What do cellulose and glycogen have in common
These molecules are also vital parts of macromolecular structures that store and transmit genetic information i. They are the basis of biological polymers that impart strength to various structural components of organisms e. In biochemistry, carbohydrates are often called saccharides , from the Greek sakcharon , meaning sugar, although not all the saccharides are sweet.
The simplest carbohydrates are called monosaccharides , or simple sugars. They are the building blocks monomers for the synthesis of polymers or complex carbohydrates, as will be discussed further in this section.
Monosaccharides are classified based on the number of carbons in the molecule. General categories are identified using a prefix that indicates the number of carbons and the suffix — ose , which indicates a saccharide; for example, triose three carbons , tetrose four carbons , pentose five carbons , and hexose six carbons Figure 1.
The hexose D-glucose is the most abundant monosaccharide in nature. Other very common and abundant hexose monosaccharides are galactose , used to make the disaccharide milk sugar lactose , and the fruit sugar fructose. Figure 1. Monosaccharides are classified based on the position of the carbonyl group and the number of carbons in the backbone.
Monosaccharides of four or more carbon atoms are typically more stable when they adopt cyclic, or ring, structures. Glucose, for example, forms a six-membered ring Figure 2. Figure 2. Note in these cyclic structural diagrams, the carbon atoms composing the ring are not explicitly shown. Two monosaccharide molecules may chemically bond to form a disaccharide. The name given to the covalent bond between the two monosaccharides is a glycosidic bond.
Glycosidic bonds form between hydroxyl groups of the two saccharide molecules, an example of the dehydration synthesis described in the previous section of this chapter:. Common disaccharides are the grain sugar maltose , made of two glucose molecules; the milk sugar lactose , made of a galactose and a glucose molecule; and the table sugar sucrose , made of a glucose and a fructose molecule Figure 3. Polysaccharides, also called glycans , are large polymers composed of hundreds of monosaccharide monomers.
Unlike mono- and disaccharides, polysaccharides are not sweet and, in general, they are not soluble in water. Like disaccharides, the monomeric units of polysaccharides are linked together by glycosidic bonds. Polysaccharides are very diverse in their structure. Three of the most biologically important polysaccharides— starch , glycogen , and cellulose —are all composed of repetitive glucose units, although they differ in their structure Figure 4.
Cellulose consists of a linear chain of glucose molecules and is a common structural component of cell walls in plants and other organisms. The characteristic blue-violet color that appears when starch is treated with iodine is due to the formation of the amylose-iodine complex. This color test is sensitive enough to detect even minute amounts of starch in solution. The helical structure of amylopectin is disrupted by the branching of the chain, so instead of the deep blue-violet color amylose gives with iodine, amylopectin produces a less intense reddish brown.
Dextrins are glucose polysaccharides of intermediate size. The shine and stiffness imparted to clothing by starch are due to the presence of dextrins formed when clothing is ironed.
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. Dextrins are more easily digested than starch and are therefore used extensively in the commercial preparation of infant foods.
In the human body, several enzymes known collectively as amylases degrade starch sequentially into usable glucose units. Glycogen is the energy reserve carbohydrate of animals.
Like starch in plants, glycogen is found as granules in liver and muscle cells. When fasting, animals draw on these glycogen reserves during the first day without food to obtain the glucose needed to maintain metabolic balance. 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 treated with iodine, glycogen gives a reddish brown color.
Glycogen can be broken down into its D-glucose subunits by acid hydrolysis or by the same enzymes that catalyze the breakdown of starch. In animals, the enzyme phosphorylase catalyzes the breakdown of glycogen to phosphate esters of glucose. 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. Cellulose, a fibrous carbohydrate found in all plants, is the structural component of plant cell walls.
The largest use of cellulose is in the manufacture of paper and paper products. Like amylose, cellulose is a linear polymer of glucose. As a result, cellulose exhibits little interaction with water or any other solvent. Cotton and wood, for example, are completely insoluble in water and have considerable mechanical strength. Now compare your answer with the one below.
The enzymes that build up and break down glycogen and starch act on the free ends of the polysaccharides. Having a great deal of branching ensures that plants and animals can quickly add to their energy supply when energy is plentiful, or break it down the storage molecules when energy is in short supply.
Polysaccharides Polysaccharides are long chains of monosaccharides linked by glycosidic bonds.
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