1

CO2 water Figure 6.3 Movements of carbon in glucose metabolism.

Aerobic TransPort Respiration

The Anaerobic World

We're now in a position to explain why oxygen can be so toxic and at the same time so necessary. Both characteristics derive from oxygen's high redox potential. For the first billion years following the origin of life, oxygen gas was scarce, and bacteria living then could not take advantage of its high redox potential as we do presently. Without oxygen to accept electrons, other molecules have to serve that role, and organisms of the time were very creative in finding alternative electron acceptors to keep the juice flowing. The requirements for an alternative electron acceptor are simple: it simply must have a higher redox potential than the "food" molecule from which it draws electrons. When oxygen is not present, a broad array of different molecules may serve as electron acceptors and electron donors. For example, during the breakdown of glucose, with six carbons (a C6 compound), one of the crucial reactions produces two molecules of a C, compound, pyruvic acid. Pyruvic acid, in turn, can accept the electrons released by glucose oxidation, forming lactic acid or ethanol. Pyruvic acid thus substitutes for oxygen as an electron acceptor (Fig. 6.4). "Internal" oxidation-reduction reactions like these are called fermentation, and they are common to both bacteria and eukaryotes. The problem for fermenters, of course, is all that energy left in the chemical bonds of lactate and ethanol, energy that cannot be used to do work because there is no electron acceptor powerful enough. Organisms can also use compounds outside the metabolic pathways as electron acceptors, in so-called anaerobic respiration. For example, nitrate (NO,-) is a common electron acceptor for anaerobes, in which it is reduced to nitrogen gas (N2). Similarly, sulfate (SO4-2) can receive electrons, producing elemental sulfur (S), hydrogen sulfide (H2S, the "rotten egg" smell), or other reduced sulfur compounds. Anaerobic respiration thus results in a net oxidation of the glucose, but in the absence of oxygen.

Alternative electron acceptors have their own redox potentials (Table 6.2), and there is no reason in principle why the electron acceptor of one organism cannot,

Fermentation

Glucose

Respiration

Glucose electrons

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