There are over 100 elements known, but only six are seen in all living organisms.

These are carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S).


Structural Role

Functional Role

Carbon (C)

Molecular “backbone” (four bonds)

Multiple functional groups.

Hydrogen (H)

“Accessory” structural (one bond)

Transferring from one molecule to another or in solution as H+.

Nitrogen (N)

Forms specialized functional entities such as amines and ammonia molecules (three or four bonds)

Specialized bonds (amino acids/proteins), hydrogen bonding in DNA/RNA, and special functions.

Oxygen (O)

Specialized functional structures (two bonds)

Often develops partial charge involved in hydrogen bonding, component of high-energy phosphate bonds for biological energy storage.

Phosphate (P)

High energy and special molecular structures (four to six bonds)

Biological energy storage, essential component of DNA and RNA structure and function.

Sulfur (S)

Strong structural bonds (two bonds)

Reversible bonding of important structural biological molecules.


These six elements can bond with themselves (e.g., C—C and O—O) and with each other (C—H, O—H, and C—N).

Elements form functional groups, including H (partially charged and ionic), OH−, COOH, amine, P, S—S, COH, and C=O.

These functional groups serve structural and functional roles as well as specialized roles in very specific biological actions and reactions for functions in the human body.