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As I understand during anaerobic respiration, the electrons received from the decomposition of glucose are transferred to nicotinamide adenine dinucleotide and flavin adenine dinucleotide. As there is no oxygen the electrons are then transferred to something else, either Lactic Acid or Ethanol (in the case of yeast).

So why do animals produce Lactic Acid and not Ethanol?

Also, (I know this is silly, but) what would happen if they did?

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  • $\begingroup$ exact answer: in.answers.yahoo.com/question/index?qid=20110227125518AAh0WrC $\endgroup$ – Devashish Das Aug 1 '14 at 9:01
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    $\begingroup$ @DevashishDas. I dont think that link answers the question. Also, I am not sure what OP intends to ask- a. How is it that alcohol is not produced in animals b. Why (or for what greater reason) is it not produced. Answer to a. is trivial: animals don't have that enzyme. $\endgroup$ – WYSIWYG Aug 1 '14 at 11:41
  • $\begingroup$ @WYSIWYG: It was the first result on google to similar to the question. $\endgroup$ – Devashish Das Aug 1 '14 at 12:27
  • $\begingroup$ Strictly speaking electrons from NADH are not transferred to something else, but are transferred back to the carbon skeleton from which they came. (There is no 'external' electron acceptor in the glycolytic schemes described by the OP). There is a difference in free energy, though. This is one of the great (apparent) paradoxes of glycolysis. In addition, another paradox is that we do have alcohol dehydrogenase in liver, which we use to break down ethanol. Why? To break down alcohol? But they why do horses also have alcohol dehydrogenase? (Horse liver adh is very well characterized). $\endgroup$ – user1136 Oct 26 '18 at 12:37
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Consider this from the point of view of integrating whole body metabolism.

The body operates on a fuel economy built around glucose. Active muscle produces lactate (C3) which is exported to the liver through the bloodstream. Liver converts lactate to glucose (C6) which is then exported to the tissues.

What if muscle produced ethanol (C2)? Liver metabolises this to acetate. This could be converted to acetyl CoA and so on to fatty acids, but there is no way to achieve net synthesis of glucose from acetate. Muscle can of course use fatty acids, so you could conceive of an alternative system based upon this. It may be that this is less efficent from the point of view of energy useage - I haven't checked this because there is a more compelling argument against using ethanol.

According to this paper, lactate levels in the bloodstream can reach 15 - 25 mM during exercise. If muscle exported ethanol instead of lactate then blood alcohol content (BAC) would reach the same levels.

The middle of that range, 20 mM ethanol, is equivalent to a BAC of 0.076% which is just below the legal limit for driving in the USA and the UK, and above the legal limit in many European countries. This BAC is associated with effects upon reasoning, depth perception and peripheral vision. So, from an evolutionary position it really wouldn't make sense to have a metabolic system based around sending this much ethanol through the blood.

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    $\begingroup$ Our metabolic system would be different if this would be our way... $\endgroup$ – Chris Aug 1 '14 at 14:54
  • $\begingroup$ yeah I agree with Chris totally $\endgroup$ – WYSIWYG Aug 2 '14 at 15:34
  • $\begingroup$ But the question wasn't about a hypothetical alternative metabolic system, it was about the existing one using lactate and not ethanol as an end product of fermentation. So yes, if our metabolic sytem was constructed differently it would be ... different. $\endgroup$ – Alan Boyd Aug 2 '14 at 15:41

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