Most people know about cholesterol because of its ability to cause heart disease and atherosclerosis. However, cholesterol has a number of other important functions in the body. Aside from its role in making steroid hormones, cholesterol is also an important component of the membrane that surrounds all cells in the human body.
Cell Membrane Components
Cells are surrounded by a membrane that is made out of lipids. Specifically, the cell membrane contains a double layer of lipids called phospholipids (because they have a phosphate molecule attached to them). Cholesterol is another lipid which can be found within the cell membrane and, according to Choleterol-and-Health.com, it can represent 50 percent of the membrane in some cells.
One role of cholesterol is to help give the cell membrane extra support. Cholesterol is more rigid than some of the other lipids in the membrane. As Cytochemistry.net explains, cholesterol helps to immobilize some of the lipid molecules around them. This extra rigidity makes the cell membrane stronger and makes it harder for small molecules to pass through the membrane. The presence of cholesterol allows the cell membrane to be strong enough to hold the cell together and to serve as an effective barrier to ions.
Despite the fact that cholesterol is more rigid than some of its neighboring lipids, cholesterol also has an important role in keeping the cell membrane fluid. Cholesterol helps generate some extra space between the lipids, which keeps them from gelling together into their "crystalline" state. This allows lipids to move freely throughout the membrane as needed.
An article in PNAS (Proceeding of the National Academy of Sciences) explains that cholesterol also has an important role in the construction of special parts of cell membranes called lipid rafts. Lipid rafts are made up of high amounts of cholesterol and special kinds of lipids called sphingolipids. These rafts allow some sections of the membrane to be distinct from other areas.
Lipid Raft Function
Lipid rafts are important for many cellular actions. Some lipid rafts are needed in order to export proteins out of the cell. Other lipid rafts are used to anchor specific proteins in the membrane and keep protein clusters together. Some pathogens (like viruses and bacteria) also target lipid rafts in order to get into the cell.