Lipid Metabolism

When glucose is going to be converted into fat, glycolysis occurs and pyruvic acid is converted into acetyl CoA. Some of the glycolytic intermediates—phosphoglyceraldehyde and dihy-

Cell Respiration and Metabolism 115

droxyacetone phosphate—do not complete their conversion to pyruvic acid, however, and acetyl CoA does not enter a Krebs cycle. The acetic acid subunits of these acetyl CoA molecules can instead be used to produce a variety of lipids, including cholesterol (used in the synthesis of bile salts and steroid hormones), ketone bodies, and fatty acids (fig. 5.12). Acetyl CoA may thus be considered a branch point from which a number of different possible metabolic pathways may progress.

In the formation of fatty acids, a number of acetic acid (two-carbon) subunits are joined together to form the fatty acid chain. Six acetyl CoA molecules, for example, will produce a fatty acid that is twelve carbons long. When three of these fatty acids condense with one glycerol (derived from phosphoglycer-aldehyde), a triglyceride (also called triacylglycerol) molecule is produced. The formation of fat, or lipogenesis, occurs primarily in adipose tissue and in the liver when the concentration of blood glucose is elevated following a meal.

Fat represents the major form of energy storage in the body. One gram of fat contains 9 kilocalories of energy, compared to 4 kilocalories for a gram of carbohydrates or protein. In a nonobese 70-kilogram (155-pound) man, 80% to 85% of the body's energy is stored as fat, which amounts to about 140,000 kilocalories. Stored glycogen, by contrast, accounts for less than 2,000 kilocalories, most of which (about 350 g) is stored in skeletal muscles and is available for use only by the muscles. The liver contains between 80 and 90 grams of glycogen, which can be converted to glucose and used by other organs. Protein accounts for 15% to 20% of the stored calories in the body, but protein is usually not used extensively as an energy source because that would involve the loss of muscle mass.

The ingestion of excessive calories increases fat production. The rise in blood glucose that follows carbohydrate-rich meals stimulates insulin secretion, and this hormone, in turn, promotes the entry of blood glucose into adipose cells. Increased availability of glucose within adipose cells, under conditions of high insulin secretion, promotes the conversion of glucose to fat (see figs. 5.11 and 5.12). The lowering of insulin secretion, conversely, promotes the breakdown of fat. This is exploited for weight reduction by low-carbohydrate diets.

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Essentials of Human Physiology

Essentials of Human Physiology

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