Diversity

As a group, prokaryotes use an impressive array of compounds to obtain energy in the form of ATP. This remarkable versatility enables them to occupy a wide variety of environmental habitats, including those where eukaryotes cannot exist (figure 11.1). Recall from chapter 6 that non-photosynthetic cells obtain energy by removing electrons from a reduced compound, the energy source, and transferring them to another compound, the terminal electron acceptor. These organisms are called chemotrophs, to distinguish them from phototrophs, which harvest energy from sunlight. Unlike chemotrophic eukaryotes, prokaryotes are not confined to using only organic compounds as an energy source. Those that obtain energy by oxidizing organic chemicals are termed chemoorganotrophs; those that obtain energy by oxidizing inorganic chemicals are chemolithotrophs. These terms and others used to describe organisms according to their metabolic capabilities are defined in table 11.3, on page 274. ■ chemotroph, p. 91 ■ phototroph, p. 91 ■ terminal electron acceptor, p. 134

Prokaryotes are also unique in the wide array of electron acceptors they may employ. Aerobic respiration, using molecular oxygen (O2) as a terminal electron acceptor, generates the most ATP. Prokaryotes may also use anaerobic respiration, however, transferring electrons to an inorganic molecule such as sulfur, sulfate, nitrate, or nitrite rather than O2. Many other prokaryotes are fermentative, using an internally generated organic compound such as pyruvate as a terminal electron acceptor. ■ anaerobic respiration, pp. 137,149 ■ fermentation, pp. 137,151

Understanding the impact of metabolic diversity involves recognizing the relative energy gains of different types of metabolism, which depend on both the energy source and the electron acceptor. The oxidation of an energy source that has a high tendency to lose electrons, coupled with the reduction of an electron acceptor that has a high tendency to gain electrons, releases the most energy (see figure 6.25). The released energy can be cap-

Figure 11.1 An Extreme Environment Inhabited by Certain Prokaryotes

A hot spring in Yellowstone National Park.

Figure 11.1 An Extreme Environment Inhabited by Certain Prokaryotes

A hot spring in Yellowstone National Park.

tured in the form of ATP. Recognizing the differences in energy gain helps explain why some types of metabolism are prevalent in some environments but not others. ■ oxidation-reduction reactions, p. 134

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