Chemical reactions in which electrons are transferred from one atom or molecule to another are referred to as oxidation–reduction reactions (“redox reactions” for short). Oxidation is the loss of electrons, and reduction is the gain of electrons. The loss and gain of electrons always occur together in a coupled oxidation–reduction reaction: Electrons are transferred from one molecule to another so that one molecule loses electrons and one molecule gains those electrons. The molecule that loses electrons is oxidized and the molecule that gains electrons is reduced.

We can illustrate oxidation–reduction reactions by looking at the role played by electron carriers in cellular respiration. Two important electron carriers are nicotinamide adenine dinucleotide and flavin adenine dinucleotide. These electron carriers exist in two forms—an oxidized form (NAD+ and FAD) and a reduced form (NADH and FADH₂). When fuel molecules such as glucose are catabolized, some of the steps are oxidation reactions. These oxidation reactions are coupled with the reduction of electron carrier molecules. The reduction reactions in which electrons are transferred to electron carriers can be written as:

NAD+ + 2e– + H⁺ → NADH

FAD + 2e– + 2H+ → FADH₂

Here, NAD+ and FAD accept electrons, resulting in the production of their reduced forms, NADH and FADH₂. Note that in redox reactions involving organic molecules such as NAD+ or FAD, the gain (or loss) of electrons is often accompanied by the gain (or loss) of protons (H+). As a result, reduced molecules can easily be recognized by an increase in C–H bonds, and the corresponding oxidized molecules by a decrease in C–H bonds.

In their reduced forms, NADH and FADH₂ can donate electrons. The oxidation of NADH and FADH₂ allows electrons (and energy) to be transferred to the electron transport chain:

NADH → NAD+ + 2e– + H+

FADH₂ → FAD + 2e– + 2H+

In addition, these reactions produce NAD+ and FAD, which can then accept electrons from the breakdown of fuel molecules. In this way, electron carriers act as shuttles, transferring electrons derived from the oxidation of fuel molecules such as glucose to the electron transport chain.

Cellular respiration can itself be understood as a redox reaction, even though it consists of many steps. In aerobic respiration, glucose is oxidized, releasing carbon dioxide, and at the same time oxygen is reduced, forming water:

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Let’s first consider the oxidation reaction (Fig. 7.2a). In glucose, there are many C–C and C–H covalent bonds, in which electrons are shared about equally between the two atoms. By contrast, in carbon dioxide, electrons are not shared equally. The oxygen atom is more electronegative than the carbon atom, so the electrons are more likely to be found near the oxygen atom. As a result, carbon has partially lost electrons to oxygen and is oxidized.

Let’s now consider the reduction reaction (Fig. 7.2b). In oxygen gas, electrons are shared equally between two oxygen atoms. In water, the electrons that are shared between hydrogen and oxygen are more likely to be found near oxygen because oxygen is more electronegative than hydrogen. As a result, oxygen has partially gained electrons and is reduced.

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Glucose is a good electron donor because its oxidation to carbon dioxide releases a lot of energy, whereas oxygen is a good electron acceptor because it has a high affinity for electrons. In cellular respiration, glucose is not oxidized all at once to carbon dioxide. Instead, it is oxidized slowly in a series of reactions, some of which are redox reactions. In this way, energy is released in a controlled manner. Some of this energy can be used to synthesize ATP directly and some of it is stored temporarily in reduced electron carriers and then used to generate ATP by oxidative phosphorylation.

Quick Check 1 For each of the following pairs of molecules, indicate which member of the pair is reduced and which is oxidized, and which has more chemical energy and which has less chemical energy: NAD+/NADH; FAD/FADH₂; CO₂/C₆H₁₂O₆.