Biochemistry is the study of the chemistry of living organisms. Biomolecules are often sorted into four categories: (1) peptides and proteins, (2) carbohydrates, (3) nucleic acids, and (4) lipids.

The term protein comes from the Greek proteios, which means "of first importance." Biochemistry, for many years, was almost synonymous with the study of proteins because these compounds serve the broadest array of functions of any class of biomolecules, including:

Structure: The actin and myosin in muscles, the collagen in skin and bone, and the keratins in hair, horn, and hoof are examples of proteins whose primary function is to produce the structure of the organism.
Catalysis: Most chemical reactions in living systems are catalyzed by enzymes, which are proteins.
Control: Many proteins regulate or control biological activity. Insulin, for example, controls the rate at which sugar is metabolized.
Energy: Some proteins, including the casein in milk and the albumin in eggs, are used primarily to store food energy.
Transport: O2 is carried through the bloodstream by hemoglobin and myoglobin. Other proteins transport sugars, amino acids, and ions across the cell membranes.
Protection: The first line of defense against viruses and bacteria are the antibodies produced by the immune system, which are based on proteins.

The term carbohydrate reflects the fact that many of the compounds in this category have the empirical formula CH2O--they are literally "hydrates of carbon." Carbohydrates are the primary source of food energy for most living systems. They include simple sugars such as glucose (C6H12O6) and sucrose (C12H22O11) as well as polymers of these sugars such as starch, glycogen, and cellulose. Carbohydrates are produced from CO2 and H2O during photosynthesis and are therefore the end products of the process by which plants capture the energy in sunlight.

The name nucleic acid was originally given to a class of relatively strong acids that were found in the nuclei of cells. As monomers, nucleic acids such as adenosine triphosphate (ATP) are involved in the process by which cells capture food energy and make it available to fuel the processes that keep cells alive. As polymers, they store and process the information that allows the organism to grow and eventually reproduce.

Compounds are classified as proteins, carbohydrates, and nucleic acids on the basis of similarities in their structures. Lipids, on the other hand, are defined on the basis of their physical properties. Any molecule in a biological system that is soluble in nonpolar solvents is classified as a lipid (from the Greek lipos, fat). The lipid known as cholesterol, for example, is virtually insoluble in water, but it is soluble in a variety of nonpolar solvents-- including the nonpolar region between the inner and outer surfaces of a membrane.

return to top