Leo,

I don't know if yeast carry out respiration & fermentation
simultaneously, but I doubt they do to any significant degree.

As I understand it, yeast ferment preferentially over respiration,
even though fermentation is a far less efficient energy producer per
sugar molecule. The reason seems to be that they can ferment at a much
faster rate than they can respire, and this more than compensates for
the lower ATP yield. I suppose it's a "Fast nickels vs. slow dimes" kind
of thing.

I had always assumed that yeast would switch to respiration if oxygen
were present, because of the higher energy yield per sugar molecule, but
a very knowledgeable researcher type informed me of the above.

Hope this helps.

--


Mike MTM, Cokesbury, New Jersey, USA