Reduction of CO2 in the Calvin cycle

The Calvin cycle, also known as the light-independent reactions or the dark reactions, is the second stage of photosynthesis. It occurs in the stroma of the chloroplasts and is responsible for the fixation and reduction of carbon dioxide (CO2) to form glucose.

Reduction phase of the Calvin cycle

During the reduction phase of the Calvin cycle, the fixed CO2 molecule is converted into a molecule of triose phosphate, which can be further used to synthesize glucose and other carbohydrates. This reduction requires energy and reducing power in the form of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH).

Photochemically made, high energy chemical intermediates

The high energy chemical intermediates used in the reduction phase of the Calvin cycle are produced during the light-dependent reactions, or the light reactions, which occur in the thylakoid membranes of the chloroplasts. These reactions involve the absorption of light energy by chlorophyll and the subsequent generation of ATP and NADPH.

Options and their analysis

a) 2 ATP + 2 NADPH: This option provides both the necessary energy and reducing power for the reduction of CO2 in the Calvin cycle. The ATP supplies the energy, while NADPH supplies the electrons and hydrogen ions for the reduction process. This option is correct.

b) 1 ATP + 1 NADPH: This option does not provide enough energy or reducing power for the reduction of CO2. It lacks one ATP and one NADPH, which are required for the process. This option is incorrect.

c) 1 ATP + 2 NADPH: This option provides enough reducing power, but lacks sufficient energy for the reduction of CO2. One additional ATP is needed to supply the necessary energy. This option is incorrect.

d) 2 ATP + 1 NADPH: This option provides enough energy, but lacks sufficient reducing power for the reduction of CO2. One additional NADPH is needed to supply the necessary electrons and hydrogen ions. This option is incorrect.

Therefore, the correct answer is option 'a' - 2 ATP + 2 NADPH. Both the ATP and NADPH are essential for the reduction phase of the Calvin cycle, providing the necessary energy and reducing power to convert CO2 into triose phosphate.