Of the 100 plus known elements, two to three dozen can be found in most organisms. Carbon is in every organic compound. Carbon's unique ability to form four stable bonds means it can form more molecular structures than any other atom. Hydrogen and oxygen, bound as water, play an essential role in living cells. Water is the primary solvent in which organic molecules are sustained in cells.
Most biochemical reactions occur in the watery, or aqueous, phase.
Hydrogen and oxygen also play vital roles in producing energy in most cells. Every protein has nitrogen in its molecular backbone. Phosphorus plays a key role in the energy and information systems of the cell.