The Chemistry of Life

A tree, a mushroom, a bacterium and a human being look nothing alike, yet underneath they are built from the same small set of ingredients, assembled by the same kinds of chemical reactions. The study of this shared chemistry is called biochemistry, and it starts with one element: carbon.

Why carbon?

A carbon atom can form four strong bonds with other atoms at the same time, including with other carbon atoms. That lets it build long chains, branching trees and closed rings, sometimes with millions of atoms. No other element is nearly as flexible. Add hydrogen, oxygen and nitrogen, plus a little phosphorus and sulphur, and you have almost everything a living thing is made of. Together, oxygen, carbon, hydrogen and nitrogen account for about 96 percent of your body's mass.

Life builds its large molecules in a clever way: by joining small units, like beads on a string. A few kinds of bead, arranged in different orders and lengths, make an enormous variety of molecules. There are four main families.

Carbohydrates: fuel and framework

Carbohydrates are sugars and the chains made from them. Glucose, a sugar with six carbon atoms, is the fuel your cells burn to release energy. Plants make glucose by photosynthesis, then link thousands of them together. Joined one way, they form starch, an energy store found in potatoes, rice and bread. Joined another way, they form cellulose, the tough fibre of plant cell walls and the most common organic molecule on the planet. You cannot digest cellulose, but it is the fibre that keeps your gut healthy.

Lipids: stores and walls

Lipids include fats, oils and waxes. They do not mix with water, and that is their great strength. Fats store energy in a compact form, holding about twice as much per gram as sugar, which is why animals use them for long-term reserves. A special kind of lipid, with a water-loving head and two oily tails, forms the membrane around every cell. In water these molecules line up tail to tail into a thin double layer that seals the cell's contents in and controls what passes through. Some hormones, such as testosterone and oestrogen, are lipids too.

Proteins: the workers

Proteins are chains of building blocks called amino acids. There are twenty kinds, and a protein may contain hundreds of them in a precise order. That order makes the chain fold into a particular three-dimensional shape, and the shape decides what the protein does. Proteins are the busiest molecules in your body. Enzymes speed up chemical reactions; haemoglobin carries oxygen in your blood; antibodies fight infection; keratin makes hair and nails; collagen holds skin and bones together; and the proteins actin and myosin make your muscles move.

Nucleic acids: the instructions

DNA and RNA are nucleic acids, long chains of units called nucleotides. The order of four kinds of nucleotide along a DNA strand spells out the recipe for every protein the organism can make. RNA copies those recipes and carries them to the cell's protein factories. DNA is also how the instructions pass from parents to children.

Why it matters

These four families explain nutrition, medicine and much of biology. A balanced diet supplies the carbohydrates, fats, proteins and other molecules your body cannot make for itself. Many medicines work by fitting into the shape of a particular protein. And the fact that every living thing uses the same molecules is powerful evidence that all life on Earth shares a common origin.