| | Metabolism is the complete set of chemical reactions that occurs in living cells. These processes are the basis of life, allowing cells to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolic reactions yield energy, an example being the breakdown of food in cellular respiration. Anabolic reactions, on the other hand, use this energy to construct components of cells such as proteins and nucleic acids.
The chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed to another by a sequence of enzymes. Enzymes are crucial to metabolism because they allow cells to drive desirable but thermodynamically unfavorable reactions by coupling them to favorable ones. Enzymes also allow the regulation of metabolic pathways in response to changes in the cell's environment or signals from other cells.
The metabolism of an organism determines which substances it will find nutritious and which it will find poisonous. For example, some prokaryotes use hydrogen sulfide as a nutrient, yet this gas is poisonous to animals. The speed of metabolism, the metabolic rate, also influences how much food an organism will require.
A striking feature of metabolism is the similarity of the basic metabolic pathways between even vastly different species. For example, the series of chemical steps in a pathway such as the citric acid cycle is universal among living cells as diverse as the unicellular bacteria Escherichia coli and huge multicellular organisms like elephants. This shared metabolic structure is most likely the result of the high efficiency of these pathways, and of their early appearance in evolutionary history.
|