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I have been wondering about life using a different "Final" electron acceptor(replacing oxygen), but every thing I can find in my research say that oxygen produces more ATP because of its higher affinity for electrons.

But as far as I know, in the electron transport chain, the ATP produced is because of protons going through the ATP synthase because of an imbalance in protons(the protons being generated by the electrons moving through the ETC). But since all the protons are generated by the time the electrons have made it through and are ready to be accepted by oxygen, then shouldn't the number of protons generated be the same regardless of that final electron acceptor? And if the number of protons produced is the same, then shouldn't the same amount of ATP be produced?

Why, despite the same number of protons being generated by electrons in the ETC, do other "Final" electron acceptors (such as Fe3+) besides oxygen produce less ATP?

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    $\begingroup$ Welcome to Biology StackExchange! Can you please include some references for your statements, that is: which sources did you use in research that has these claims? $\endgroup$
    – Domen
    Jan 13, 2023 at 22:14

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Short answer: The "worse" final electron acceptor will just not accept the electron, and electrons will just get back up in the electron transport chain, shutting down oxidative phosphorylation.

Long answer: This is related to a deceptively dissimilar question: "Why do cells throw away perfectly good organics in fermentation?"

To really explain this, you'll have to know some quantum mechanics.

enter image description here

Every nucleus, atom, molecule and ion has orbitals (MO theory) , little "slots" for electrons to exist in (shown as horizontal lines). Orbitals have distinct "energy levels", thanks to the electromagnetic interaction between the electron, the nucleus (nuclei) and other electrons. Typically, electrons will exist in the lowest energy level not occupied by another pair of electrons. Electrons release energy when they move down and absorb energy when they move up. A free electron ($n= \infty $) will release more energy moving to the lowest energy level ($n=1$) when that lowest energy level is more negative, and will have to absorb more energy to become free again. This is what is meant when we say an electron acceptor has "higher affinity for electrons". enter image description here The ENTIRE gimmick of the electron transport chain is that the electron moves to progressively higher affinity acceptors, releasing energy in the process. By the time the electron reaches the penultimate of the electron transport chain, its energy level is so low/the acceptor affinity is so high (equivalent) that it takes another acceptor with even higher affinity to attract that electron.

But what if the cell just... throw the electron away? Well, that would completely defeat the point. For the electron to become free it needs to be provided with energy. More energy than what the electron transport chain has extracted for sure, because the electron in the food molecule isn't even free to begin with.

Ok, but WHAT IF the cell use the electron to build organics instead? Every cell requires an energy source, a carbon source and an electron source. Unfortunately, that won't work in this case. Organics in biology usually requires electrons with energy equivalent to that of NADH. Since that is the very beginning of the electron transport chain here, it won't work.

The ONLY productive way to throw an electron away is to give it an acceptor and then throw that away too.

So that's why Fe3+ etc cannot accept an electron from the penultimate of the aerobic electron transport chain. Because you'd need to provide energy to the electron to do so.

Back to the question I proposed in the beginning. "Why do cells throw away perfectly good organics in fermentation?". By definition, fermentation means using an organic terminal electron acceptor. What usually happens is that cells do this clever maneuver where they rip the electron from an high energy position in the molecule and puts it in another position with lower energy. enter image description here

Notice the cycling of the electron carrier NADH.

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    $\begingroup$ The items of information assembled in your answer may be individually correct, but imho they miss the wood for trees, and they do not address the actual question. The wood is that the potential to obtain energy from oxidation depends on the negative magnitude of ΔG, and the question is not why no energy is produced if Fe(III) replaces O2 — dissimilatory oxidation of NADH by Fe(III) exists (ecampusontario.pressbooks.pub/bioc2580/chapter/…) — but why it is less than with O2 This is not obvious as the ΔG is not significantly different, despite the assertion in the Q. $\endgroup$
    – David
    Jan 20, 2023 at 15:55
  • $\begingroup$ I highly doubt that the ΔG°' is "not significantly different". ΔE˚' is. However, oxygen accepts 2 electron per water, while ferric iron accepts 1 per iron. Using ΔG°' = -nFΔE°', where n is the number of electrons transfered... Yeah you can see where this is going. $\endgroup$ Jan 21, 2023 at 3:34
  • $\begingroup$ Ok. Point taken. But nevertheless, the fact remains that the overall ΔG is sufficient to reduce Fe(III) in dissimilatory oxidation, and your answer does not acknowledge this fact or address the energetics of this process. I have not answered myself because I have not yet tracked down sources for the details of the components of the ETC in dissimilatory iron-reducing bacteria. If you wish to apply your chemical knowledge to the actual situation it will save me the effort. $\endgroup$
    – David
    Jan 21, 2023 at 13:35

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