Lipids are essential for active transport, enzyme activity, and membrane formation. They are insoluble in water and have two distinct areas with different affinities for water and oil. Membrane lipids arrange themselves in a bilayer, with their hydrophilic heads prominent and hydrophobic tails in the middle. Membrane lipids are the most predominant molecules in living membranes, and the major classes are phospholipids, glycolipids, sphingolipids, and cholesterol. Phospholipids are the most common and form when a phosphate group bonds to a diglyceride. Glycolipids are formed when a carbohydrate chain binds to a phospholipid and serve as markers for cellular recognition. Cholesterol stabilizes the membrane and makes it less fluid. Sphingolipids form lipid rafts, which are important in cell signaling and recognition.
Membrane lipids are integral to 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 oil is added 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 non-polar and will not dissolve in water. It is this distinct feature of lipids that causes the structure of living membranes. When a membrane forms, the membrane lipids arrange themselves in a bilayer. A bilayer consists of two sheets of membrane lipids with their hydrophilic heads prominent and hydrophobic tails in the middle of the membrane bilayer.
All membranes of living organisms, both around and inside cells, are made primarily of lipids and proteins. Membrane lipids are the most predominant molecules in the membrane. Some proteins are scattered throughout the lipid layer, while others are attached to its surface.
Most membrane lipids are formed from glycerol which binds to three fatty acid chains through a covalent bond. The resulting molecules are called glycerides. Sphingolipids are the exception to this rule as they form when sphingosine covalently bonds to fatty acid chains instead of glycerol. Covalent bonding occurs when oppositely charged atoms share pairs of electrons.
Phospholipids are the most common of the membrane lipids. They form when a phosphate group, which contains phosphorus, bonds to a diglyceride and another simple organic molecule. A diglyceride consists of 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 fatty acids are hydrophobic and remain in the middle of the membrane bilayer.
Glycolipids are formed when a carbohydrate chain binds to a phospholipid. The carbohydrate chain is located on the outer layer of the membrane bilayer. In this way, the carbohydrate chain serves as a marker to allow for cellular recognition. Furthermore, glycolipids provide energy, which is stored in the carbohydrate chain. Finally, glycolipids help stabilize the membrane and provide a place for it to attach to other cells or tissues.
When cholesterol is present in a membrane, it binds weakly with the phospholipids on both sides of the membrane. By binding to adjacent phospholipids, cholesterol stabilizes them and in turn stabilizes the entire membrane. With higher amounts of cholesterol, the membrane becomes less fluid, or able to move freely, and mechanically stronger. The amount of cholesterol present in membranes varies according to the type of cell. Plants contain no cholesterol, so they rely on the cell wall for the stability of their cells.
Finally, sphingolipids are mostly found in the outer layer of the bilayer. There is a very uneven distribution of this type of lipid throughout the bilayer. Sphingolipids form lipid rafts, which are important in cell signaling and recognition. Cholesterol is sometimes found next to or near sphingolipids to stabilize the cell membrane that surrounds them.
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