Question

Carbons that are fixed in the Calvin Cycle can be converted into all of the following except:

a. Glycogen

b. G3P

c. Cellulose

d. Glucose

Which phase of the Calvin cycle are electrons added to carbons?

a. Regeneration

b. Oxidative phosphorylation

c. Fixation

d. Reduction

Answer 1

• Calvin cycle is a cyclic pathway of dark reaction during photosynthesis. It is also called C3 cycle since, it results in the formation of the first stable 3-carbon compound, called 3-Phosphoglyceric Acid (3-PGA).
• This cycle consists of three main phases, namely- carboxylation, reduction and regeneration.
• Carboxylation is the addition of carbon dioxide to RuBP (ribulose biphosphate) catalysed by the enzyme, named RuBP carboxylase-oxygenase or RuBisCO. This results in the formation of two molecules of three carbon compound, called 3-PGA.
• ATP and NADPH are used to reduce 3-PGA into Glyceraldehyde-3-phosphate (G3P) . This is the reduction reaction.
• In regeneration step, the RuBP molecule is regenerated so that the C3 cycle continues.
• To remove one glucose molecule (has 6 carbon atoms) from the cycle, 6 molecules of carbon dioxide has to be fixed, for which, the cycle has to occur 6 times.

1. The G3P is used to produce, glucose, starch and cellulose. Glycogen is not a storage product for energy source in plants, but it is in animals. Therefore, option 'a' is the answer.

2. Reduction is a chemical reaction, in which electrons are added to an atom. In Calvin cycle, 3PGA gets reduced into G3P. Therefore, answer is option 'd'.