What is the theoretical quantum requirement (number of photons) needed to evolve one molecule of molecular oxygen during oxygenic photosynthesis?
Answer and explanation
Correct answer: D. 8 photons
- Step 1: Evolution of one molecule of requires the removal of 4 electrons from two water molecules ().
- Step 2: Each electron must be excited twice: once by a photon in Photosystem II and once by a photon in Photosystem I (the Z-scheme).
- Step 3: Therefore, transferring 4 electrons from to requires absorbed photons (quanta). The quantum yield is .
Answer
8 photons
Why not the others: '2 photons' could only excite one electron through one photosystem; '4 photons' accounts only for excitation in a single photosystem without the two-step series arrangement; '18 photons' confuses the number of ATP required for glucose synthesis with the quantum requirement for oxygen evolution.
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