Carbohydrates are molecules that contain carbon, hydrogen, and oxygen, with the concentration of hydrogen and oxygen atoms in a 2;1 ratio. Abundant energy is locked in their many carbon-hydrogen bonds. Plants, algae, and some bacteria produce carbohydrates by the process of photosynthesis. Most organisms use carbohydrates as an important fuel, breaking these bonds and releasing energy to sustain life.
Among the least complex of the carbohydrates are the simple sugars or monosaccharides (MON-oh-SACK-uh-rides). This word comes from two Greek words meaning "single" (monos) and "sweet" (saccharon) and reflects the fact that monosaccharides are individual sugar molecules. Some of these sweet-tasting sugars have as few as three carbon atoms. The monosaccharides that play a central role in energy storage, however, have six. The primary energy-storage molecule used by living things is glucose (C6H12O6), a six-carbon sugar with seven energy-storing carbon-hydrogen bonds.
Figure 9 Structure of glucose molecule. (a) The structural formula of glucose in its linear form and (b) as a ring structure. (c) Space-filling model of glucose. (Hydrogen, blue; Oxygen, red; Carbon. black).
Notice in Figure 9 that glucose, like other sugars, exists as a straight chain or as a ring of atoms.
Glucose is not the only sugar with the formula C6H12O6. Other monosaccharides having this same formula are fructose and galactose. Because these molecules have the same molecular formula as glucose but are put together slightly differently, they are called isomers, or alternative forms, of glucose. Your taste buds can tell the difference: Fructose is much sweeter than glucose.
Two monosaccharides linked together form a disaccharide (dye-SACK-uh-ride). Many organisms, such as plants, link monosaccharides together to form disaccharides, which are less readily broken down while they are transported within the organism. Sucrose (table sugar) is a disaccharide formed by linking a molecule of glucose to a molecule of fructose by dehydration synthesis as shown in Figure 10. It is the common transport form of sugar in plants. Lactose, or milk sugar (glucose 1 galactose), is a disaccharide produced by many mammals to feed their young.
Figure 10 Disaccharides are formed from monosaccharides by dehydration synthesis.
Not only do organisms unlock, use, and transport the energy within carbohydrate molecules, they store this energy. To do this, however, organisms must convert soluble sugars such as glucose to an insoluble form to be stored. Sugars are made insoluble by joining together into long polymers called polysaccharides. Plants store energy in polysaccharides called starches. The starch amylose, for example, is made up of hundreds of glucose molecules linked together in long, unbranched chains. Most plant starch is a branched version of amylose called amylopectin (AM-ih-low-PECK-tin). Animals store glucose in highly branched polysaccharides called glycogen (GLYE-ko-jen) (Figure 11).
Figure 11 Glycogen, a starch. Glycogen storage granules can be seen in the cytoplasm of many types of cells, such as liver cells, muscle cells, and certain types of white blood cells. The electron micrograph shows a neutrophil, a type of white blood cell that is abundant in the body and that phagocytizes foreign material.
The chief component of plant cell walls is a polysaccharide called cellulose. Cellulose is chemically similar to amylose but is bonded in a way that most organisms cannot digest (Figure 12). For this reason, cellulose works well as a biological structural material and occurs widely in this role in plants. The structural material in insects, many fungi, and certain other organisms is a modified form of cellulose called chitin (KITE-n). Chitin is a tough, resistant surface material that is also relatively indigestible.
Figure 12 Stucture of cellulose. (a) Cellulose fibers from a ponderosa pine. (b) Macrofibrils compose each fiber. (c) Each macrofibril is composed of bundles of microfibrils. (d) Microfibrils, in turn, are composed of bundles of cellulose chains. Cellulose fibers can be very strong; this is one reason why wood is such a good building material.