Cell membrane carbohydrates are recognition sites

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-associated carbohydrates may be covalently bonded to lipids or to proteins:

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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-dimensional shapes. An oligosaccharide of a specific shape on one cell can bind to a complementary shape on an adjacent cell. This binding is the basis of cell–cell adhesion.