The carbohydrates include simple sugars as well as polysaccharides. These simple sugars, such as glucose, are the major nutrients of cells. As discussed in Chapter 3, their breakdown provides both a source of cellular energy and the starting material for the synthesis of other cell constituents. Polysaccharides are storage forms of sugars and form structural components of the cell. In addition, polysaccharides and shorter polymers of sugars act as markers for a variety of cell recognition processes, including the adhesion of cells to their neighbors and the transport of proteins to appropriate intracellular destinations.
The basic formula for simple sugars (monosaccharides) is (CH2O)n, from which the name carbohydrate is derived (C = “carbo” and H2O = “hydrate”). The six-carbon (n = 6) sugar glucose (C6H12O6) is especially important in cells, since it provides the principal source of cellular energy (Figure 2.5). Other simple sugars have between three and seven carbons, with three- and five-carbon sugars being the most common. Sugars containing five or more carbons can cyclize to form ring structures, which are the predominant forms of these molecules within cells. As illustrated in Figure 2.5, the cyclized sugars exist in two alternative forms (called α or β), depending on the configuration of carbon 1.
Figure 2.5 Structure of glucose Glucose is a simple six-carbon sugar. It can cyclize to form a ring in two alternative forms (α and β), depending on the configuration of carbon 1.
Monosaccharides can be joined together by dehydration reactions in which H2O is removed and the sugars are linked by a glycosidic bond between two of their carbons (Figure 2.6). If only a few sugars are joined together, the resulting polymer is called an oligosaccharide. If a large number (hundreds or thousands) of sugars are involved, the resulting polymers are macromolecules called polysaccharides. Two common polysaccharides glycogen and starch are the storage forms of carbohydrates for energy utilization in animal and plant cells, respectively. Both glycogen and starch are composed entirely of glucose molecules in the α configuration (Figure 2.7). The principal linkage is between carbon 1 of one glucose and carbon 4 of a second. In addition, both glycogen and one form of starch (amylopectin) contain occasional α-1,6 linkages, in which carbon 1 of one glucose is joined to carbon 6 of a second. As illustrated in Figure 2.7, these linkages lead to the formation of branches resulting from the joining of two separate α-1,4 linked chains. Such branches are present in glycogen and amylopectin, although another form of starch (amylose) is an unbranched molecule.
Figure 2.6 Formation of a glycosidic bond Two glucose molecules in the α configuration are joined by a bond
between carbons 1 and 4, which is therefore called an α-1,4 glycosidic bond.
The structures of glycogen and starch are thus basically similar, as is their function: to store glucose. Cellulose, in contrast, has a quite distinct function as the principal structural component of the plant cell wall. Perhaps surprisingly, then, cellulose is also composed entirely of glucose molecules. The glucose residues in cellulose, however, are in the β rather than the α configuration, and cellulose is an unbranched polysaccharide (see Figure 2.7). The linkage of glucose residues by β-1,4 rather than α-1,4 bonds causes cellulose to form long extended chains that pack side by side to form fibers of great mechanical strength. The animal parallel of cellulose is chitin, a polysaccharide of modified glucose residues linked by β bonds, which forms the exoskeletons of crabs and insects.
Figure 2.7 Structure of polysaccharides Polysaccharides are macromolecules consisting of hundreds or thousands of simple sugars. Starch, glycogen, and cellulose are all composed entirely of glucose residues, which are joined by α-1,4 glycosidic bonds in starch and glycogen but by β-1,4 bonds in cellulose. One form of starch (amylopectin) and glycogen also contain occasional α-1,6 bonds, which serve as branch points by joining two separate α-1,4 chains.
In addition to their roles in energy storage and cell structure, oligosaccharides and polysaccharides are important in a variety of informational processes. For example, oligosaccharides are frequently linked to proteins, where they play important roles in protein folding and serve as markers to target proteins for transport to the cell surface or incorporation into different sub cellular organelles. Oligosaccharides and polysaccharides also serve as markers on the surface of cells, playing important roles in cell recognition and the interactions between cells in tissues of multicellular organisms.