Question

In photosynthesis, the electron that travels through the electron transport chain in the thylakoid membrane comes from____1____ . In cellular respiration, a molecule of supplies the electron that travels through the electron transport chain____2____.
1 2
atp atp
carbon dioxide carbon dioxide
water water
nadh nadh

Answer 1

Answers:
1) Water
2) NADH

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1) Water donates the initial electron in photosynthesis. The process of photolysis splits the water into H+ and O2 (oxygen, a byproduct of photosynthesis), and releases electrons which are taken up by photosystem II.

2) The electron transport chain is in the last step of cellular respiration, oxidative phosphorylation. NADH donates the electrons, which are used to pump H+ against the gradient into the intermembrane space of the mitrochondria. These H+ flow back across the membrane through ATP synthase to generate ATP.