Metabolism is the means by which living things change molecules, including both anabolic reactions (those that build molecules) and catabolic reactions (those that break down molecules).
Metabolism includes all processes in the cell that change molecules into other types of molecules. These processes build molecules or break molecules down. Metabolism is the process by which cells, and therefore organisms, obtain energy from food. Additionally, metabolism is the process by which organisms build the molecules required to carry out the processes of life and remove nitrogenous wastes produced during metabolic processes.
Metabolic reactions are the chemical processes occurring in living organisms that are necessary to maintain life. These reactions rely on catalysts, such as metabolic enzymes, which are chemical agents that speed up reactions without being consumed during the process. Metabolic enzymes, as the name suggests, aid in metabolism and also detoxification of substances in the body. This acceleration of chemical reactions by a catalyst is called catalysis. Each catalyst is specific to the reaction it catalyzes; so each catalyst drives only a single process. For example, hydrolases break down materials by the addition of water, transaminases are involved in the addition or removal of amino acids, and lactate dehydrogenase converts lactate to pyruvic acid.
There are two types of metabolic reactions: anabolic and catabolic. An anabolic reaction builds large molecules from smaller molecules. In anabolic reactions, energy is added to the reactants (the substances that undergo changes during a reaction) in order to produce the products (the substances produced during a reaction). In the simplest terms, an anabolic reaction is one in which energy is added to reactants to form the products.
An example of an anabolic reaction is synthesis (the reaction of simple materials with each other to produce chemical compounds), such as the synthesis of carbohydrates in plants and the synthesis of proteins in animals.
Catabolic reactions are the opposite of anabolic reactions. A catabolic reaction breaks down large molecules into smaller molecules, releasing energy in the process. In simple terms, a catabolic reaction is one in which energy is released as reactants break down into products.
Some examples of catabolic reactions include cellular respiration, fermentation, and glycolysis. Cellular respiration is the process by which biochemical energy from nutrients is converted into adenosine triphosphate (ATP) and waste products. Fermentation is the process by which one substance breaks down into simpler substances. Yeast and bacteria are usually involved in the fermentation process. The breakdown of glucose by enzymes is completed through a process known as glycolysis. In addition to releasing energy, glycolysis produces pyruvic acid.