[ptryon]

Hi,

If you add up the electrons produced in the aerobic respiration of glucose (via glycolysis and the Krebs cycle) you get 14e produced per glucose molecule and if you add up the electrons required to reduce oxygen to water you only need 12e per glucose molecule. What happens to the two surplus electrons?

Electrons produced by the oxidation of glucose:

2 NADH in Glycolysis
2 NADH per glucose in the link reaction (Producing Acetyle CoA from Pyruvate)
6 NADH produced in the Krebs cycle- per glucose
2 FADH₂ produced in the Krebs cycle- per glucose

Total NADH = 10, Totals e = 10
Total 2 FADH₂ Total e = 4 (note each FAD requires 2e for its reduction)
Overall 14 e lost in the oxidation of glucose

Electrons required for the reduction of oxygen in the present of hydrogen ions:

3O₂ + 12H⁺ + 12e  6H₂O

Overall 12 e required for the reduction of oxygen per glucose

[RedViper9]

This is a system of two equations.

1 Glucose 14e^-
12e^- + 6H⁺ + 3O₂ 6H₂O

12*7 = 84 = 14*6

6 Glucose 84e^-
84e^- + 42H⁺ + 21O₂ 42H₂O

Therefore:
6 Glucose + 42H⁺ + 21O₂ 42H₂O

No electrons are missing.

[Borek]

Therefore:
6 Glucose + 42H⁺ + 21O₂ 42H₂O

No electrons are missing.

Perhaps I am missing something, but none of the equations you wrote is balanced as written. So even if you are right on some level, you have not proved anything yet.

[Babcock_Hall]

One very large problem I see in the accounting is that NAD is an obligate 2-electron acceptor and NADH is an obligate 2-electron donor (NAD also accepts one proton in the conversion to NADH).

I find assigning oxidation numbers to carbon helpful when I work problems like this: O.N(C) = (# of bonds to oxygen) - (# of bonds to hydrogen).  Treat other electronegative atoms such as nitrogen as if they were oxygen.  I think that you will find that the number of electrons produced when glucose is converted into carbon dioxide is much larger than 14.  Try it and see.

[ptryon]

Thank you for your suggestion of analysing the problem using oxidation numbers. I will give that a try and see if it helps.

I am still interested in the electrons that are transferred to the electron transport chain (ETC) and eventually to the oxygen molecules to make water. The stoichiometry of the electron transport chain should balance too. Surely every glucose molecule needs to supply the correct number of electrons to yield the production of 6 water molecules (i.e. 12e) If NAD accepts 2 electrons doesn't this exasperate the problem even further? This means that 24e are being fed into the electron transport chain per glucose and therefore 12 water molecules could be produced per glucose(!?)

[AWK]

Check carefully your biochemical scheme and compare it with a few others. May be something is missing or you something omitted.
And compare with
https://biochemistryquestions.wordpress.com/2008/08/30/energetic-balance-of-the-total-oxidation-of-one-mol-of-glucose-up-to-co2-and-h2o-understanding-the-contradictions/

[Babcock_Hall]

If you assign oxidation numbers to glucose and CO₂, you can confirm that 24 electrons are removed from the carbon atoms.  I am not following how you found the value of 12 for the number of water molecules.  It sounds too high.

[ptryon]

It all goes back to the number of NADs and FADs that seem to be produced in the process of glycolysis, the link reaction and the Krebs cycle.

Almost every source I consult agrees with the following facts. For each glucose molecule:

- 10 NADs are reduced
- 2 FADs are reduced
- All these NADs and FADs make their way to the membrane where they are oxidised and their electrons enter into the ETC (24 electrons if I am not mistaken?)

So whats the problem? At the other end O₂ molecules sit there and gobble up the electrons to form water. It seems like rather too many electrons are being pumped into the ETC- because we only produce 6H₂O molecules per glucose (according to the net equation for respiration). But this would only require 12e.

Annoyingly almost every source I consult seems to focus on the ATP yield (I can see why this is of more interest)- but as a chemist I am curious to know what happens to these sexy little electrons :-)

[AWK]

In the link given by me there is a clear information that there are two slightly different processes and the difference lies in a small number of ATP molecules.

