Nadh

oxidative phosphorylation

substrate-level phosphorylation

Pyruvate Pyruvate

6.5 Fermentation

Fermentation is used by organisms that cannot respire, either because a suitable inorganic terminal electron acceptor is not available or because they lack an electron transport chain. Escherichia coli is a facultative anaerobe that has the ability to use any of three ATP-generating options—aerobic respiration, anaerobic respiration, and fermentation; the choice depends in part on the availability of terminal electron acceptors. In contrast, members of a group of aerotolerant anaerobes called the lactic acid bacteria lack the ability to respire; they only ferment, regardless of the presence of oxygen (O2). Because they can grow in the presence of oxygen but never use it as a terminal electron acceptor, they are sometimes called obligate fermenters. The situation is different for obligate anaerobes that

use fermentation pathways; they cannot even grow in the presence of O2, and many are rapidly killed in its presence.

In general, the only ATP-yielding reactions of fermentation are those of glycolysis, and involve substrate-level phos-phorylation. The other steps function primarily to consume reducing power, providing a mechanism for recycling NADH (figure 6.22). If this reduced carrier were not recycled, no NAD+ would be available to accept electrons in subsequent rounds of glycolysis, blocking that ATP-generating pathway. To consume reducing power, fermentation pathways use an organic molecule as a terminal electron acceptor. In some cases, pyru-vate serves directly as the electron acceptor; in others, it is first converted to another compound before being reduced.

The end products of fermentation are significant for a number of reasons. Because a given type of organism uses a characteristic fermentation pathway, end products can sometimes be

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152 Chapter 6 Metabolism: Fueling Cell Growth

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