBP required for the cycle to repeat itself.

Hence, for every hexose molecule synthesized, six CO₂ are fixed, phosphates transferred from ATP and electrons from NADPH.


P.S. The essay main points are in the previous post (19 Oct '11) so you may want to refer to it for more effective revision instead.