In the early nineteenth century, as chemistry became recognized as a scientific discipline, a distinction was made between inorganic and organic chemistry. Organic compounds (those containing carbon and hydrogen) were thought to be made only in living systems. However, in 1828, Friedrich Wöhler in Germany heated an inorganic compound, ammonium carbamate, and made an organic one, urea, found naturally in animal urine. Wöhler's experiment showed that the chemistries of the living and nonliving worlds are continuous:
At the end of the nineteenth century, a parallel controversy arose as organic chemists debated whether an intact, living cell was needed to carry out biochemical reactions. Hans Büchner in Germany reproduced the synthesis of ethanol with a cell‐free extract of brewer's yeast, showing that reactions of living systems can be reproduced in vitro (literally, in glass), that is, away from a living system. Reactions in living cells occur because they are catalyzed by enzymes — the very word enzyme is derived from the Greek word for yeast, zymos.
Biochemistry became a distinct science in the early twentieth century. In the United States, it arose from the merger of physiological chemistry and agricultural chemistry. Contemporary biochemistry has three main branches:
- Metabolism is the study of the conversion of biological molecules, especially small molecules, from one to another—for example, the conversion of sugar into carbon dioxide and water, or the conversion of fats into cholesterol. Metabolic biochemists are particularly interested in the individual enzyme‐catalyzed steps of an overall sequence of reactions (called a pathway) that leads from one substance to another.
- Structural Biochemistry is the study of how molecules in living cells work chemically. For example, structural biochemists try to determine how the three‐dimensional structure of an enzyme contributes to its ability to catalyze a single metabolic reaction.
- Molecular Genetics is concerned with the expression of genetic information and the way in which this information contributes to the regulation of cellular functions.
These distinctions are somewhat artificial, as contemporary biochemistry is intimately connected with other branches of biology and chemistry, especially organic and physical chemistry, physiology, microbiology, genetics, and cell biology.