Cells are defined by membranes. After all, membranes physically separate cells from their external environment. In addition, membranes define spaces within many cells that allow them to carry out their diverse functions.

Lipids are the main component of cell membranes. They have properties that allow them to form a barrier in an aqueous (watery) environment. Proteins are often embedded in or associated with the membrane, where they perform important functions such as transporting molecules. Carbohydrates can also be found in cell membranes, usually attached to lipids (glycolipids) and proteins (glycoproteins).