Calvin cycle or dark reaction of photosynthesis is a light-independent reaction in which products of the light reaction are consumed . In which step of the Calvin cycle these products are not consumed ?

Options

A. Carboxylation
B. Reduction
C. Regeneration
D. Both (B) and (C)

Answer

**Step-by-Step Solution:**

1. **Understanding the Calvin Cycle**: The Calvin cycle, also known as the dark reaction of photosynthesis, consists of three main stages: carboxylation, reduction, and regeneration. It is important to note that while it is termed a "dark reaction," it does not necessarily occur only in the absence of light; rather, it does not directly require light to proceed.

2. **Identifying the Products of Light Reactions**: The light-dependent reactions of photosynthesis produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). These products are essential for the Calvin cycle as they provide the energy and reducing power necessary for the reactions.

3. **Analyzing Each Step of the Calvin Cycle**:
- **Carboxylation**: This is the first step of the Calvin cycle where carbon dioxide (CO2) is fixed into an organic molecule. The enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco) catalyzes the reaction between CO2 and ribulose 1,5-bisphosphate (RuBP). Importantly, during this step, ATP and NADPH are not consumed.

- **Reduction**: In this step, the 3-phosphoglycerate (3-PGA) molecules formed during carboxylation are converted into glyceraldehyde-3-phosphate (G3P). This process requires the input of ATP and NADPH, which are the products of the light reactions.

- **Regeneration**: This final step involves the regeneration of RuBP from G3P, allowing the cycle to continue. This step also requires ATP, again utilizing the products of the light reactions.

4. **Conclusion**: Since the question asks for the step in the Calvin cycle where the products of the light reaction (ATP and NADPH) are not consumed, the correct answer is the **carboxylation** step.

**Final Answer**: The step of the Calvin cycle where the products of the light reaction are not consumed is **carboxylation**.

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