A main function of lipids is the formation of biological membranes. Cells are surrounded by a thin layer of lipids. The layer is made up of a special type of lipid that has both hydrophobic and hydrophilic properties. The hydrophilic ends of these molecules face the water-filled environment inside the cells and the watery environment outside the cells. A hydrophobic region exists inside the two layers. The membrane that surrounds the cells is rich in proteins and other lipids such as cholesterol.
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Cell Membrane" src="http://www.cancerquest.org/images/New_Membrane.jpg" title="Lipid Bilayer" />
Most chemicals can not cross the lipid bilayer. Water and some other small molecules can freely pass through the membrane while other molecules must be actively transported via protein channels embedded in the membrane. Membranes also contain a combination of the biomolecules that have been described so far. As seen above, proteins may be coupled to carbohydrates to form glycoproteins. Glycoproteins are important in the cell:cell interactions discussed previously, and changes in the amounts or types of these proteins are seen in cancer. Similarly, a combination of lipids and carbohydrates lead to the formation of glycolipids.
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