Which of the following is responsible for the production of oxygen during photosynthesis? A. Calvin cycle B. Photolysis of water C. Cyclic photophosphorylation D. Reduction of NADP+
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Remember, the oxygen produced in photosynthesis comes from the photolysis of water, which occurs in the light reactions. The Calvin cycle, on the other hand, is responsible for fixing carbon and producing glucose but does not release oxygen.
Photosynthesis converts light energy to chemical energy (glucose) in plants, occurring in two stages: light reactions and the Calvin cycle. The light reactions, in chloroplast thylakoids, use chlorophyll to capture light energy, producing ATP and NADPH. These molecules fuel the Calvin cycle in the stroma, which fixes carbon dioxide to create glucose.
Examining the options:
1. A) Calvin cycle: Also called dark or light-independent reactions, this occurs in the stroma. It uses ATP and NADPH from light reactions to fix CO2 and produce glucose. Oxygen is not a product; it is generated during the light reactions.
2. B) Photolysis of water: This light reaction process splits water using light energy in thylakoid membranes. It produces oxygen, protons (H\(^+\)), and electrons (e\(^-\)). The released oxygen atoms form the atmospheric oxygen byproduct.
The equation is:
\[
2H_2O \xrightarrow{\text{light energy}} 4H^+ + 4e^- + O_2
\]
This is the direct source of photosynthetic oxygen.
3. C) Cyclic photophosphorylation: Electrons cycle through photosystem I, producing ATP but not NADPH or oxygen. Since no water photolysis occurs, oxygen is not generated.
4. D) Reduction of NADP+: NADP+ is reduced to NADPH during light reactions (non-cyclic photophosphorylation), essential for the Calvin cycle. Oxygen is not produced; it originates from water photolysis.
Therefore, the correct answer is B) Photolysis of water, the direct source of oxygen in photosynthesis.
