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Glycolipids and Cholesterol
The cell membranes of eukaryotic cells and some bacterial cells consist of three types of lipids - phospholipids, glycolipids and cholesterol molecules. Glycolipids are sugar-containing lipid molecules that resemble sphingolipids in that they contain a sphingosine molecule. The sphingosine is attached to a fatty acid via an amide bond at the nitrogen while a sugar molecule (monosaccharide or oligosaccharide) is attached via a glycosidic bond to the primary alcohol. The fatty acid and the long, unsaturated hydrocarbon chain of the sphingosine give the glycolipid its hydrophobic properties while the sugar component gives the glycolipid its hydrophilic properties. Therefore, the polar sugar molecule always extend out towards the aqueous extracellular environment. The simplest glycolipid is called a cerebroside. Cholesterol molecules are steroids, which means they contain four rings fused together. On one side of the fused rings is a hydrocarbon tail while the other side contains a polar hydroxyl group. The polar hydroxyl group interacts with the polar heads of the nearby phospholipids and points towards the aqueous environment. On the other hand, the fused rings and the hydrocarbon tail lies inside the membrane, parallel with respect to the tails of the phospholipids. Although cholesterol molecules are found in some bacterial (i.e. mycoplasma), cholesterol is predominantly a eukaryotic membrane lipid.
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