Number of water molecules produced by cellular respiration

[biofan]

Hi.
I have a question regarding the number of water molecules produced by cellular respiration. At the level of the electron transport system (ETS), the equation regarding O2 as the final electron acceptor is: 1/2O2 + 2é + 2H+ ---­ H20. Considering that the 10 NADH molecules entering the ETS will contribute to 10 é pairs (and hence to the consumption of 10/2 = 5 O2 molecules), and that the 2 FADH2 molecules entering the ETS will contribute to 2 é pairs (and use 2/2 = 1 O2 molecule). Then what about water? According to this equation, a total of 12 molecules of water would be produced. How should we relate this to the general equation of glucose breakdown: glucose + 6 O2 ---- 6 CO2 + 6 H20 + E ??? This should mean that 6 water molecules are consumed at some other point to account for the global equation??
Any help would be appreciated, thanks.

[sebast18]

Hi, in fact cellular respiration does produce a part of the water the body needs, but it is a quite unsignificant amout. I could open my books and tell you the exact amount but your equation seemed right.

[biofan]

Thanks Sebast.
Sorry if my question lacked in clarity. I wasn't interested so much about the idea of water production per se, but rather about the understanding of the equations (trying to balance the general equation that states that 6 molecules of water will be formed for each glucose molecule broken down, with the equation that accounts for the oxygen consumption (and water production) at the end of the ETS. This equation states that for 1/2 molecule of O2 consumed by accepting an electron pair, there will be 1 molecule of water formed. Hence, considering that the 10 NADH molecules entering the ETS will contribute 10 electron pairs to the ETS, and the 2 FADH2 molecules contributes to 2 electron pairs; 12 molecules of water will be formed for each glucose molecule being broken down... I then presume that somewhere upstream in the pathways (glycolysis? transition reaction? Krebs cycle?), there should be 6 molecules of water being consumed, but where?

[MrMistery]

Well, i can tell you this:
Glycolysis also produces one molecule of water from 2-phosphoglycerate to phosphoenolpiruvate.(so 2 extra molecules of water are produced this way). On the other hand the Krebs cycle uses up 2 molecules of water per glucose when it turns fumarate into malate.
The answer to your question lies at the electron transport chain. i have been killing my brains trying to figure out how. I know 1000% that for every 2 NADH or FADH2 molecules one water molecule is produced. What i can not understand is how the heck?!? Cause every NADH molecule carries 2 electrons, and it should be enough to reduce one water molecule. Sorry, i just never realised this problem before. I will post it here as soon as i have an answer

[biofan]

Thanks Mr Mistery. It is one of those questions that seems so obvious at first sight that you just don't pay attention to it... just like the majority of textbooks on the subject I've consulted thus far! I don't know if this is a similar phenomenon to the one in the journalistic circle, where it sometimes seems like everybody is copying each other, but it seems like no one seemed to have noticed that things didn't balance out in terms of water production... Now I don't intend to say that things don't happen this way (as I said; there should be an overall consumption of 6 molecules of water somewhere upstream of the ETS to account for it), but rather point out the fact that the most common textbooks on the subject should have mentionned it. Hoping someone can help me figure out this one... Thanks in advance!

[sham]

Kreb cycle uses 3 molecules of H2O for every pyruvate that enters it.
Equation : C3H4O3 + 3H2O = 3CO2 + 10 H (for ETS). Yes, 1 molecule of H2O will be releases when fumarate turn to malate. The other 2 H2O, I am not sure.
Sham

[MrMistery]

krebs cycle uses H2O? Where? Maybe it says that 3 water molecules take place in the Krebs cycle. Cause conversion of citrate to isocitrate is done by extracting and then inserting a water molecule.