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MCB 303: Fermentation
After glucose has been broken down into pyruvic acid, the pyruvic acid can be
completely broken down in respiration, as previously described, or it can be
converted to an organic product in fermentation, whereupon NAD+ and NADP+ are
regenerated and can enter another round of glycolysis. Fermentation can be defined
in several ways, but we define it here as a process that:
1. Releases energy from sugars or other organic molecules, such as amino acids,
organic acids, purines, and pyrimidines;
2. Does not require oxygen;
3. Does not require the use of the Krebs cycle or an electron transport chain;
4. uses an organic molecule as the final electron acceptor;
5. Produces only small amounts of ATP (only one or two ATP molecules for each
molecule of starting material) because much of the original energy in glucose
remains in the chemical bonds of the organic end-products, such as lactic acid or
ethanol.
During fermentation, electrons are transferred (along with protons) from reduced
coenzymes (NADH, NADPH) to pyruvic acid or its derivatives. Those final electron
acceptors are reduced to the end- products shown below. An essential function of
the second stage of fermentation is to ensure a steady supply of NAD+ and NADP+
so that glycolysis can continue. In fermentation, ATP is generated only during
glycolysis. Microorganisms can ferment various substrates; the end-products depend
on the particular microorganism, the substrate, and the enzymes that are present
and active. Chemical analyses of these end-products are useful in identifying
microorganisms.
