In green plants, what is the correct sequence of photosystems during the light-dependent reactions of photosynthesis, and what does each photosystem primarily produce?

• A. Photosystem II acts first, producing ATP, followed by Photosystem I, which produces NADPH.
• B. Photosystem I acts first, producing ATP, followed by Photosystem II, which produces NADPH.
• C. Photosystem I acts first, producing NADPH, followed by Photosystem II, which produces ATP.
• D. Photosystem II acts first, producing NADPH, followed by Photosystem I, which produces ATP.

Answer: (A)

Verified step by step guidance

1. Understand that the light-dependent reactions of photosynthesis occur in the thylakoid membranes of chloroplasts and involve two photosystems: Photosystem II (PSII) and Photosystem I (PSI). These photosystems work together to convert light energy into chemical energy in the form of ATP and NADPH.
2. Recall that Photosystem II (PSII) is the first photosystem to act. It absorbs light energy, which excites electrons in chlorophyll molecules. These high-energy electrons are passed to the electron transport chain, and water molecules are split (photolysis) to replace the lost electrons, releasing oxygen as a byproduct.
3. As the excited electrons move through the electron transport chain, their energy is used to pump protons (H⁺) into the thylakoid lumen, creating a proton gradient. This gradient drives ATP synthesis via ATP synthase, producing ATP.
4. Next, the electrons from Photosystem II are transferred to Photosystem I (PSI). PSI absorbs light energy, further exciting the electrons. These high-energy electrons are then used to reduce NADP⁺ to NADPH, a key molecule for the Calvin cycle.
5. Summarize the sequence: Photosystem II acts first, producing ATP, followed by Photosystem I, which produces NADPH. This sequence is essential for providing the energy and reducing power needed for the light-independent reactions of photosynthesis.