Although living cells are primarily made up of water, a number of other molecules are also abundant. Gigantic molecules, called macromolecules, populate a cell and provide it with important functions for life. For example, macromolecules provide structural support, a source of stored fuel, the ability to store and retrieve genetic information, and the ability to speed biochemical reactions. Four major types of macromolecules—proteins, carbohydrates, nucleic acids, and lipids—play these important roles in the life of a cell. In this tutorial, we examine the structures and functions of carbohydrates and lipids.


Carbohydrates are a large group of molecules that all have similar atomic compositions—with the general formula Cn(H2O)n—but differ greatly in size, chemical properties, and biological functions. Carbohydrates include the relatively small glucose molecule and the enormous glycogen molecule, which may consist of hundreds of thousands of glucose monomers. Carbohydrates are energy-rich. Many, such as glycogen, provide energy-storage functions. Other carbohydrates, such as cellulose—a component of plant cell walls—serve primarily structural roles in a cell.

Lipids—colloquially called fats—are hydrocarbons (composed of C and H atoms) that are insoluble in water because of their many nonpolar covalent bonds. Lipids are diverse in structure and function, but all have in common that they are hydrophobic. The most common units of lipids are triglycerides. A triglyceride contains three fatty acid molecules and one glycerol molecule. The long hydrocarbon chain of a fatty acid is very hydrophobic because of its abundant C–H and C–C bonds.

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Textbook Reference: Concept 2.2 Atoms Interact and Form Molecules