Energy transfer from P680 to oxygen-evolving complex

Question

This topic is confusing me.

As I understand it, in the PS2, photons are used to split water into molecular oxygen + protons and electrons in the oxygen-evolving complex. To do this, a photon is used to excite an electron in the P₆₈₀, this electron is then released to go through the electron transport chain and somehow the oxygen-evolving complex gets energy from it to make water endure a "photochemical reaction" as described in my biology book.

But how the electron can transfer its energy to the oxygen-evolving complex if it went directly to the electron transport chain? how the energy produced in the P₆₈₀ transported to the Oxygen-evolving complex?

I would appreciate simple terms as I'm still a college student.

Answer 1

You have to think the other way around: don't pay attention to where the electron from P₆₈₀ is going.

Instead of that, pay attention to this fact: after PSII absorbs a photon and triggers the loss of an electron from a P₆₈₀ chlorophyll a molecule, this P₆₈₀ has now an electron deficit. That is, this P₆₈₀ has lost one electron.

This P₆₈₀ without one electron is now called P₆₈₀⁺. And here comes the important information, which answers your question: According to Lodish (2002),

P₆₈₀⁺, the photochemically oxidized reaction-center chlorophyll of PSII, is the strongest biological oxidant known. (emphasis mine)

And that explains what (thermodynamically) drives O₂-evolving complex (still according to Lodish, 2002):

The reduction potential of P₆₈₀⁺ is more positive than that of water, and thus it can oxidize water to give O₂ and H⁺ ions. (emphasis mine)

I hope those terms are simple enough for you. Finally, for better understanding all this, have a look at the concept of reduction potential (which, as a college student, I believe you have to study).

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Source:

• Lodish, H. (2002). Molecular cell biology. 4th ed. New York, N.Y.: Freeman.