Question

The main function of the Calvin cycle is

• A. To use NADPH${}_2$ to release oxygen.
• B. To transport RuBP out of the chloroplast.
• C. To build simple sugars from carbon dioxide.
• D. To split water molecules and release oxygen gas.
• E. To use ATP to release carbon dioxide.

Answer 1

The correct option is C To build simple sugars from carbon dioxide.

Calvin cycle or C3 pathway is the major pathway for fixation of carbon dioxide in green plants. It represents phase-II i.e. dark reaction of photosynthesis. It takes place in the stroma of the chloroplasts. Calvin cycle is described under three stages:

1. Carboxylation of RuBP: In this process there is fixation of atmospheric CO₂ into a stable organic compound PGA (Phosphoglyceric acid) with the help of enzyme RuBisCO.

2. Reduction of CO₂: The 3-C PGA then undergoes reduction with the help of the assimilatory power to form 3-C Phosphoglyceraldehyde (PGAL). NADPH₂ provides the hydrogen and ATP supplies energy for reduction. Enzyme triosephosphate dehydrogenase catalyses the reaction. Some molecules of PGAL are converted into another triosephosphate called Dihydroxy Acetone Phosphate (DHAP) in the presence of enzyme phosphotriose isomerase.

The formation of sugars (end products of photosynthesis), the 3-C triose phosphates (i.e., PGAL and DHAP) form 6-C hexose sugar fructose-1,6-bisphosphate in the presence of enzyme aldolase which is then dephosphorylated first to fructose monophosphate and then to fructose (6-C) in the presence of enzyme phosphatase. Some fructose monophosphate molecules may be isomerised into glucose (6-C) monophosphate by the enzyme isomerase and then into glucose. The hexose sugar may be further converted to sucrose or to starch and are stored in storage cells.

3. Regeneration of RuBP: The 5-C RuBP is constantly required for fixation of CO₂ in the calvin cycle. It is regenerated through another chain of reactions.

Hence sugars are formed in calvin cycle.

So, the correct answer is 'To build simple sugars from carbon dioxide'.