In order to understand the impact of biochemistry on biology, one must understand the chemical elements of living matter and the complete structures of many biological compounds - amino acids, sugars, lipids, nucleotides, vitamins, and hormones - and their behavior during metabolic reactions. Essential understanding of biochemistry requires knowledge of the stoichiometry and mechanisms of a large number of reactions. In addition, an understanding of the basic thermodynamic principles is essential for learning how plants derive energy from sunlight (photosynthesis) and how animals derive energy from food (catabolism).

Living creatures on the earth are composed mainly of a very few elements, principally carbon, hydrogen, oxygen, and nitrogen (C, H, O, N), as shown in Figure 1.4. Life is not built on these four elements alone. Many other elements are necessary for terrestrial organisms (Table 1.1). A "second tier" of essential elements includes sulfur and phosphorus, which form covalent bonds, and the ions Na⁺, K⁺, Mg²⁺, Ca²⁺, and Cl^-.

Sulfur is an important constituent of proteins, and phosphorus plays essential roles in energy metabolism and the structure of nucleic acids. Beyond the first two tiers of elements (which correspond roughly to the most abundant elements of the first two rows of the periodic table), we come to those that play quantitatively minor, but often indispensable, roles. As Table 1.1 shows, most of these third- and fourth-tier elements are metals, and some serve as aids to catalysis of biochemical reactions.

See also: Polypeptides, Polysaccharides, Nucleic Acids, Amino Acids, Sugars, Nucleotides