Membrane lipids are integral for active transport across the membrane, many types of enzyme activity and membrane formation. Lipids are a group of compounds that include fats and oils and are insoluble in water, but soluble in alcohol. In other words, lipids do not dissolve in water, which is clearly shown when you add oil to water – they mix, but remain separate. There are many different types of lipids, but the major classes of membrane lipids are phospholipids, glycolipids, sphingolipids and cholesterol.
Lipids are amphipathic because each molecule has two distinct areas with different affinities for water and oil. The hydrophilic area of the molecule is polar, so it is attracted to water. The hydrophobic region is not polar and will not dissolve in water. It is this distinct characteristic of lipids that causes the structure of living membranes. When a membrane is formed, the membrane lipids arrange into a bilayer. A bilayer is made up of two sheets of membrane lipids with their hydrophilic heads pointing out and hydrophobic tails in the middle of the membrane bilayer.
All membranes in living organisms, both around the cells and within them, are mostly made up lipids and proteins. The membrane lipids are the most predominant molecules in the membrane. Some proteins are interspersed throughout the lipid layer, while others are attached to its surface.
Most membrane lipids are formed from glycerol linking to three fatty acid chains through covalent bonding. The resulting molecules are called glycerides. Sphingolipids are the exception to this rule as they are formed when sphingosine covalently bonds to the fatty acid chains instead of the glycerol. Covalent bonding occurs when oppositely charged atoms share electron pairs.
Phospholipids are the most common of the membrane lipids. They are formed when a phosphate group, which contains phosphorous, is bonded to a diglyceride and another simple organic molecule. A diglyceride is made up from two glyceride molecules. In a phospholipid, the head containing the phosphate group is polar, so it is attracted to water. The long hydrocarbon chains of the fatty acids are hydrophobic and stay in the middle of the membrane bilayer.
Glycolipids are formed when a carbohydrate chain attaches to a phospholipid. The carbohydrate chain is found on the outside layer of the membrane bilayer. This way, the carbohydrate chain acts as a marker to allow cellular recognition. Also, glycolipids provide energy, which is stored in the carbohydrate chain. Finally, glycolipids help stabilize the membrane and provide a place for attaching to other cells or tissues.
When cholesterol is present in a membrane, it binds weakly with phospholipids on either side of it. By binding to the adjacent phospholipids, the cholesterol stabilizes them and in turn stabilizes the entire membrane. With greater amounts of cholesterol, the membrane becomes less fluid, or able to move freely, and more mechanically stronger. The amount of cholesterol found in membranes varies due to cell type. Plants contain no cholesterol, so rely on the cell wall for stability of their cells.
Finally, sphingolipids are found mostly in the outer layer of the bilayer. There is a very uneven distribution of this type of lipid across the bilayer. Sphingolipids form lipid rafts, which are important in cell signalling and recognition. Cholesterol is sometimes found beside or near the sphingolipids to stabilize the cell membrane around them.