Biochemistry is the study of the substances occurring in living organisms and the reactions in which they are involved. Biochemistry is a science on the border between biology and organic chemistry. The main constituents of living matter are water, carbohydrates, lipids, and proteins. The total chemical activity of the organism is known as its metabolism. Plants use sunlight as an energy source to produce carbohydrates from carbon dioxide and water by photosynthesis. The carbohydrates are them stored as starch, and are used for structural purposes, as in the cellulose of plant cell walls, or oxidized through a series of chemical reactions including the citric acid cycle, the energy released being stored as ATP (adenosine triphosphate). In animals energy is stored mainly as lipids, which as well as forming fat deposits are components of all cell membranes.


Proteins have many functions, of which metabolic regulation is perhaps the most important. Enzymes, which control almost all biochemical reactions, and some hormones are possible. Plants synthesize proteins using simpler nitrogenous compounds from the soil. Animals obtain proteins from food and break them down by hydrolysis to amino acids. New proteins are made according to the pattern determined by the sequence of nucleic acids in the genes.

Many reactions occur in all cells and may be studied in simple systems. Methods used by biochemists and chemists are similar and include labeling with radioactive isotopes and separation techniques such as chromatography, used to analyze very small amounts of substances, and the high-speed centrifuge. Molecular strictures may be determined by X-ray diffraction.


Landmarks in biochemistry include the synthesis of urea by Wöhler (1828), the pioneering research of von Liebig, Pasteur, and Bernard, and more recently the elucidation of DNA by James Watson and Francis Crick (1953).