The reactions of Calvin cycle not directly dependent on light, but they usually do not occur at night - Why?

Options

A. Night is often too cold for these reactions to occur
B. `Counderset(2)` concentration in night is too high for these reactions to occur
C. Plants usually open their stomata at night
D. Calvin cycle is dependent on the products of light reaction

Answer

### Step-by-Step Solution:

1. **Understanding the Calvin Cycle**: The Calvin cycle, also known as the dark reactions or biosynthetic reactions, is a part of photosynthesis that does not directly require light. It utilizes the products of the light reactions (ATP and NADPH) to convert carbon dioxide (CO2) into glucose.

2. **Products of Light Reactions**: The light reactions of photosynthesis occur in the thylakoid membranes of chloroplasts and produce ATP and NADPH. These products are essential for the Calvin cycle to function.

3. **Dependence on Light Reactions**: Although the Calvin cycle itself does not need light to occur, it is dependent on the products generated during the light reactions. Therefore, without light, ATP and NADPH are not produced.

4. **Why Calvin Cycle Does Not Occur at Night**: At night, there is no light available to drive the light reactions. Consequently, ATP and NADPH are not produced, making it impossible for the Calvin cycle to proceed.

5. **Environmental Conditions**: While it is true that environmental conditions such as temperature and CO2 concentration can influence plant metabolism, the primary reason the Calvin cycle does not occur at night is the absence of light, which is necessary for the production of ATP and NADPH.

6. **Conclusion**: The Calvin cycle does not occur at night primarily because it relies on the products of the light reactions, which are not available in the absence of light.