Fermentation
Fermentation is the partial oxidation of glucose or another organic compound to
release energy; it uses an organic molecule as an electron acceptor rather than an
electron transport chain. In a simple fermentation reaction, NADH reduces pyru-
vic acid from glycolysis to form lactic acid. Another example involves two reac-
tions. The first is a decarboxylation reaction where CO 2 is given off (this is the
CO 2 that causes bakery items to rise), followed by a subsequent reduction reac-
tion that produces ethanol.
The essential function of fermentation is the regeneration of NAD+for glycol-
ysis so ADP molecules can be phosphorylated to ATP. The benefit of fermentation
is that it allows ATP production to continue in the absence of O 2. Microorganisms
that ferment can grow and colonize in an anaerobic environment.
Microorganisms produce a variety of fermentation products. The products of
fermentation of cells are waste products of the cells, but many are useful to
humans. These include ethanol(the alcohol that humans can drink, like in beer,
wine, and liquor), acetic acid(vinegar), and lactic acid(found in cheese, sauer-
kraut, and pickles). Other fermentation products are very harmful to humans. An
example is the bacterium Clostridium perfringens, which ferments hydrogen and
is associated with gas gangrene. Gangrene is involved in necrosis or the “death”
of muscle tissue.
Other Catabolic Pathways
There are two other important catabolic pathways. These are lipid catabolism
and protein catabolism. Both of these convert substances (lipids and proteins)
into ATP, providing the microorganism with energy.
LIPID CATABOLISM
Lipids, including fats, which consist of glycerol and fatty acids, can be involved
in ATP production. Enzymes called lipaseshydrolyze the bonds attaching the
glycerol to the fatty acid chains. Glycerol is converted to dihydroxyacetone phos-
phate (DHAP), which is oxidized to pyruvic acid in glycolysis. The fatty acids
are broken down in catabolic reactions called beta-oxidation. In beta-oxidation,
CHAPTER 5 The Chemical Metabolism^97