Question

Q21). The reactions of the Calvin cycle are NOT directly dependent on light, but they usually do NOT occur at night.

A) The Calvin cycle depends on ATP and NADPH, which are not as available at night.

B) Carbon dioxide concentrations decrease at night.

C) It is often too cold at night for these reactions to take place.

D) Plants usually do not open their stomata at night.

Answer 1

The Calvin cycle depends on ATP and NADPH from the light-dependent reactions and thus typically does not occur at night when these resources are not readily available due to the absence of photosynthesis.

Step-by-step explanation:

The Calvin cycle, also known as the light-independent reactions of photosynthesis, is not directly dependent on light, which can be misleadingly referred to as the 'dark reactions.' These reactions utilize ATP and NADPH produced by the light-dependent reactions to fix carbon dioxide into organic molecules like glyceraldehyde-3-phosphate (G3P). The Calvin cycle does not usually occur at night because, during the night, plants typically have reduced ATP and NADPH since these molecules are generated during the light-dependent reactions when photosynthesis is active. Furthermore, plants often close their stomata at night to conserve water, which limits the availability of CO2 for the Calvin cycle.

Therefore, the correct answer would be A) The Calvin cycle depends on ATP and NADPH, which are not as available at night. It relies on the products of the light-dependent reactions, which occur during daylight. Hence, the Calvin cycle typically takes place when these energy carriers are freshly supplied by the light-dependent processes.