Question

The compound formed as net gain in carbon assimilation is:
A. 3-C DHAP of third CO$_2$ fixation
B. GAP of second CO$_2$ fixation
C. Erythrose 4- phosphate reacting with DHAP
D. RuBP

Answer 1

Carbon fixation in Calvin- Benson cycle.

Complete step-by-step answer:
1. The first stable intermediate product in the Calvin- Benson cycle is 3- phosphoglycerate, at the end of the cycle it has to be regenerated.
2. During the regeneration, 12 mols. of 3- phosphoglycerate are phosphorylated by 12 mols. of ATP to form 12 mols. of 1,3 - bisphosphoglycerate in presence of enzyme phosphoglycerate kinase. The molecules of ATP get converted to ADP (dephosphorylated)
3- phosphoglycerate + ATP ----> 1,3 – bisphosphoglycerate + ADP
3. The 12 mols. of 1,3 - bisphosphoglycerate are reduced to 12 mols. of 3-phosphoglyceraldehyde (PGA) in presence of the coenzyme NADPH (nicotinamide adenine dinucleotide phosphate). The reaction is catalyzed by the enzyme triose phosphate dehydrogenase. Inorganic phosphate is released and the NADPH is dehydrogenated.
1,3 – bisphosphoglycerate + NADPH ----> 3-phosphoglyceraldehyde + NADP
4. 3- phosphoglyceraldehyde (PGA) is isomerized to dihydroxyacetone phosphate (DHAP) in presence of the enzyme triose phosphate isomerase.
3- phosphoglyceraldehyde <-----> Dihydroxyacetone phosphate
Thus, the carbon fixation is inadvertently related to the formation of DHAP.

So, the correct answer is “Option A 3-C DHAP of third CO$_2$ fixation”.

Note:
1. In C4 plants, the Calvin- Benson cycle occurs after carbon dioxide is added to phosphoenolpyruvate (PEP) to form oxaloacetate (OAA).
2. C3 plants don’t suppress photorespiration as they are open during the day and closed during the night in terms of photosynthesis. They live in cold and wet climates that are characterized to medium/ low light intensities