Metabolism | Research & Encyclopedia Articles

Metabolism

The term metabolism refers to all the chemical changes that take place in the body's tissues when the cells are producing both energy and essential new organic materials. While these metabolic activities are many and varied, most of them fall into two broad categories: anabolic processes and catabolic ones. These two quite different processes take place constantly and simultaneously.

Anabolism is the cell's synthesizing or building-up phase. Through anabolic reactions, simple substances--usually the molecules of glucose, fatty acids, and amino acids that have been derived from foodstuffs--are combined in various ways to form more complex substances, such as the new cellular material needed for growth and tissue maintenance. By combining amino acids, for instance, the cells can form structural proteins and use them to repair or replace worn-out tissues. The cells can also form functional proteins, such as enzymes, antibodies, and hormones.

Catabolism, the cell's degradative or breaking-down phase, is almost exactly the reverse. Through catabolic reactions, complex compounds--proteins, fats, and carbohydrates--are broken down primarily to produce the energy needed for all metabolic activities. Not all the energy is used up at once: as complex nutrient molecules are broken down and oxidized--or burned--as fuel, only about 60 percent of the energy released is used for immediate needs. The rest is stored in the chemical bonds that link certain atoms, most notably those of the phosphate adenosine triphosphate (ATP). When the body needs energy, enzymes break these chemical bonds and release the stored energy for use by the cells.

Enzymes help regulate most metabolic activities. For example, hormones secreted by special cells in the pancreas help determine whether metabolic reactions will be largely anabolic, as is usually the case soon after a meal is eaten, or catabolic, generally during periods when additional energy is needed. And thyroxine, a hormone secreted by the thyroid gland, is one of several hormones that help determine the rate at which these activities will occur--the metabolic rate or, roughly, the rate at which the body uses up energy in the course of its various metabolic reactions.

The metabolic rate is influenced by a number of factors, such as the individual's age, sex, level of activity, state of health, and, of course, the amount of the hormone thyroxine he or she secretes. Probably the most influential factors, though, are the temperature of the surrounding environment and the calorigenic effects of the foods most recently eaten. Because an individual's metabolic rate can give physicians a great deal of important information, it often needs to be measured. And since almost all the energy used by the body is eventually converted to heat, the metabolic rate is usually calculated by measuring the amount of heat loss an individual displays during resting (or basal) conditions. The person's basal metabolic rate (BMR) is then judged to be normal or abnormal by comparing it to standardized rates--that is, rates that reflect the average BMRs of healthy individuals of various ages taken under identical and standardized conditions.

Back in the 1830s, the German physiologist Theodor Schwann coined the word metabolism for the chemical changes that take place in living tissues. Fittingly enough, it was during Schwann's lifetime that many important metabolic concepts were first formulated. In 1828, for instance, Friedrich Wöhler discovered that urea, an organic product present in urine, could be manufactured in the laboratory from inorganic chemicals, a discovery that stimulated interest in studying the body's own chemicals. Later, Wöhler also showed that a chemical taken by mouth somehow had combined with other chemicals when it appeared in the urine, one of the first studies that demonstrated chemical changes definitely could--and did--occur inside the body.

In 1842, Justus von Liebig showed that animal heat stemmed almost entirely from the oxidation of recently consumed foods. Liebig, who also believed that not all foods provided equal amounts of heat, pioneered a number of studies aimed at determining the caloric values of different foods. In the 1850s, Claude Bernard, the noted French physiologist, made an important contribution to the study of metabolism. For one thing, Bernard discovered a starch-like substance in the liver that he named glycogen. He went on to show that glycogen was composed of simple blood sugars stored in the liver and could be broken down into glucose when needed. This process, Bernard argued, indicated that the body's various mechanisms appeared to work together, in an integrated fashion, to maintain a constant and well-balanced inner environment.

In the 1890s, German physiologist Max Rubner (1854-1932) showed that the energy used and released by animals followed the same basic chemical principles that inanimate systems were already known to follow, and Eduard Buchner's experiments with yeast proved that metabolic processes in organisms resulted strictly from chemical reactions and were not energized by a "life force." In the years that followed, numerous scientists--among them Hermann von Helmholtz, German physiologist Eduard Pflüger (1829-1910), and Arthur Harden--were able to establish most of the important metabolic parameters.

During the twentieth century, therefore, many metabolic studies began to center around the concept of intermediary metabolism. The concept, introduced in the 1860s by Karl von Voit, held that chemical reactions were often much longer and more complicated than had previously been realized, and that it was vital for chemists to search for the intermediate steps that helped a chemical reaction get from one stage to another. Among those scientists working on intermediary metabolism were two German physiologists, Gustav Embden (1874-1933) and Otto Meyerhof, who, in 1933, were able to work out the complicated sequences of chemical reactions that are involved in the breakdown of glycogen. Four years later, Hans Krebs introduced a model of what he called the citric acid cycle, a central feature of one of the body's major catabolic pathways. And in the late 1940s and 1950s, German biochemist Fritz Lipmann (1899-) made a number of valuable contributions to intermediary metabolism through his discovery of coenzyme A and by his emphasis on the vital role played by phosphates in various metabolic pathways.

One focus of recent research is the role of metabolism in the aging process. Studies have found that reducing calorie intake substantially, but not to the point of malnutrition, extends the life span of animals ranging from spiders and fleas to mice and monkeys. The increased longevity goes beyond the effects of simply avoiding diseases that are linked to obesity. Scientists are now trying to better understand this observation.