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Cholesterol is an important component of animal cell membranes, comprising up to half the total lipids in some cells. Because it has a rigid ring structure, cholesterol lends strength to cell membranes and helps the cell hold its shape. Cholesterol also helps to maintain the permeability of the cell membrane to small molecules, and may play a part of protecting cells from freeze damage. Hence a cell without cholesterol in its membrane would not hold its shape well and would have difficulty with passage of ions across the membrane.
Proteins have many functions in the cell membrane. Some proteins are on the membrane surface; these allow attachment of cells to surfaces or to one another. Proteins also are the main component of the major histocompatibility complex, a group of chemical markers on the outside of each cell that allow the immune system to distinguish self from non-self.
Proteins embedded in the cell membrane function in transport across the membrane. Some work as pores to allow molecules to pass in and out freely by diffusion, while others perform active transport, attaching to specific molecules and moving them across the cell membrane against the concentration gradient. A cell without proteins in the membrane would be unable to move most molecules across the membrane and would quickly die.
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