Field of science concerned with chemical substances and processes that occur in plants, animals, and microorganisms. It involves the quantitative determination and structural analysis of the organic compounds that make up cells (proteins, carbohydrates, and lipids) and of those that play key roles in chemical reactions vital to life (e.g., nucleic acids, vitamins, and hormones). Biochemists study cells' many complex and interrelated chemical changes. Examples include the chemical reactions by which proteins and all their precursors are synthesized, food is converted to energy (see metabolism), hereditary characteristics are transmitted (see heredity), energy is stored and released, and all biological chemical reactions are catalyzed (see catalysis, enzyme). Biochemistry straddles the biological and physical sciences and uses many techniques common in medicine and physiology as well as those of organic, analytical, and physical chemistry.

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Physiological chemistry is the chemistry of the organs and tissues of the body and of the various physiological processes incident to life. Physiological chemistry is essentially the precursor to modern biochemistry. In the nineteenth century, physiological chemistry dealt primarily with extracellular chemistry, such as the chemistry of digestion and other body fluids. Modern biochemical methods have allowed a much broader study including the chemistry of proteins and nucleic acids.

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