Polysaccharides:- Some Homopolysaccharides Are Stored Forms of Fuel
The most important storage polysaccharides are starch in plant cells and glycogen in animal cells. Both poly saccharides occur intracellularly as large clusters or granules (Fig. 7–14). Starch and glycogen molecules are heavily hydrated, because they have many exposed hydroxyl groups available to hydrogen-bond with water. Most plant cells have the ability to form starch, but it is
FIGURE 7–14 Electron micrographs of starch and glycogen granules. (a) Large starch granules in a single chloroplast. Starch is made in the chloroplast from D-glucose formed photosynthetically. (b) Glycogen granules in a hepatocyte. These granules form in the cytosol and are much smaller (~0.1 m) than starch granules (~1.0 μm).
especially abundant in tubers, such as potatoes, and in seeds. Starch contains two types of glucose polymer, amylose and amylopectin (Fig. 7–15). The former consists of long, unbranched chains of D-glucose residues connected by (α 1 →4) linkages. Such chains vary in molecular weight from a few thousand to more than a million. Amylopectin also has a high molecular weight (up to 100 million) but unlike amylose is highly branched. The glycosidic linkages joining successive glucose residues in amylopectin chains are (α1 →4); the branch points (occurring every 24 to 30 residues) are (α1 →6) linkages. Glycogen is the main storage polysaccharide of animal cells. Like amylopectin, glycogen is a polymer of (α1 →4)-linked subunits of glucose, with (α1 →6)-linked branches, but glycogen is more extensively branched (on average, every 8 to 12 residues) and more compact than starch. Glycogen is especially abundant in the liver, where it may constitute as much as 7% of the wet weight; it is also present in skeletal muscle. In hepatocytes glycogen is found in large granules (Fig. 7–14b), which are themselves clusters of smaller granules com posed of single, highly branched glycogen molecules with an average molecular weight of several million. Such glycogen granules also contain, in tightly bound form, the enzymes responsible for the synthesis and degradation of glycogen.
Because each branch in glycogen ends with a nonreducing sugar unit, a glycogen molecule has as many nonreducing ends as it has branches, but only one reducing end. When glycogen is used as an energy source, glucose units are removed one at a time from the nonreducing ends. Degradative enzymes that act only at nonreducing ends can work simultaneously on the many branches, speeding the conversion of the polymer to monosaccharides.
Why not store glucose in its monomeric form? It has been calculated that hepatocytes store glycogen equivalent to a glucose concentration of 0.4 M. The actual con centration of glycogen, which is insoluble and con tributes little to the osmolarity of the cytosol, is about 0.01 μM. If the cytosol contained 0.4 M glucose, the osmolarity would be threateningly elevated, leading to osmotic entry of water that might rupture the cell (see Fig. 2–13). Furthermore, with an intracellular glucose concentration of 0.4 M and an external concentration of about 5 mM (the concentration in the blood of a mam mal), the free-energy change for glucose uptake into cells against this very high concentration gradient would be prohibitively large.
Dextrans are bacterial and yeast polysaccharides made up of (α1→6)-linked poly-D-glucose; all have (α1n3) branches, and some also have (α 1→2) or (α 1→4) branches. Dental plaque, formed by bacteria growing on the surface of teeth, is rich in dextrans. Syn thetic dextrans are used in several commercial products (for example, Sephadex) that serve in the fractionation of proteins by size-exclusion chromatography (see Fig. 3–18b). The dextrans in these products are chemically cross-linked to form insoluble materials of various porosities, admitting macromolecules of various sizes.