[Arkcon]

Pytron: You are facing a problem that comes up quite often here on the Chemical Forums, and AWK: has given you the best answer.  The reason electrons seem to be missing or ATP yield seems to be variable is because of what Babcock_Hall: said, the amount of FAD and NAD that reacts, and how efficiently they transfer electrons is variable from species to species, tissue to tissue, and random organism needs.  You even said it -- most if your references agree...so some don't.  The question is why.

[Babcock_Hall]

So whats the problem? At the other end O₂ molecules sit there and gobble up the electrons to form water. It seems like rather too many electrons are being pumped into the ETC- because we only produce 6H₂O molecules per glucose (according to the net equation for respiration). But this would only require 12e.

Annoyingly almost every source I consult seems to focus on the ATP yield (I can see why this is of more interest)- but as a chemist I am curious to know what happens to these sexy little electrons :-)

To reduce six O₂ to the oxidation state that they have in water or CO₂ also requires 24 electrons.

[ptryon]

Firstly thank you to everyone who has posted answers :-)

I am sorry to say I am not yet clear on what happens to these electrons.

AWK thank you for the link, it gives a great explanation of why the ATP yield varies, but it doesn't seem to discuss the fate of the electrons transferred to NAD and FAD in their reduction- apart from stating that the electrons enter the respiratory chain (what I referred to as the electron transport chain). So we're left with the same problem- it looks like 24e are being pumped into this chain- enough to produce 12 water molecules per glucose- not 6 water molecules that is required stoichiometrically. Sorry if I am missing something here- please do let me know if I am...

Babcock hall: I see two problems here- firstly as far as I can tell the NADs and FADs dont have a role in producing CO₂ but I have not examined the mechanism in detail (if they do I'd love to know as this could be the solution). The other problem is that you only need 12e^- to reduce 6O₂ molecules at the end of the ETC. Have a look at the balanced equation:

6O₂ + 12H⁺ + 12e^- 6H₂O

So what happens to the other 12 electrons.

[ptryon]

Pytron: You are facing a problem that comes up quite often here on the Chemical Forums, and AWK: has given you the best answer.  The reason electrons seem to be missing or ATP yield seems to be variable is because of what Babcock_Hall: said, the amount of FAD and NAD that reacts, and how efficiently they transfer electrons is variable from species to species, tissue to tissue, and random organism needs.  You even said it -- most if your references agree...so some don't.  The question is why.

The ATP yield does seem to vary from source to source- but from my reading it seems like there is a strong agreement on the NAD and FAD yield- including the article that you claim answers the question.
If you think about it the NADs and FADs shouldn't vary for full aerobic respiration since they are the oxidising reagents for a complete oxidation reaction.

[Yggdrasil]

Firstly thank you to everyone who has posted answers :-)

I am sorry to say I am not yet clear on what happens to these electrons.

AWK thank you for the link, it gives a great explanation of why the ATP yield varies, but it doesn't seem to discuss the fate of the electrons transferred to NAD and FAD in their reduction- apart from stating that the electrons enter the respiratory chain (what I referred to as the electron transport chain). So we're left with the same problem- it looks like 24e are being pumped into this chain- enough to produce 12 water molecules per glucose- not 6 water molecules that is required stoichiometrically. Sorry if I am missing something here- please do let me know if I am...

Babcock hall: I see two problems here- firstly as far as I can tell the NADs and FADs dont have a role in producing CO₂ but I have not examined the mechanism in detail (if they do I'd love to know as this could be the solution). The other problem is that you only need 12e^- to reduce 6O₂ molecules at the end of the ETC. Have a look at the balanced equation:

6O₂ + 12H⁺ + 12e^- 6H₂O

So what happens to the other 12 electrons.

Question is not what happened to the other 12 electrons, but what happened to the other 6 waters.

You are correct that breakdown of glucose will theoretically feed 24 electrons into the electron transport chain to produce 12 water molecules.  Yet, according to the balanced chemical reaction, oxidation of glucose to CO₂ should produce only 6 water molecules.  What's the source of this discrepancy?  6 water molecules get used up in the breakdown of glucose, yielding a net production of only 6 water molecules.

[ptryon]

Yggdrasil thank you for your reply.

At what point are these 6 water molecules used? I haven't spotted these on any reaction schemes I have seen (but admittedly, these are all generally massively simplified and don't show any mechanistic details). I'd love to know more.