5 Carbohydrates

1. 5 Carbohydrates

2. Objectives • Describe the basic structure and function of sugars. • Name three polysaccharides and describe their functions.

3. SugarsA 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 CH2O. At the core of most sugar molecules found in nature are carbon skeletons that have a ring shape

4. Monosaccharides Simple sugars contain just one sugar unit and are called monosaccharides (mahn oh SAK uh rydz). Glucose, fructose, and galactose are examples of monosaccharides. (Notice that the names of sugars end in the suffix -ose.)

5. 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.

6. Glucose molecules that are not used immediately by cells are usually incorporated into larger carbohydrates, or they may be used to make fat molecules.

7. 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.

9. PolysaccharidesLong polymer chains made up of simple sugar monomers are called polysaccharides (pah lih SAK uh rydz), or complex carbohydrates • starch is a polysaccharide found in plant cells that consists entirely of glucose monomers. starch chains serve as sugar stockpiles. • Potatoes, rice, and corn are examples of foods rich in starch.

11. 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.

12. animals such as turkeys (and humans) store excess sugar in the form of a polysaccharide called glycogen • Some polysaccharides in plants, such as cellulose, serve as building materials. They protect cells and stiffen the plant, preventing it from flopping over. • . 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.

13. 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 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

14. 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.