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Several questions about electron transport chain and charged molecules

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I always thought that atoms and molecules tend to hold no charge unless some force is involved.

  1. I'm learning about the electron transport chain in particular, and about redox reactions in general, and it seems that there are continual changes in positive and negative charges. Is there something powering all of these changes in charge, or am I mis-remembering the thing about atoms and molecules wanting to remain neutral?
  2. I understand that if the molecules are polar, the water molecules in the cytosol could pull them apart, resulting in charges, but what would cause them to continually gain and lose electrons, as in the case of NAD+ being transformed to NADP and then back to NAD+ again, continually?
  3. My biology textbook says that NAD+ gains an H and an electron from another H. Why doesn't NAD+ just gain two Hs and become NADH2+? What is powering the separation of the H electron from its proton?

I read something about electron flow and batteries, where the electrons sort of move around freely, but I don't understand what would cause that to happen in a cell. Help?

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I assume you know most of the chemiosmotic theory so with that in mind I will try to answer your question.

1) The first point you make is very easy to answer if a reduction reaction occurs then an oxidation reaction must occur as well that's why it is called a redox reaction they must happen in pairs. The electrons in the reaction are passed from a donor to an acceptor.

2) Molecules are changing for ATP production. ATP production is fueled by the proton motive force that is hydrogen ions (cations) diffusing through ATP synthase converting ADP and phosphate to ATP. The hydrogen ions move from the high concentration inside the cell to the low concentration outside the cell then through ATP synthase as mentioned above. The hydrogen ions are generated by redox reactions such as NADH ---> NAD+ + H+ + 2e- This reaction would be oxidation because you are losing electrons and gaining a hydrogen ion (this is only half of the reaction remember you would still need the reduction). The way I remember this is that our metabolism (ATP production) is oxidative which means compounds like NADH will be oxidised in order to produce ATP in the long run. Hopefully that will answer your second point.

3) If you read more about the chemiosmotic theory you will realise all these electrons produced will be passed along the carrier proteins in order drive the movement of the hydrogen ions across the membrane.


I tried to answer your questions as best I could, hopefully answered some of your questions. I find it a lot easier if you draw a big diagram of what is going on especially for this topic. Hopefully others will reply and be able to explain things better than I could good luck!

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The hydrogen ions move from the high concentration inside the cell to the low concentration outside the cell then through ATP synthase as mentioned above.


Small but important correction, via the electron transport chain protons are transported to the outside of the cell, i.e. the movement through the synthase is from the outside to the inside (i.e. the other way round).

As otherwise noted the charge transfers are the result of redox-reactions that are catalyzed by enzymes. With regards to NADH2, this is a matter of notation and charge conservation.

The correct equations would be NAD (+) +2e- + 2H(+) -> NADH + H(+) Only one hydrogen delivered from the oxidized molecule is actually bound.

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Another way to think about NAD is to look at it as a hydride ion acceptor, a hydride ion being a proton and two electrons. NADH is a hydride ion donor. Likewise NADP is a hydride ion accepter, and it becomes NADPH. The original post has a typographical error with respect to NADP.

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