Carbohydrates (kar"bo-hi'dratz) provide much of the energy that cells require. They also supply materials to build certain cell structures, and they often are stored as reserve energy supplies.
Carbohydrates are water-soluble molecules that contain atoms of carbon, hydrogen, and oxygen. These molecules usually have twice as many hydrogen as oxygen atoms, the same ratio of hydrogen to oxygen as in water molecules (H2O). This ratio is easy to see in the molecular formulas of the carbohydrates glucose (CbH^Ob) and sucrose (C12H22O11).
Carbohydrates are classified by size. Simple carbohydrates, or sugars, include the monosaccharides (single sugars) and disaccharides (double sugars). A monosac-charide may include from three to seven carbon atoms, occurring in a straight chain or a ring (fig. 2.10). Mono-saccharides include glucose (dextrose), fructose, and galactose. Disaccharides consist of two 6-carbon units. Sucrose (table sugar) and lactose (milk sugar) are disac-charides (fig. 2.11a and b).
Complex carbohydrates, also called polysaccharides, are built of simple carbohydrates (fig. 2.11c). Cellulose is a polysaccharide made of many glucose molecules, which humans cannot digest . It is important as dietary "fiber." Plant starch is another example. Starch molecules consist of highly branched chains of glucose molecules connected differently than in cellulose. Humans easily digest starch.
Animals, including humans, synthesize a polysaccharide similar to starch called glycogen. Its molecules also consist of branched chains of sugar units; each branch consists of a dozen or fewer glucose units.
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