Photosynthetic Electron Flow—Final Electron Acceptor of Water-Splitting Electrons During photosynthesis, electrons released by the photolysis (splitting) of water ultimately reduce which molecule?

Introduction / Context:
In oxygenic photosynthesis, water is split at photosystem II to provide electrons, protons, and O2. The path of these electrons through the photosynthetic electron transport chain determines which molecules become reduced and how energy is stored for carbon fixation.

Given Data / Assumptions:

• Photolysis occurs at PSII's oxygen-evolving complex.
• Electrons travel from PSII → plastoquinone → cytochrome b6f → plastocyanin → PSI → ferredoxin.
• Ferredoxin-NADP+ reductase catalyzes the terminal reduction of NADP+ to NADPH.

Concept / Approach:
The ultimate purpose of linear electron flow is to generate NADPH (reducing power) and ATP (energy). Electrons from water end up reducing NADP+ to NADPH at the stromal side of PSI via ferredoxin. ATP is produced by chemiosmosis but does not accept electrons. O2 is produced from water and released; it is not reduced. Rubisco catalyzes CO2 fixation and is not an electron sink. Plastoquinone is an intermediate carrier, not the final destination.

Step-by-Step Solution:

Verification / Alternative check:
Measuring stromal NADPH accumulation upon illumination confirms that NADP+ is the terminal electron acceptor in linear flow.

Why Other Options Are Wrong:

• ATP: synthesized via ATP synthase; does not accept electrons.
• O2: generated from water; not reduced in this context.
• Rubisco: catalytic CO2-fixing enzyme, not an electron carrier.
• Plastoquinone: a mobile intermediate; electrons move past it to PSI and ferredoxin.

Common Pitfalls:
Confusing the proton gradient (for ATP) with electron flow; assuming O2 is an electron acceptor here.

Final Answer:
NADPH (i.e., they reduce NADP+ to NADPH)