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

Human muscle cells that are working so strenuously that their production of pyruvic acid exceeds the oxygen supply begin to produce lactic acid. In this condition of "oxygen debt," the muscle cells are forced to utilize solely the anaerobic pathway, which provides fewer ATPs per glucose molecule than does aerobic respiration. The accumulation of lactic acid contributes to the feeling of muscle fatigue and cramps. Walking after cramping at the end of a race can make a runner feel better by hastening the depletion of lactic acid.

Carbohydrate Storage

Metabolic pathways are usually interconnected in ways that enable certain molecules to enter more than one pathway. For example, carbohydrate molecules from foods may enter catabolic pathways and be used to supply energy, or they may enter anabolic pathways and be stored or be converted to nonessential amino acids (fig. 4.13).

Excess glucose in cells may enter anabolic carbohydrate pathways and be linked into storage forms such as glycogen. Most cells can produce glycogen, but liver and muscle cells store the greatest amounts. Following a meal, when blood glucose concentration is relatively high, liver cells obtain glucose from the blood and synthesize glycogen. Between meals, when blood glucose concentration is lower, the reaction is reversed, and glucose is released into the blood. This mechanism ensures that cells throughout the body have a continual supply of glucose to support cellular respiration.

Glucose can also react to form fat molecules, which are later deposited in adipose tissues. This happens when a person takes in more carbohydrates than can be stored as glycogen or are required for normal activities. Because the body has an almost unlimited capacity to perform this type of anabolism, overeating, even if mostly carbohydrates, can result in becoming obese (overweight).

Although this section has dealt primarily with the metabolism of glucose, lipids and proteins can also be broken down to release energy for ATP synthesis. In all three cases, the final process is aerobic respiration, and the most common entry point is into the citric acid cycle as acetyl CoA (fig. 4.14). These pathways are described in detail in chapter 18 (pp. 741-743).

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