From my understanding, alcohol fermentation takes place in yeast and lactate production takes place in humans.

These two pathways take place only when there is insufficient oxygen, because the other parts of metabolism (TCA cycle, ETC) can't take place as they happen in the mitochondria, which requires O2.

For alcohol fermentation, there is production of carbon dioxide while lactic acid fermentation does not produce carbon dioxide. CO2 is produced when there is an oxidation of one carbon molecule. So my main question is: since there is a lack of O2, how does yeast produces CO2 while humans do not?

Also, how does NAD+ recycle all the time when glycolysis is happening?



1 Answer 1


Glycolysis needs a steady supply of NAD+ to happen - this is the driver for the anaerobic oxidation to lactate and ethanol, although this is energetically much less favorable than the complete oxidation. But without oxygen there is no other way to keep the glycolysis active for at least some energy supply.

The difference is located in the enzymes available for the conversion of the pyruvate. This is the Lactate dehydrogenase in humans (and other mammals) and the Pyruvate decarboxylase in yeast. The first catalyzes the reaction from Pyruvate to Lactate, the second from Pyruvate to Acetaldehyde and CO2, the Acetaldehyde is subsequently converted to Ethanol. Only the second step produces NAD+.

See the illustration (from here) for further understanding:

enter image description here

The CO2 produced in this reaction does not occur due to oxidation, but is released from the decarboxylation of the Pyruvate. See the illustration below (from here):

enter image description here

In the production of lactate no decarboxylation is happening which allows the backreaction from lactate to pyruvate once enough oxygen is present again.

  • $\begingroup$ Where the 2 NAD+ accepts electrons from to recycle back to NADH ? $\endgroup$
    – user307640
    Commented Jun 8, 2017 at 10:44
  • $\begingroup$ You might want to add that since there is no decarboxylation in production of lactate, there is no CO2 evolved. Also that glucose already contains oxygen, so there is no need of O2 in producing CO2 from pyruvate (just saw that you missed these points from the question :) $\endgroup$ Commented Jun 8, 2017 at 11:15
  • $\begingroup$ @user307640 Each molecule of glucose makes two molecules of pyruvate, which allows to run the process twice. $\endgroup$
    – Chris
    Commented Jun 8, 2017 at 11:46
  • 1
    $\begingroup$ @another'Homosapien' Good point. $\endgroup$
    – Chris
    Commented Jun 8, 2017 at 11:47
  • 2
    $\begingroup$ @another'Homosapien' None taken, that's ok. Something clear for yourself is not necessarily clearly explained for somebody else. Criticism helps here. $\endgroup$
    – Chris
    Commented Jun 9, 2017 at 9:07

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