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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Perspective, Fourth Edition

152 Chapter 6 Metabolism: Fueling Cell Growth

Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

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