electrons pyruvic acid lactic acid pyruvic acid acetic acid

oxidation electron product acceptor

Figure 6.4 Fermentation versus respiration of glucose. Only respiration results in a net oxidation.

at the same time, serve as an electron donor for another. Essentially, the chemical "cast-offs" of organisms that cannot completely metabolize a food molecule may be taken up and used by another kind of organism (Fig. 6.5). This can lead to complicated and mutually dependent communities of microbes. Bacterial fermenters, for example, produce a host of compounds as by-products of the breakdown of large organic molecules. When the food molecule is glucose, the products typically are C2 compounds like ethanol or acetic acid. When the food molecules are fats, larger compounds are produced, like propionic acid (C3) or butyric acid (C4), or even one-carbon acids, like formic acid. These compounds can, in turn, serve as electron donors, that is, food, for still other organisms. For example, a large class of bacteria, known as the acetogens, exploits these chemical "cast-offs." Ace-togens come in three basic varieties: fermenters, which produce acetic acid (the acid in vinegar, usually designated by its anionic form, acetate) and carbon diox-ide;the CO2-reducing acetogens, which take formic acid and hydrogen gas and make acetate;and the H2-producing acetogens, which use propionic and butyric

Table 6.2 Substrates and redox potentials of common biological reactions found in eukaryotes and bacteria.

Oxidation/reduction couple Eo (mV) Comment

Table 6.2 Substrates and redox potentials of common biological reactions found in eukaryotes and bacteria.

O2 / H2O

+ 815


Fe+3 / Fe+2

+ 780

Iron bacteria

NO2-/ NH/

+ 440

Ammonium-oxidizing bacteria

NO3- / NO2-

+ 420

Nitrite-oxidizing bacteria

SO-2 / S

+ 50

Sulfur bacteria

Fumarate / succinate

+ 33

Intermediate in aerobic oxidation of glucose

Oxaloacetate / malate

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