Cell membranes also contain cholesterol in the phospholipid bilayer. In some membranes there are only a few cholesterol molecules, but in others there are as many cholesterols as phospholipids according to Audesirk & Audesirk. Cholesterol makes the bilayer stronger, more flexible but less fluid, and less permeable to water-soluble substances such as ions and monosaccharides.

Ideas about cell structure have changed considerably over the years. Early biologists saw cells as simple membranous sacs containing fluid and a few floating particles. Today's biologists know that cells are infinitely more complex than this. There are many different types, sizes, and shapes of cells in the body.