Metabolism represents all of the chemical activity in a cell, including reactions that build molecules (anabolism) and those that break molecules down (catabolism). Anabolic reactions require energy, whereas catabolic reactions provide it. Thus, metabolism describes how cells transform energy through a variety of chemical reactions, which are often made more efficient with the help of enzymes.
Metabolism is the management of energy in cells and provides three key functions:
To produce energy, macromolecules from food must be broken down into smaller molecules—through a catabolic pathway. This, in turn, provides energy to construct larger molecules from smaller building blocks—through an anabolic pathway. In other words, the potential energy in food—comprised of the chemical energy stored in the bonds between atoms—can be converted into kinetic energy that can be used for cellular reactions. Enzymes are essential molecular tools in metabolic pathways, as they greatly speed up many chemical reactions by reducing the amount of required energy.
Catabolism is the breakdown of macromolecules for any purpose. This includes the degradation of food molecules into smaller molecules that can be used as building blocks, a process that releases energy that is transferred to ATP. Protein digestion is an example of catabolism. For the body to utilize the protein we eat, it must be broken down from large protein molecules into smaller polypeptides, and then into individual amino acids.
Excess amino acids that are broken down for removal release, nitrogen-containing ammonia. This ammonia is toxic at high levels, and thus must be converted into a safer form that organisms can handle and dispose of. In humans, ammonia is combined with carbon dioxide and converted into urea before being eliminated from the body in the form of urine. Other organisms use different types of nitrogenous waste, such as uric acid in birds and reptiles. Compared to urea, uric acid requires much less water to be released from the body and therefore has adaptive value certain conditions.
Anabolic pathways construct larger molecules from smaller building block molecules, using energy (in the form of ATP). For example, protein anabolism involves stringing together amino acids to form polypeptides. The synthesized polypeptides then fold into three-dimensional protein structures. Excess amino acids can be used to make triglycerides and stored as fat, or converted into glucose and used to make ATP. Thus, both the anabolic and catabolic pathways are necessary for maintaining energy balance.
Another, less well-known example of anabolism is the production of condensed tannins in seeds. Seeds that are eaten by animals can be protected from digestion if their seed coats contain dark-colored, condensed tannins. Plants produce tannins by linking anthocyanin molecules, using the same dehydration reactions used to build polypeptides.
