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Figure 2.10 Structures of glucose (top) and fructose (bottom) molecules. The numbers of atoms and locations of bonds are easy to see in the upper linear diagrams, but when these molecules are in solution, they are in the form of rings, as shown in the lower diagrams.

a starch molecule to become available as an energy source in cells, it has to be hydrolyzed; that is, it has to be broken up into individual glucose molecules through the restoration of a water molecule for each unit.

Throughout the world, starches are major sources of carbohydrates for human consumption—the principal starch crops being potatoes, wheat, rice, and corn in temperate areas and cassava and taro in tropical areas.

Cellulose, the chief structural polymer in plant cell walls, is a polysaccharide consisting of 3,000 to 10,000 unbranched chains of glucose molecules. Although cellulose is very widespread in nature, its glucose units are bonded together differently from those of starch, and most animals digest it much less readily than they do starch. Organisms that do digest cellulose, such as the protozoans living in termite guts, caterpillars, and some fungi, produce special enzymes capable of facilitating the breakdown of bonds between the carbons and the glucose units of the cellulose; the organisms then can digest the released glucose.

Lipids Lipids are fatty or oily substances that are mostly insoluble in water because they have no polarized components. They typically store about twice as much energy as similar amounts of carbohydrate and play an important role in the longer-term energy reserves and structural components of cells. Like carbohydrates, lipid molecules contain carbon, hydrogen, and oxygen, but there is proportionately much less oxygen present. Examples of lipids include fats, which are solid at room temperature (Fig. 2.11), and oils, which are liquid. Oil molecules are formed from sugars and are composed of a unit of glycerol—a three-carbon compound that has three hydroxyl (-OH) groups—with three fatty acids attached to the glycerol unit. A fatty acid has a carboxyl (-COOH) group at one end and typically has an even number of carbon atoms to which hydrogen atoms can become attached.

Most fatty acid molecules consist of a chain with 16 to 18 carbon atoms. If hydrogen atoms are attached to every available attachment point of these fatty acid carbon atoms, as in most animal fats such as butter and those found in meats, the fat is said to be saturated. If there is at least one double bond between two carbons and there are fewer hydrogen atoms attached, the fat is said to be unsaturated. If there are three or more double bonds between the carbons of a fatty acid, as in some vegetable oils such as those of canola, olive, or safflower, the fat is said to be polyunsaturated. Unsaturated vegetable oils can become saturated by bubbling hydrogen gas through them, as is done in the manufacture of margarine. Human diets high in saturated fats often ultimately lead to clogging of arteries and other heart diseases, while diets low in saturated fats promote better health. However, some fat in the diet appears to be essential to normal animal and human absorption of nutrients, and there is concern that consumption of "fake" fat introduced to the public in the late 1990s could lead to health problems.

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