O

Fumarate

▲ FIGURE 2-25 Conversion of succinate to fumarate. In this oxidation reaction, which occurs in mitochondria as part of the citric acid cycle, succinate loses two electrons and two protons. These are transferred to FAD, reducing it to FADH2.

ric) ions, a reaction that occurs as part of the process by which carbohydrates are degraded in mitochondria. Each oxygen atom receives two electrons, one from each of two Fe2 +

ions:

Thus Fe2+ is oxidized, and O2 is reduced. Such reactions in which one molecule is reduced and another oxidized often are referred to as redox reactions. Oxygen is an electron acceptor in many redox reactions in aerobic cells.

Many biologically important oxidation and reduction reactions involve the removal or the addition of hydrogen atoms (protons plus electrons) rather than the transfer of isolated electrons on their own. The oxidation of succinate to fumarate, which also occurs in mitochondria, is an example (Figure 2-25). Protons are soluble in aqueous solutions (as H3O+), but electrons are not and must be transferred di-

Oxidized: NAD4

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