How do the High-Energy Electrons During Chemiosmosis Come from Water?

Question

On Pg. 199 of Campbell, this is stated.

Both* work by way of chemiosmosis, but in chloro- plasts, the high-energy electrons dropped down the transport chain come from water, while in mitochondria, they are extracted from organic molecules (which are thus oxidized).

*Both refers to the to chemiosmosis in mitochondria and in chloroplasts (during photosynthesis).

My question is, how do the high-energy electrons in the transport chain for chemiosmosis come from water? Aren’t these high-energy electrons derived from light harvesting done by the light-harvesting complexes?

Answer 1

The light harvesting complexes use light energy to "lift" electrons to a higher energy state from which they can be used to reduce organic molecules (by way of an electron transport chain and NADPH), leaving a "hole" or cation behind in the LHC and thereby oxidizing it. If these complexes were not re-reduced by something, they would become non-functional after absorption of a single photon. So, they are re-reduced by removing electrons from water to generate molecular oxygen, which happens in photosystem II. So the photosystems I and II act as a series of "pumps" that make otherwise thermodynamically "uphill" (unfavorable) electron transfer from water to NADPH spontaneous, but they do not act as a net source of electrons--water does.