Biochemistry, study of the substances found in living organisms, and of the chemical reactions underlying life processes. This science is a branch of both chemistry and biology; the prefix bio- comes from bios, the Greek word for "life." The chief goal of biochemistry is to understand the structure and behavior of biomolecules. These are the carbon-containing compounds that make up the various parts of the living cell and carry out the chemical reactions that enable it to grow, maintain and reproduce itself, and use and store energy.
A vast array of biomolecules is present in the cell. The structure of each biomolecule determines in what chemical reactions it is able to participate, and hence what role it plays in the cell's life processes. Among the most important classes of biomolecules are nucleic acids, proteins, carbohydrates, and lipids.
Nucleic acids are responsible for storing and transferring genetic information. They are enormous molecules made up of long strands of subunits, called bases, that are arranged in a precise sequence. These are "read" by other components of the cell and used as a guide in making proteins.
Proteins (see Protein) are large molecules built up of small subunits called amino acids. Using only 20 different amino acids, a cell constructs thousands of different proteins, each of which has a highly specialized role in the cell. The proteins of greatest interest to biochemists are the enzymes (see Enzyme), which are the "worker" molecules of the cell. These enzymes serve as promoters, or catalysts, of chemical reactions.
Carbohydrates (see Carbohydrate) are the basic fuel molecules of the cell. They contain carbon, hydrogen, and oxygen in approximately equal amounts. Green plants and some bacteria use a process known as photosynthesis to make simple carbohydrates (sugars) from carbon dioxide, water, and sunlight. Animals, however, obtain their carbohydrates from foods. Once a cell possesses carbohydrates, it may break them down to yield chemical energy or use them as raw material to produce other biomolecules.
Lipids are fatty substances that play a variety of roles in the cell. Some are held in storage for use as high-energy fuel; others serve as essential components of the cell membrane.
Biomolecules of many other types are also found in cells. These compounds perform such diverse duties as transporting energy from one location in the cell to another, harnessing the energy of sunlight to drive chemical reactions, and serving as helper molecules (cofactors) for enzyme action. All these biomolecules, and the cell itself, are in a state of constant change. In fact, a cell cannot maintain its health unless it is continually forming and breaking down proteins, carbohydrates, and lipids; repairing damaged nucleic acids; and using and storing energy. These active, energy-linked processes of change are collectively called metabolism. One major aim of biochemistry is to understand metabolism well enough to predict and control changes that occur in cells. Biochemical studies have yielded such benefits as treatments for many metabolic diseases, antibiotics to combat bacteria, and methods to boost industrial and agricultural productivity. These advances have been augmented in recent years by the use of genetic engineering techniques.