Which statement accurately describes how the two photosystems function in the light reactions of photosynthesis?

• A. Photosystem II is only involved in cyclic electron flow, while Photosystem I is responsible for splitting water molecules.
• B. Photosystem II absorbs light first, initiating electron flow that ultimately leads to ATP and NADPH production, while Photosystem I re-energizes electrons for NADPH synthesis.
• C. Both photosystems function independently, each producing ATP and NADPH without sharing electrons.
• D. Photosystem I absorbs light first and passes electrons to Photosystem II, which then produces both ATP and NADPH.

Answer: (B)

Verified step by step guidance

1. Step 1: Begin by understanding the role of photosystems in the light reactions of photosynthesis. Photosystems are protein-pigment complexes located in the thylakoid membrane of chloroplasts. They are responsible for capturing light energy and driving electron flow.
2. Step 2: Clarify the sequence of events in the light reactions. Photosystem II (PSII) absorbs light first, exciting electrons and initiating the electron transport chain. These electrons are passed to Photosystem I (PSI) via carrier molecules.
3. Step 3: Explain the specific functions of each photosystem. Photosystem II is responsible for splitting water molecules (photolysis), releasing oxygen, protons, and electrons. The electrons are used to generate ATP through chemiosmosis as they move through the electron transport chain.
4. Step 4: Describe the role of Photosystem I. Photosystem I re-energizes the electrons using light energy and facilitates their transfer to NADP⁺, forming NADPH, which is a key molecule for the Calvin cycle.
5. Step 5: Address the incorrect statements in the problem. Photosystem II is not involved in cyclic electron flow, and Photosystem I does not split water molecules. Both photosystems work together in a coordinated manner, rather than independently, to produce ATP and NADPH.