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1 Objectives Describe the basic structure and function of sugars. Name three polysaccharides and describe their functions. Key Terms carbohydrate monosaccharide disaccharide polysaccharide starch glycogen cellulose Have you ever heard athletes talk about "carbs"? What are they referring to? "Carbs," which is short for carbohydrates, include the small sugar molecules dissolved in sport drinks as well as the long starch molecules in pasta and potatoes. Carbohydrates can be called upon as an energy source just minutes after a meal, or can be stored away for later use. They are an important source of energy for athletes (and everyone else). In this section you will learn how carbohydrates are used by living things. Sugars A carbohydrate is an organic compound made up of sugar molecules. Sugars contain the elements carbon, hydrogen, and oxygen in the ratio of 1 carbon: 2 hydrogen: 1 oxygen. The molecular formula of any carbohydrate is a multiple of the basic formula CH 2 O. At the core of most sugar molecules found in nature are carbon skeletons that have a ring shape. Monosaccharides Simple sugars contain just one sugar unit and are called monosaccharides (mahn oh SAK uh rydz). Glucose, fructose, and 1 of 5 10/5/11 6:49 AM

2 galactose are examples of monosaccharides. (Notice that the names of sugars end in the suffix -ose.) One or more of these simple sugars are found in many sweet things you eat. Honey, for example, contains both glucose and fructose. Glucose exists in both straight-chain and ring-shaped forms. Figure 5-6 shows the complete molecular structure of glucose in its ring form, as well as a simplified diagram that shows only its core ring. This book will use the simplified representation of sugars in most diagrams. Figure 5-6 The complete structural diagram of the monosaccharide glucose (left) shows all its atoms. The simplified representation (right) shows just the core ring formed by some of the carbon and oxygen atoms. Ring shapes are common in sugar molecules found in nature. Sugar molecules, particularly glucose, are the main fuel supply for cellular work. Similar to an automobile engine burning gasoline, cells break down glucose molecules and extract their stored energy. This process is described in detail in Chapter 7. Cells also use the carbon skeletons of monosaccharides as raw material for manufacturing other kinds of organic molecules. Glucose molecules that are not used immediately by cells are usually incorporated into larger carbohydrates, or they may be used to make fat molecules. Disaccharides Using the dehydration reaction, cells construct a disaccharide (dy SAK uh ryd) or "double sugar," from two monosaccharides. The most common disaccharide is sucrose (Figure 5-7). Sucrose consists of a glucose molecule linked to a fructose molecule. Sucrose is a major carbohydrate in plant sap, and it nourishes all the parts of the plant. For example, the sap of maple trees (maple 2 of 5 10/5/11 6:49 AM

3 syrup) contains sucrose. Table sugar is sucrose processed from the stems of sugarcane or the roots of sugar beets. Once consumed, sucrose can be broken down into glucose and fructose and used right away. The body can also store glucose in larger molecules for later use. Figure 5-7 Sucrose is a disaccharide (double sugar) consisting of two monosaccharides linked together. Polysaccharides Long polymer chains made up of simple sugar monomers are called polysaccharides (pah lih SAK uh rydz), or complex carbohydrates (Figure 5-8). For example, starch is a polysaccharide found in plant cells that consists entirely of glucose monomers. Inside a plant cell, such as a potato cell, starch chains branch and coil up like the loops of a telephone cord. The starch chains serve as sugar stockpiles. Plant cells, like animal cells, need sugar for energy to perform work, and as raw material for building other molecules. When plants break down starch molecules, the stored glucose becomes available. Humans and most other animals are also able to use plant starch as food by breaking it down within their digestive systems. Potatoes, rice, and corn are examples of foods rich in starch. 3 of 5 10/5/11 6:49 AM

4 Figure 5-8 Glycogen, cellulose, and starch are three types of polysaccharides found in food. Though all three polymers are composed of the same monomer, glucose, the way the glucose monomers link together is different for each. Animal cells do not contain starch. Instead, animals such as turkeys (and humans) store excess sugar in the form of a polysaccharide called glycogen (GLY kuh jun). Glycogen, like starch, is a chain of many glucose monomers. However, a glycogen polymer is more highly branched than a starch polymer. In humans, most glycogen is stored as granules in liver and muscle cells. When the body needs energy, it breaks down these glycogen granules, releasing glucose. Some polysaccharides in plants, such as cellulose, serve as building materials. They protect cells and stiffen the plant, preventing it from flopping over. Like starch and glycogen, cellulose is also made of glucose monomers. Multiple cellulose chains are linked together with hydrogen bonds, forming cable-like fibers in the tough walls that enclose plant cells, such as the cells of broccoli stems. Most animals, including people, cannot digest cellulose because they lack the molecule necessary to break the bonds between the glucose monomers in cellulose. Therefore, cellulose from plant foods, commonly 4 of 5 10/5/11 6:49 AM

5 referred to as "fiber," passes unchanged through your digestive system. Cellulose helps keep your digestive system healthy, but it does not serve as a nutrient. In contrast, some organisms, such as cows and termites, can derive nutrition from cellulose. Microorganisms that inhabit their digestive tracts break down the cellulose, making glucose available to the cow or termite. Almost all carbohydrates are hydrophilic. This is due to the many hydroxyl groups in their sugar units. (Recall that the hydroxyl functional group attracts water.) Therefore, monosaccharides and disaccharides dissolve readily in water, forming sugary solutions. Cellulose and some forms of starch, however, are such large molecules that they do not dissolve in water. If they did, then your cotton T-shirt or jeans, which are mostly cellulose, would dissolve the first time you were caught in a rainstorm! But though they are not soluble in water, note that these large carbohydrates are still hydrophilic. That is one reason why cotton bath towels can absorb so much water. Concept Check Explain the difference between a monosaccharide and a disaccharide. Give an example of each. 2. Compare and contrast starch, glycogen, and cellulose. 3. How do animals store excess glucose molecules? Copyright 2006 by Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights reserved. 5 of 5 10/5/11 6:49 AM