In addition to lipids and proteins, the cell membrane contains carbohydrates (see Figure 6.1). The carbohydrates are located on the outer surface of the cell membrane and serve as recognition sites for other cells and molecules, as you will see in Key Concept 6.2.
Membrane-
A glycolipid consists of a carbohydrate covalently bonded to a lipid. Extending out from the cell surface, the carbohydrate may serve as a recognition signal for interactions between cells. For example, the carbohydrates on some glycolipids change when cells become cancerous. This change may allow white blood cells to target cancer cells for destruction.
A glycoprotein consists of one or more short carbohydrate chains covalently bonded to a protein. The bound carbohydrates are oligosaccharides, usually not exceeding 15 monosaccharide units in length (see Key Concept 3.3). A proteoglycan (see Key Concept 5.4) is a more heavily glycosylated protein: it has more carbohydrate molecules attached to it, and the carbohydrate chains are often longer than they are in glycoproteins. The carbohydrates of glycoproteins and proteoglycans often function in cell recognition and adhesion.
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The “alphabet” of monosaccharides on the outer surfaces of membranes can generate a large diversity of messages. Recall from Key Concept 3.3 that monosaccharides are simple carbohydrates, often containing five or six carbons in a ring structure, which can bond with one another in various configurations. They may form linear or branched oligosaccharides with many different three-