Carbon, Nitrogen, Oxygen
Carbon, nitrogen and oxygen are the main components of all proteins and other biomolecules and are, as such, essential bioelements.
In the form of "inorganic" ions like carbonate (CO32-), bicarbonate (HCO3-) or ammonium (NH4+), they play an important role in the stabilization of the pH-levels of blood.
All oxygen in the earth's atmosphere is the product of photosynthesis in plants and thus has a biological source. It is released in photosystem II in plant cells.
Nitrogen atoms are often used to coordinate metal atoms.
Along with the imidazole residue of the amino acid histidine, porphyrin (e.g. in haem complexes), chlorin (e.g. in chlorophyll) and corrin (e.g. in cobalamines) ring systems play a very important role.