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aTacoParty t1_iwo8azb wrote

Like nearly everything in biology, water exists in a dynamic equilibrium. Constantly shifting between H2O and H+ & OH-. The mitochondrial matrix is an aqueous environment (IE watery) so there is a constant supply of H+ and OH-. The electron transport chain scoops these protons and sends them across the inner mitochondrial membrane to create the proton gradient.

Additional protons are also provided by the oxidation of NADH to NAD+ & H+.

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AngrySc13ntist t1_iwomisc wrote

Almost, but not quite. The proton gradient refers to the difference in concentration of H+ on each side of the membrane. It's ostensibly what allows for the ETC to transport those protons in the first place, by following their own gradient and harnessing some energy in the process to make ATP.

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aTacoParty t1_iwop1i4 wrote

I think you may have the first 4 complexes of the ETC and ATP synthase mixed up (though ATP synthase is sometimes consider complex V). The ETC pumps protons** across the inner mitochondrial membrane from the matrix to the intermembrane space to create the proton gradient. The protons then flow down their gradient across the membrane (IMS to matrix) via ATP synthase which generates ATP in the process.

The first diagram from the Khan academy article about oxphos does a great job at showing how the protons flow and how each complex contributes. https://www.khanacademy.org/science/ap-biology/cellular-energetics/cellular-respiration-ap/a/oxidative-phosphorylation-etc

EDIT: electrons -> protons

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danby t1_iwpk812 wrote

> The ETC pumps electrons across the inner mitochondrial membrane from the matrix to the intermembrane space to create the proton gradient.

This absolutely does not happen.

Electrons are transported from complex to complex arranged as a chain hence why it is called the ElectronTransport Chain. And indeed this is how it is illustrated in both the 1st two diagrams of your link, where the "path" of the electron(s) is between complexes within the membrane and not in to the intermembrane space. You'll note in the 2nd diagram that at the end the "free" electrons are passed from complex IV to Oxygen in the matrix because oxygen is the final acceptor of electrons in the ETC. This is where the electrons end up, not in the intermembrane space. Free electrons are not accumulated in the intermembrane space (I'm not even sure physics allows free electrons to accumulate outside of some exceptionally rarefied circumstances)

The oxidisation of free FADH and NADH provides high energy electrons at the start of the chain, as the electrons transition between complexes they lose energy and this free energy is "used" to move the H+ ions in "solution" within the matrix to the intermembrane space. That is, the ETC complexes pump protons across the inner mitochondrial member to the intermembrane space. This establishes a concentration gradient as H+ ions are highly concentrated in the intermembrane space.

Also the whole system wouldn't work if the intermembrane space was filled with free electrons while complexes I - IV were "pumping" H+ ions there. If that were the case the H+ ions would immediately neutralise and become hydrogen atoms. ATP Synthase works because it can make use of the concentration gradient between "free" H+ ions in the intermembrane space and the lack of "free" H+ ion in the mitochondrial matrix.

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aTacoParty t1_iwpvixg wrote

You're absolutely correct. I mistyped electrons instead of protons. I've edited my original comment!

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AngrySc13ntist t1_iwtch59 wrote

Ahhh, I see the problem. I had incorrectly assumed you were talking about ATP synthase, based on my very flimsy and rushed reading of your comment. Incidentally, I am of the camp that considers ATP synthase part of the ETC, but my critique of you was still based on my misperception of what you were trying to say.
Thanks for correcting my correction, science is hard and biology is confusing!

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