1.2 Elements of Life

A few elements build everything

Just six elements make up about 96% of living matter: carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S) — remember them as CHNOPS. Oxygen, carbon, hydrogen, and nitrogen alone account for most of your mass.

Carbon is the backbone

Carbon has 4 valence electrons, so it forms four covalent bonds. That lets it build long chains, branches, and rings — the skeletons of nearly every organic molecule. This bonding versatility is why life's molecules are so diverse.

Trace elements still matter

Some elements are needed only in tiny amounts — trace elements — but are still essential. Iron is needed to carry oxygen in hemoglobin; iodine for thyroid hormones. Phosphorus is central to nucleic acids, ATP, and phospholipids; nitrogen is found in proteins and nucleic acids; sulfur appears in some amino acids.

Key terms

CHNOPS

Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, Sulfur — the six main elements of life.

Valence electrons

Outer-shell electrons that determine bonding. Carbon has 4, so it forms 4 bonds.

Organic molecule

A carbon-based molecule, typically containing C–H bonds.

Trace element

An element needed in very small amounts but still essential (e.g., iron, iodine).

Role of phosphorus

Backbone of nucleic acids, the energy-carrying part of ATP, and the heads of phospholipids.

Try an AP-style question

Which cellular molecules would contain the ¹⁵N label?

1. Carbohydrates and lipids
2. Only phospholipids and ATP
3. Only nucleic acids
4. Proteins and nucleic acids