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Fermentation is a metabolic process that converts sugar to
acids, gases and/or alcohol. It occurs in yeast and bacteria,
but also in oxygen-starved muscle cells (see "Lactic acid
fermentation" below). Fermentation takes place in the absence of
oxygen, when the electron transport chain is unusable. It is
used by the cell not to generate energy directly, but to recycle
NADH into NAD+ so that glycolysis can continue, as long as
glucose is present. The energy generated by the glycolysis-
fermentation pathway, a form of substrate-level phosphorylation,
is small compared with that of oxidative phosphorylation.
Fermentation consumes NADH, which in aerobic conditions might
have been used to generate energy in the electron transport
chain. For that reason, cells generally benefit from avoiding
fermentation when oxygen is available. Exceptions include
obligate anaerobes, which cannot tolerate oxygen.  Ferme