Metabolism is the name for the sum of chemical reactions taking place in the human body. Because some reactions break large molecules into smaller pieces, while other reactions build up larger molecules from constituents, metabolism subdivides into two categories: catabolic metabolism and anabolic metabolism. Anabolic metabolism refers to reactions that build up molecules.
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The human body, to grow and maintain itself, has to take in nutrient molecules from the outside environment. This serves two purposes. First, these nutrient molecules are broken down to provide energy that cells use to engage in movement, growth and various chemical reactions. In addition, however, the products of nutrient breakdown are small molecules that cells can use as building blocks of larger molecules -- literally forming new material from the breakdown products of nutrients.
The function of anabolic metabolism is, stated simply, to build structural and functional molecules. This can take many different forms. For instance, explains Dr. Lauralee Sherwood in her book "Human Physiology," body cells are composed primarily of protein. To build new structural and functional proteins, cells need amino acids, which are the building blocks of proteins. Cells obtain these building blocks by absorbing them from the bloodstream, where they end up after the digestive tract has broken down an ingested protein into its constituent amino acids.
Anabolism is more than simply cell-building, however. The human body stores nutrient molecules for use during periods of fasting or scarce food availability. These storage forms of nutrients are made up of either fat or sugar. Sugar storage, in particular, is dependent upon anabolic reactions, note Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." The liver and muscles assemble glycogen, a long chain of glucose molecules, from ingested glucose extracted from sugars and carbohydrates.
While catabolism, or reactions that break down molecules, generally yields energy, anabolism generally requires energy. This, explain Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry," is one of the reasons that body cells require a constant supply of nutrients that they can burn for energy. Even something as simple as producing glycogen, the storage form of sugar, requires large amounts of energy that has to come from breakdown of nutrient molecules.
Catabolic reactions power anabolic reactions in more than one way. In addition to literally providing the energy required to run anabolic reactions, catabolic reactions also supply another necessary ingredient -- electrons. In chemistry, electrons are the "glue" that holds atoms together through chemical bonds. To make new bonds, explain Drs. Campbell and Farrell, as the body needs to do when it builds large molecules from smaller ones, cells require a source of electrons. Catabolic, or breakdown, reactions supply these electrons, and keep anabolism running.