Electron fate across photosynthesis and respiration Assume the full sequence of photosynthesis followed by cellular respiration. Electrons that begin in H2O at the start of the light reactions ultimately end up attached to which molecule at the end of aerobic respiration?

Introduction / Context:
Energy flow in biology involves electron transfers that begin with water oxidation in photosynthesis and end with oxygen reduction in respiration. Tracing the path of electrons clarifies how light energy is stored and later released to power life processes.

Given Data / Assumptions:

• During the photosynthetic light reactions, water is split and electrons are transferred through photosystems.
• Photosynthesis ultimately stores reducing power in molecules that support carbohydrate synthesis.
• In aerobic respiration, oxygen is the terminal electron acceptor, forming water.

Concept / Approach:

At photosystem II, H2O is oxidized: electrons flow via plastoquinone, cytochrome b6f, plastocyanin, photosystem I, and ferredoxin to reduce NADP+ to NADPH. In the Calvin cycle, NADPH helps reduce CO2 into carbohydrates. When organisms respire these carbohydrates, electrons pass through NADH/FADH2 to the mitochondrial electron transport chain where O2 is reduced to H2O. Thus, electrons that started in water return to water after fueling metabolism, with O2 serving as the terminal acceptor in respiration.

Step-by-Step Solution:

Verification / Alternative check:

Mass balance across the combined processes shows that O2 is reduced to H2O in mitochondria, consistent with classic respirometry and isotope tracing experiments.

Why Other Options Are Wrong:

NADPH does not persist as the final sink; it is consumed. Ethanol production is anaerobic fermentation, not the aerobic sequence. O2 is not produced by electron transport; rather, it is reduced. Bicarbonate formation is not the terminal step of respiratory electron flow.

Common Pitfalls:

Confusing the oxygen evolved from water splitting with the oxygen reduced in respiration; they are different O2 molecules in most ecosystems.

Final Answer:

O2 to make new H2O