Biochemistry is a field of science which emphasizes study of the molecular level of life. The reactions, chemical substances and processes that occur in plants, animals, and microorganisms are all targets for study. Specifically, biochemistry involves the quantitative determination and structural analysis of the organic compounds that comprise the basic constituents of cells (proteins, carbohydrates, and lipids) and of those that play a key role in chemical reactions vital to life (nucleic acids, vitamins, and hormones). Biochemistry entails the study of all the complexly interrelated chemical changes that occur within the cell—e.g., those relating to protein synthesis, the conversion of food to energy, and the transmission of hereditary characteristics. Both the cell's degradation of substances that release energy and its buildup of complex molecules that store energy or act as substrates or catalysts for biological chemical reactions are studied in detail by biochemists. Biochemists also study the regulatory mechanisms within the body that govern these and other processes.
Biochemistry lies in the border area between the biological and physical sciences. Accordingly, it makes use of many of the techniques common to physiology and those integral to analytical, organic, and physical chemistry. The field of biochemistry has become so large that many subspecialities are recognized such as proteomics, bioinformatics and molecular biology. Taken as a whole, modern biochemistry has outgrown its earlier status of an applied science and has acquired a place among the pure, or theoretical, sciences.