Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. Glycogen (black granules) in spermatozoa of a flatworm transmission electron microscopy, scale: 0.3 m. In cellulose, glucose monomers are linked in unbranched chains by β-1,4 glycosidic linkages. A view of the atomic structure of a single branched strand of glucose units in a glycogen molecule. Because the earth is covered with vegetation, cellulose is the most abundant of all carbohydrates, accounting for over 50% of all the carbon found in the vegetable kingdom.\): Cellulose. Although the percentage of glycogen (by weight) is higher in the liver, the much greater mass of skeletal muscle stores a greater total amount of glycogen.Ĭellulose, a fibrous carbohydrate found in all plants, is the structural component of plant cell walls. The number of glucose units in cellulose ranges from 500 to 5000, depending on the source of the polysaccharide. It is produced by plants, algae, tunicates, and some bacteria, primarily as a structural polymer. In animals, the enzyme phosphorylase catalyzes the breakdown of glycogen to phosphate esters of glucose.Ībout 70% of the total glycogen in the body is stored in muscle cells. Cellulose, one of the most abundant natural polysaccharides, is a linear polymer consisting of (14)-d -glucose unit ( Figure 15 ). Amylopectin is a branched-chain polysaccharide composed of glucose units linked primarily by -1,4-glycosidic bonds. (b) Because of hydrogen bonding, amylose acquires a spiral structure that contains six glucose units per turn. Glycogen can be broken down into its D-glucose subunits by acid hydrolysis or by the same enzymes that catalyze the breakdown of starch. (a) Amylose is a linear chain of -D-glucose units joined together by -1,4-glycosidic bonds. When treated with iodine, glycogen gives a reddish brown color. Glycogen is structurally quite similar to amylopectin, although glycogen is more highly branched (8–12 glucose units between branches) and the branches are shorter. When fasting, animals draw on these glycogen reserves during the first day without food to obtain the glucose needed to maintain metabolic balance. Like starch in plants, glycogen is found as granules in liver and muscle cells. Practically all mammalian cells contain some stored carbohydrates in the form of glycogen, but it is especially abundant in the liver (4%–8% by weight of tissue) and in skeletal muscle cells (0.5%–1.0%). Glycogen is the energy reserve carbohydrate of animals. In the human body, several enzymes known collectively as amylases degrade starch sequentially into usable glucose units. The complete hydrolysis of starch yields, in successive stages, glucose: Dextrins are more easily digested than starch and are therefore used extensively in the commercial preparation of infant foods. Because of their characteristic stickiness with wetting, dextrins are used as adhesives on stamps, envelopes, and labels as binders to hold pills and tablets together and as pastes. The shine and stiffness imparted to clothing by starch are due to the presence of dextrins formed when clothing is ironed. A molecule of amylopectin may contain many thousands of glucose units with branch points occurring abo ut every 2530 units (see figure below. These branch points occur more often in glycogen.ĭextrins are glucose polysaccharides of intermediate size. Amylopectin is a branched-chain polysaccharide composed of glucose units linked primarily by -1,4-glycosidic bonds but with occasional -1,6-glycosidic bonds, which are responsible for the branching. Both amylopectin and glycogen contain branch points that are linked through α-1,6-linkages. \): Representation of the Branching in Amylopectin and Glycogen.
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