Molecules of Life
Four categories of molecules, called carbohydrates, lipids, proteins, and nucleic acids, are unique to cells. They are called macromolecules because each is composed of many subunits:
During synthesis of macromolecules, the cell uses a dehydration reaction, so called because an -OH (hydroxyl group) and an -H (hydrogen atom)-the equivalent of a water molecule-are removed as the molecule forms (Fig. 2.8a). The result is reminiscent of a train whose length is determined by how many boxcars are hitched together. To break up macromolecules, the cell uses a hydrolysis reaction, in which the components of water are added (Fig. 2.8b).
Glycerol and fatty acids
Figure 2.8 Synthesis and degradation ofmacromolecules. a. In cells, synthesis often occurs when subunits bond following a dehydration reaction (removal of H2O). b. Degradation occurs when the subunits in a macromolecule separate after a hydrolysis reaction (addition of H2O).
Carbohydrates, like all organic molecules, always contain carbon (C) and hydrogen (H) atoms. Carbohydrate molecules are characterized by the presence of the atomic grouping H-C-OH, in which the ratio of hydrogen atoms (H) to oxygen atoms (O) is approximately 2:1. Because this ratio is the same as the ratio in water, the name “hydrates of carbon” seems appropriate. Carbohydrates first and foremost function for quick, short-term energy storage in all organisms, including humans. Figure 2.9 shows some foods that are rich in carbohydrates.
If the number of carbon atoms in a carbohydrate is low (between three and seven), it is called a simple sugar, or monosaccharide. The designation pentose means a 5-carbon sugar, and the designation hexose means a 6-carbon sugar. Glucose, the hexose our bodies use as an immediate source of energy, can be written in any one of these ways:
Figure 2.9 Common foods. Carbohydrates such as bread and pasta are digested to sugars; lipids such as oils are digested to glycerol and fatty acids; and proteins such as meat are digested to amino acids. Cells use these subunit molecules to build their own macromolecules.
Other common hexoses are fructose, found in fruits, and galactose, a constituent of milk. A disaccharide (di, two; saccharide, sugar) is made by joining only two monosaccharides together by a dehydration reaction (see Fig. 2.8a). Maltose is a disaccharide that contains two glucose molecules:
When glucose and fructose join, the disaccharide sucrose forms. Sucrose, which is ordinarily derived from sugarcane and sugar beets, is commonly known as table sugar.
Complex Carbohydrates (Polysaccharides)
Macromolecules such as starch, glycogen, and cellulose are polysaccharides that contain many glucose units. Although polysaccharides can contain other sugars, we will study the ones that use glucose.
Starch and Glycogen
Starch and glycogen are ready storage forms of glucose in plants and animals, respectively. Some of the macromolecules in starch are long chains of up to 4,000 glucose units. Starch has fewer side branches, or chains of glucose that branch off from the main chain, than does glycogen, as shown in Figures 2.10 and 2.11. Flour, usually acquired by grinding wheat and used for baking, is high in starch, and so are potatoes. After we eat starchy foods such as potatoes, bread, and cake, glucose enters the bloodstream, and the liver stores glucose as glycogen. In between eating, the liver releases glucose so that the blood glucose concentration is always about 0.1%. If blood contains more glucose, it spills over into the urine, signaling that the condition diabetes mellitus exists.
The polysaccharide cellulose is found in plant cell walls. In cellulose, the glucose units are joined by a slightly different type of linkage from that in starch or glycogen. Although this might seem to be a technicality, actually it is important because humans are unable to digest foods containing this type of linkage; therefore, cellulose largely passes through our digestive tract as fiber, or roughage. It is believed that fiber in the diet is necessary to good health, and some researchers have suggested it may even help prevent colon cancer.
Figure 2.10 Starch structure and function. Starch has straight chains of glucose molecules. Some chains are also branched, as indicated. The electron micrograph shows starch granules in potato cells. Starch is the storage form of glucose in plants.
Figure 2.11 Glycogen structure and function. Glycogen is more branched than starch. The electron micrograph shows glycogen granules in liver cells. Glycogen is the storage form of glucose in humans.