The Chemistry of Life

All living organisms are built from chemical compounds, grouped into two classes.

Inorganic compounds are usually small and (with a few exceptions) contain no carbon. The most important is water, which makes up roughly 60-70% of a living cell. Water is an excellent solvent, transports dissolved substances, and helps regulate temperature. Mineral salts (such as calcium, iron and iodine) are also inorganic.

Organic compounds always contain carbon bonded to hydrogen, and are built by living things. The four main groups are:

• Carbohydrates (e.g. glucose, starch) - the main energy source; made of carbon, hydrogen and oxygen in the ratio $1:2:1$.
• Lipids (fats and oils) - long-term energy storage and insulation.
• Proteins - built from amino acid monomers; used for growth, repair and as enzymes.
• Nucleic acids (DNA and RNA) - store and carry genetic information.