Why must the first stage occur before the second stage in photosynthesis?

Why must the first stage occur before the second stage in photosynthesis?

• A. Because oxygen produced in the first stage is required.
• B. Because ATP or hydrogen produced in the first stage is required.
• C. Because carbon dioxide is not used in the first stage.
• D. Because light is required for the second stage.

Answer: (B)

The key idea is that the second stage needs energy and reducing power from the first stage. In the light-dependent reactions, light energy is used to split water and drive an electron transport chain, which generates ATP and NADPH. The Calvin cycle in the second stage uses ATP as energy and NADPH as the source of reducing power to convert carbon dioxide into carbohydrate. Without those molecules from the first stage, the carbon-fixation steps of the second stage can’t proceed. Oxygen production, CO2 usage in the first stage, or light being required for the second stage aren’t the limiting factors here—the essential point is supplying ATP and NADPH for the second stage.

The key idea is that the second stage needs energy and reducing power from the first stage. In the light-dependent reactions, light energy is used to split water and drive an electron transport chain, which generates ATP and NADPH. The Calvin cycle in the second stage uses ATP as energy and NADPH as the source of reducing power to convert carbon dioxide into carbohydrate. Without those molecules from the first stage, the carbon-fixation steps of the second stage can’t proceed. Oxygen production, CO2 usage in the first stage, or light being required for the second stage aren’t the limiting factors here—the essential point is supplying ATP and NADPH for the second stage.