During the light reactions of photosynthesis ______ is oxidized and ______ is reduced.

Question

BlackTom AI · Accepted Answer

During the light reactions of photosynthesis, electron flow is initiated by splitting water and ultimately reducing carrier molecules. 
Option 1: 'Oxygen and water' would imply oxygen is oxidized and water is reduced, which contradicts the known direction of electron flow where water loses electrons (is oxidized) and NADP+ gains electrons (is reduced).
Option 2: 'CO2 and Water' suggests carbon dioxide is the oxidized species and water is reduced, which does not reflect the events of the light reactions where CO2 is fixed later in the Calvin cycle and not involved in the direct oxidation-reduction steps of the light reactions.
Option 3: 'NADPH and CO2' states that NADPH is oxidized and CO2 is reduced, but in the light reactions NADP+ is reduced to NADPH, not NADPH being oxidized, so this reverses the actual redox changes.
Option 4: 'Water and NADP+' correctly identifies water as the substance that is oxidized (splitting to release electrons and protons) and NADP+ as the electron acceptor that is reduced to NADPH, which matches the standard depiction of the Z-scheme of photosystem I/II. The oxidation of water provides electrons, while the reduction of NADP+ stores energy in NADPH for the carbon-fixing steps downstream.
Option 5: 'Water and CO2' would imply CO2 is involved as the oxidized partner, which is not correct for the light reactions where CO2 is not directly oxidized or reduced.

In summary, only option 4 aligns with the fundamental redox changes of the light reactions: water is oxidized, and NADP+ is reduced to NADPH, while O2 is released as a byproduct.

During the light reactions of photosynthesis is oxidized and is reduced.Single choice

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During the light reactions of photosynthesis ______ is oxidized and ______ is reduced.Single choice