When discussing the basics of metabolism, it is important to first understand that metabolism is the building of large molecules from small molecules and the breaking down of large molecules into small molecules. Living things must build the molecules that carry out cellular functions through anabolic processes, which take in energy. Organisms get the energy they need to carry out the functions of life through catabolic processes, which release energy. This energy often takes the form of a molecule called adenosine triphosphate, or ATP. Adenosine triphosphate (ATP) is a nucleotide consisting of the sugar ribose, the base adenine, and three phosphate groups. It contains chemical energy in the bonds of its phosphate groups used to fuel cellular processes. When a phosphate ion is released from ATP, energy is released along with it. The molecule that remains is adenosine diphosphate, or ADP. The formation and breaking of the bonds in these molecules provides the energy that drives the processes of life.
Other molecules act as electron carriers, which harness the energy released in redox reactions (oxidation-reduction reactions that involve the transfer of charge from one molecule to another) to improve the energy efficiency of metabolic reactions.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 one another to produce chemical compounds), such as the synthesis of proteins in humans.