Metabolism (chemistry)

In digestion, fats are hydrolyzed or decomposed into their component glycerol and fatty acids. These are then synthesized to neutral fats, cholesterol compounds, and phospholipids—fats, chemically united with phosphorus, that circulate in the blood. Fat may be synthesized into body structure or stored in the tissues for withdrawal when needed. Like glucose, it is then catabolized to carbon substances that are broken down into carbon dioxide and water.

Vitamins are accessory organic compounds essential to enhancement of the metabolism of amino acids, carbohydrates, and fats in living organisms. Some organisms, notably green plants, synthesize vitamins, often in quantities greater than the organisms require. With few exceptions, animals cannot synthesize these substances and must obtain them in their food. See Nutrition, Human; Vitamin.

If an enzyme is lacking because of some hereditary defect, the chemical transformation in which it would participate is blocked. As a result, cell products fail to be synthesized or catabolized, too much of a metabolic product accumulates, causing injury to tissues, or intracellular materials fail to cross cell membranes.

Although the effects of some metabolic errors are manifested in early infancy, others may appear only in adulthood. Some inborn errors may be fatal, some may have no apparent harmful effects, and some may persist. A result of error in amino acid metabolism is phenylketonuria (PKU). This occurs in infants when metabolism of the amino acid phenylalanine is blocked; the accumulated metabolic products may cause brain damage. In carbohydrate metabolism, one error results in galactosemia, in which the enzyme required to convert galactose to glucose is absent. The consequent inability to metabolize milk sugar results in the accumulation of galactose in the blood, sometimes with damage to the brain and liver and the development of cataracts and mental retardation. See also Birth Defects; Genetic Disorders.