The monomers of polysaccharides become covalently connected by glycosidic bonds (Figure 2-1).
Carbohydrates serve several major functions in living systems. Monosaccharides and oligosaccharides serve as readily utilizable energy sources. Starch and glycogen act as macromolecular energy stores in plants and animals, respectively. Carbohydrates perform structural roles, such as cellulose in plant cell walls and chitin in the exoskeletons of arthropods. Surface carbohydrates are often complexed with proteins as glycoproteins or with lipids as glycolipids in the plasma membrane. The great potential for structural diversity and thus, specificity, makes these molecules very useful as cell-recognition markers in cellular communication and in cell-to-cell attachments.
Glycogen consists of polymers of glucose units joined by α(1→4) linkages and forms branched chains by α(1→6) linkages. Starch has fewer α(1→6) linkages than glycogen.
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