Question

(i) How many turns of dark reaction yield one molecule of glucose ?
(ii) Explain main chemical steps that take place in the dark reaction or Calvin cycle of photosynthesis.

Answer 1

(i) Three turns.

(ii) The Calvin cycle of photosynthesis can be divided into three main stages: carbon fixation, reduction, and regeneration of the starting molecule.

1. Carbon fixation

A CO₂ molecule combines with a five-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP). This results in a six-carbon compound that splits into two molecules of a three-carbon compound, 3-phosphoglyceric acid (3-PGA). This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, or rubisco.

2. Reduction

In the second stage, ATP and NADPH are required to convert the 3-PGA molecules in the first stage into molecules of a three-carbon sugar, glyceraldehyde-3-phosphate (G3P).

Moreover, this stage is called 'reduction' stage because NADPH donates electrons to, or reduces, a three-carbon intermediate to make G3P.

3. Regeneration

Some G3P molecules go to make glucose, while others get recycled to regenerate the RuBP acceptor. Regeneration requires ATP and involves a complex network of reactions.