Electron Transport and Oxidative Phosphorylation

Built into the foldings, or cristae, of the inner mitochondrial membrane are a series of molecules that serve as an electron-transport system during aerobic respiration. This electron-transport chain of molecules consists of a protein containing flavin mononucleotide (abbreviated FMN and derived from the vitamin riboflavin), coenzyme Q, and a group of iron-containing pigments called cytochromes. The last of these cytochromes is cytochrome a3, which donates electrons to oxygen in the final oxidation-reduction reaction (as will be described shortly). These molecules of the electron-transport system are fixed in position within the inner mitochondrial membrane in such a way that they can pick up electrons from NADH and FADH2 and transport them in a definite sequence and direction.

H

O

HC

-C-

H

Acetyl CoA (C2)

COOH I

HO-C-COOH I

COOH Citric acid (C6)

COOH I

HO-C-COOH I

COOH Citric acid (C6)

COOH Oxaloacetic acid (C4) 8

COOH I

COOH Malic acid (C4)

COOH Oxaloacetic acid (C4) 8

COOH I

C COOH

h2o-

H COOH

HOOC H

Fumaric acid (C4)

COOH Succinic acid (C4)

NADH

COOH I

COOH Isocitric acid (C6)

NADH

H COOH

HOOC H

Fumaric acid (C4)

Ketoglutaric Acid Kreb Cycle

COOH Succinic acid (C4)

Figure 5.8 The complete Krebs cycle. Notice that, for each "turn" of the cycle, one ATP, three NADH, and one FADH2 are produced.

COOH a-Ketoglutaric acid (C5)

Figure 5.8 The complete Krebs cycle. Notice that, for each "turn" of the cycle, one ATP, three NADH, and one FADH2 are produced.

Cell Respiration and Metabolism 111

Free radicals are molecules with unpaired electrons, in contrast to molecules that are not free radicals because they have two electrons per orbital. A superoxide radical is an oxygen molecule with an extra, unpaired electron. These can be generated in mitochondria through the accidental leakage of electrons from the electron-transport system. Superoxide radicals have some known, physiological functions; for example, they are produced in phagocytic white blood cells where they are needed for the destruction of bacteria. However, the production of free radicals and other molecules classified as reactive oxygen species (including the superoxide, hydroxyl, and nitric oxide free radicals, and hydrogen peroxide) have been implicated in many disease processes, including atherosclerosis (hardening of the arteries—see chapter 13). Accordingly, reactive oxygen species have been described as exerting an oxida-tive stress on the body. Antioxidants are molecules that scavenge free radicals and protect the body from reactive oxygen species. Antioxidants produced in the body cells include the enzyme superoxide dismutase, which converts superoxide radicals to hydrogen peroxide, and a tripeptide called glutathione, which functions as the major cellular scavenger of free radicals. Those ingested in the diet include ascorbic acid (vitamin C), a-tocopherol (vitamin E), and many other molecules found in different fruits and vegetables.

In aerobic respiration, NADH and FADH2 become oxidized by transferring their pairs of electrons to the electron-transport system of the cristae. It should be noted that the protons (H+) are not transported together with the electrons; their fate will be described a little later. The oxidized forms of NAD and FAD are thus regenerated and can continue to "shuttle" electrons from the Krebs cycle to the electron-transport chain. The first molecule of the electron-transport chain in turn becomes reduced when it accepts the electron pair from NADH. When the cytochromes receive a pair of electrons, two ferric ions (Fe3+) become reduced to two ferrous ions (Fe2+).

The electron-transport chain thus acts as an oxidizing agent for NAD and FAD. Each element in the chain, however, also functions as a reducing agent; one reduced cytochrome transfers its electron pair to the next cytochrome in the chain (fig. 5.9). In this way, the iron ions in each cytochrome alternately become reduced (from Fe3+ to Fe2+) and oxidized (from Fe2+ to Fe3+). This is an exergonic process, and the energy derived is used to phos-phorylate ADP to ATP. The production of ATP in this manner is thus appropriately termed oxidative phosphorylation.

The coupling is not 100% efficient between the energy released by electron transport (the "oxidative" part of oxidative phosphorylation) and the energy incorporated into the chemical bonds of ATP (the "phosphorylation" part of the term). This difference in energy escapes the body as heat. Metabolic heat production is needed to maintain our internal body temperature.

NADH

NADH

Cytochrome a3

Oxidative Phorylation

Fe Fe

, Cytochrome a Ci and c

■ Figure 5.9 Electron transport and oxidative phosphorylation. Each element in the electron-transport chain alternately becomes reduced and oxidized as it transports electrons to the next member of the chain. This process provides energy for the formation of ATP. At the end of the electron-transport chain, the electrons are donated to oxygen, which becomes reduced (by the addition of two hydrogen atoms) to water.

Oxidized Fe'

CoQ Reduced

Fe Fe

, Cytochrome a Ci and c

Cytochrome a3

■ Figure 5.9 Electron transport and oxidative phosphorylation. Each element in the electron-transport chain alternately becomes reduced and oxidized as it transports electrons to the next member of the chain. This process provides energy for the formation of ATP. At the end of the electron-transport chain, the electrons are donated to oxygen, which becomes reduced (by the addition of two hydrogen atoms) to water.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • rian
    Which molecules continue on the electron transport chain?
    8 years ago
  • dino
    What is the white blood cell electron transport chain?
    8 years ago

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