Yeast are classified as facultative anaerobes, which means they are capable of
both aerobic and anaerobic respiration. When oxygen is unavailable, yeast carry out
fermentation, a type of anaerobic respiration. The difference between aerobic and
anaerobic respiration lies in how the NADH + H+ produced in glycolysis is converted
back to NAD+
. In aerobic respiration the hydrogens (electrons) from NADH +H+ are
passed to oxygen in the electron transport chain yielding approximately 2.5 ATPs per
NADH + H+ while in fermentation the hydrogens (electrons) are passed on to
acetaldehyde to form ethanol yielding no ATPs per NADH + H+ as follows:
Step 1 - Pyruvate (from glycolysis) → Acetaldehyde + CO2
Step 2 - Acetaldehyde Ethanol
NADH + H+ NAD+
Humans have made use of the byproducts of fermentation for centuries - the CO2
to make bread rise and the ethanol in beer and wine. From the yeast's viewpoint both
CO2 and ethanol are waste products and in fact ethanol is toxic, killing the yeast
organisms when it reaches a concentration between 14-18%. This is why the percentage
of alcohol in wine and beer doesn't exceed approximately 16%. In order to produce
beverages with higher concentrations of alcohol (liquors), the fermented products must be
distilled.
Fermentation produces only 2 ATPs per glucose molecule (via glycolysis).
Aerobic respiration produces 30 - 32 ATPs from a molecule of glucose. Thus the ability
of yeast to live in the absence of oxygen comes at a price - fermentation produces 19-fold
fewer ATPs per glucose molecule than does aerobic respiration.