Metabolism is the set of chemical processes which occur in cells of living organisms to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories: 1) catabolism, which breaks down organic matter to harvest energy in cellular respiration; 2) anabolism, which uses energy to construct components of cells such as proteins and nucleic acids.
Metabolism comprises all the chemical reactions by which molecules taken into an organism are broken down to produce energy and by which energy is used to build up complex molecules. All metabolic reactions fall into one of two general categories: catabolic and anabolic reactions, or the processes of breaking down and building up, respectively. An example of metabolism from daily life takes place in the process of taking in and digesting nutrients.
Catabolism and anabolism share an important common sequence of reactions known collectively as the citric acid cycle, the tricarboxylic acid cycle, or the Krebs cycle. Named after the German biochemist Sir Hans Adolf Krebs (1900-1981). The citric acid cycle is a series of chemical reactions in which tissues use carbohydrates, fats, and proteins to produce energy; it is part of a larger series of enzymatic reactions known as oxidative phosphorylation. In the latter reaction, glucose is broken down to release energy, which is stored in the form of ATP—a catabolic sequence.
The chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, by a sequence of enzymes. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy and will not occur by themselves, by coupling them to spontaneous reactions that release energy. As enzymes act as catalysts they allow these reactions to proceed quickly and efficiently. Enzymes also allow the regulation of metabolic pathways in response to changes in the cell's environment or signals from other cells.
Metabolism Explained by Professor (Video)