Biomolecules

Every living thing, for all its complexity, is built from a handful of types of chemical molecule. These biomolecules (carbohydrates, lipids, proteins and nucleic acids) are the materials and machinery of life. Enzymes are the special proteins that make life's chemistry possible. Understanding them bridges chemistry and biology.

Two classes of biomolecule mainly handle energy and structure:

• Carbohydrates are the sugars and starches. Simple sugars like glucose are the body's quick fuel. Complex carbohydrates like starch (in plants) and glycogen (in animals) store energy, and cellulose builds plant cell walls.
• Lipids are the fats and oils. They store energy in a compact form, cushion and insulate the body, and, crucially, form the membranes that surround every cell.

Two further classes handle function and information:

• Proteins are built from smaller units called amino acids. These chains fold into specific shapes. Proteins do almost all the work of the cell: they build tissues (like muscle), transport substances, defend the body (antibodies) and act as enzymes.
• Nucleic acids are DNA and RNA. They carry the genetic information (the instructions for making proteins and for inheritance), written in a code of just four "letters" (bases).

These four classes (carbohydrates, lipids, proteins, nucleic acids) are often called the molecules of life.

Amino acids are not only building blocks of proteins. Some amino-acid compounds are used as artificial sweeteners, sugar substitutes that taste sweet without the bulk of ordinary sugar. The best known is aspartame, a compound made from two amino acids.

The reason aspartame is a low-calorie sweetener follows from its intense sweetness:

• Many times sweeter than sugar: Aspartame is roughly 200 times sweeter than table sugar (sucrose). A very tiny quantity is enough to sweeten a food or drink.
• Same energy per gram, fewer total calories: Each gram of aspartame, like other amino acids, releases about the same energy on oxidation as protein does. But because only a minute amount is used, the total calories it contributes are very few.

So aspartame is not low-calorie because it lacks energy. It is low-calorie because the sweetening dose is so small that the energy it adds is negligible.

Among the most important proteins are the enzymes. An enzyme is a biological catalyst that greatly speeds up a chemical reaction in the body without being used up itself.

Life's reactions, from digesting food to copying DNA, would be far too slow without enzymes. Each enzyme is highly specific. It acts on one particular substance (its substrate), rather like a lock and key. Enzymes work best at a particular temperature and pH. So fever or extreme conditions can disrupt the body. Without enzymes, the chemistry of life simply could not run fast enough to keep us alive.