What would happen to the light reactions of photosynthesis if the Calvin cycle stopped converting NADPH to NADP\(^+\)?

• A. The light reactions would start producing oxygen instead of NADPH.
• B. The light reactions would produce more ATP to compensate for the lack of NADP\(^+\).
• C. The light reactions would continue at the same rate because NADPH is not required.
• D. The light reactions would slow down due to a lack of available NADP\(^+\) to accept electrons.

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the relationship between the light reactions and the Calvin cycle in photosynthesis. The light reactions generate ATP and NADPH, which are used in the Calvin cycle to drive the synthesis of glucose. NADPH is converted back to NADP\(^+\) in the Calvin cycle, which is then reused in the light reactions.
2. Step 2: Recognize the role of NADP\(^+\) in the light reactions. NADP\(^+\) acts as an electron acceptor during the light reactions, allowing the transfer of electrons from photosystem I to form NADPH. Without NADP\(^+\), the electron transport chain cannot function efficiently.
3. Step 3: Analyze the impact of the Calvin cycle stopping its conversion of NADPH to NADP\(^+\). If NADP\(^+\) is not regenerated, the light reactions will lack the necessary electron acceptor, causing the electron transport chain to slow down.
4. Step 4: Consider why the light reactions would slow down rather than producing oxygen or more ATP. Oxygen production occurs during the splitting of water in photosystem II, but the overall rate of the light reactions depends on the availability of NADP\(^+\) to accept electrons. ATP production is also tied to the electron transport chain, which would be hindered without NADP\(^+\).
5. Step 5: Conclude that the correct answer is: 'The light reactions would slow down due to a lack of available NADP\(^+\) to accept electrons.' This is because NADP\(^+\) is essential for maintaining the flow of electrons in the light reactions.