1. Carbon (C): The backbone of all organic molecules, including proteins, carbohydrates, lipids, and nucleic acids.

2. Hydrogen (H): Found in water, carbohydrates, lipids, and proteins.

3. Oxygen (O): Essential for respiration and forms part of water, carbohydrates, lipids, and proteins.

4. Nitrogen (N): Key component of proteins and nucleic acids (DNA and RNA).

These four elements make up the vast majority of the mass of living organisms.