Ure 411

Figure

A summary of ATP synthesis by oxidative phosphorylation.

to the electron transport chain, which is a series of enzyme complexes that carry and pass electrons along from one to another. These complexes dot the folds of the inner mitochondrial membranes (see chapter 3, p. 77), which, if stretched out, may be 45 times as long as the cell membrane in some cells. The electron transport chain passes each electron along, gradually lowering the electron's energy level and transferring that energy to ATP synthase, an enzyme complex that uses this energy to phosphorylate ADP to form ATP (fig. 4.11). These reactions, known as oxidation/reduction reactions, are described further in Appendix D, A Closer Look at Cellular Respiration, pp. 1035-1036.

Neither glycolysis nor the citric acid cycle uses oxygen directly although they are part of the aerobic metabolism of glucose. Instead, the final enzyme of the electron transport chain gives up a pair of electrons that combine with two hydrogen ions (provided by the hydrogen carriers) and an atom of oxygen to form a water molecule:

Thus, oxygen is the final electron "carrier." In the absence of oxygen, electrons cannot continue to pass through the electron transport chain, and aerobic respiration grinds to a halt.

Figure 4.12 summarizes the steps in glucose metabolism. More detailed descriptions of glycolysis and the citric acid cycle may be found in Appendix D, A Closer Look at Cellular Respiration, p. 1036.

High energy electrons (e ) and hydrogen ions (H+)

Pyruvic acid

High energy electrons (e ) and hydrogen ions (H+)

High energy electrons (e ) and hydrogen ions (H+)

Pyruvic acid

High energy electrons (e ) and hydrogen ions (H+)

High energy electrons (e ) and hydrogen ions (H+)

Oxaloacetic acid

Cytosol Mitochondrion

Oxaloacetic acid

Figure

An overview of aerobic respiration, including the net yield of ATP at each step per molecule of glucose.

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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