Terminal electron acceptor in noncyclic photophosphorylation In the noncyclic (linear) electron flow of the light reactions, electrons originating from water are ultimately accepted by which molecule?

Introduction / Context:Noncyclic photophosphorylation transfers electrons from water to NADP+, generating both ATP and reducing power (NADPH) for the Calvin cycle. Identifying the terminal electron acceptor is key to understanding the function of photosystem I and downstream reductive metabolism.

Given Data / Assumptions:

• Photosystem II oxidizes water, releasing electrons, protons, and O2.
• Electrons move through cytochrome b6f and plastocyanin to photosystem I.
• Ferredoxin NADP+ reductase catalyzes the final reduction step.

Concept / Approach:

The linear pathway is water → PSII → plastoquinone → cytochrome b6f → plastocyanin → PSI → ferredoxin → NADP+. The last acceptor is NADP+, which becomes NADPH. This reductant powers CO2 fixation and reduction in the Calvin cycle.

Step-by-Step Solution:

Verification / Alternative check:

Spectroscopic and biochemical assays show accumulation of NADPH under illumination with active PSI and FNR, confirming NADP+ as the acceptor.

Why Other Options Are Wrong:

Chlorophyll a and b are photopigments, not final electron sinks. CO2 is reduced in the Calvin cycle by NADPH, not directly by the electron transport chain. Ubiquinone is a mitochondrial carrier, not a chloroplast component.

Common Pitfalls:

Confusing the terminal acceptor of chloroplast electron flow (NADP+) with that of mitochondria (O2).

Final Answer:

NADP+