Fox: Human Physiology, Eighth Edition
2. Chemical Composition of the Body
© The McGraw-H Companies, 2003
Chapter Two isomers glucose, fructose, and galactose, for example, are mono-saccharides that have the same ratio of atoms arranged in slightly different ways (fig. 2.13).
Two monosaccharides can be joined covalently to form a di-saccharide, or double sugar. Common disaccharides include table sugar, or sucrose (composed of glucose and fructose); milk sugar,
■ Figure 2.13 Structural formulas for three hexose sugars. These are (a) glucose, (b) galactose, and (c) fructose. All three have the same ratio of atoms—C6H| 2O6i The representations on the left more clearly show the atoms in each molecule, while the ring structures on the right more accurately reflect the way these atoms are arranged.
or lactose (composed of glucose and galactose); and malt sugar, or maltose (composed of two glucose molecules). When numerous monosaccharides are joined together, the resulting molecule is called a polysaccharide. Starch, for example, a polysaccharide found in many plants, is formed by the bonding together of thousands of glucose subunits. Glycogen (animal starch), found in the liver and muscles, likewise consists of repeating glucose molecules, but it is more highly branched than plant starch (fig. 2.14).
Many cells store carbohydrates for use as an energy source, as described in chapter 5. If a cell were to store many thousands of separate monosaccharide molecules, however, their high concentration would draw an excessive amount of water into the cell, damaging or even killing it. The net movement of water through membranes is called osmosis, and is discussed in chapter 6. Cells that store carbohydrates for energy minimize this osmotic damage by instead joining the glucose molecules together to form the polysaccharides starch or glycogen. Since there are fewer of these larger molecules, less water is drawn into the cell by osmosis (see chapter 6).
In the formation of disaccharides and polysaccharides, the separate subunits (monosaccharides) are bonded together covalently by a type of reaction called dehydration synthesis, or condensation. In this reaction, which requires the participation of specific enzymes (chapter 4), a hydrogen atom is removed from one monosaccharide and a hydroxyl group (OH) is removed from another. As a covalent bond is formed between the two mono-saccharides, water (H2O) is produced. Dehydration synthesis reactions are illustrated in figure 2.15.
When a person eats disaccharides or polysaccharides, or when the stored glycogen in the liver and muscles is to be used by tissue cells, the covalent bonds that join monosaccharides to form disaccharides and polysaccharides must be broken. These digestion reactions occur by means of hydrolysis. Hydrolysis (from the Greek hydro = water; lysis = break) is the reverse of dehydration synthesis. When a covalent bond joining two monosaccha-rides is broken, a water molecule provides the atoms needed to complete their structure. The water molecule is split, and the resulting hydrogen atom is added to one of the free glucose molecules as the hydroxyl group is added to the other (fig. 2.16).
When a potato is eaten, the starch within it is hydrolyzed into separate glucose molecules within the small intestine. This glucose is absorbed into the blood and carried to the tissues. Some tissue cells may use this glucose for energy. Liver and muscles, however, can store excess glucose in the form of glycogen by dehydration synthesis reactions in these cells. During fasting or prolonged exercise, the liver can add glucose to the blood through hydrolysis of its stored glycogen.
Dehydration synthesis and hydrolysis reactions do not occur spontaneously; they require the action of specific enzymes. Similar reactions, in the presence of other enzymes, build and break down lipids, proteins, and nucleic acids. In general, therefore, hydrolysis reactions digest molecules into their subunits, and dehydration synthesis reactions build larger molecules by the bonding together of their subunits.
Chemical Composition of the Body
■ Figure 2.14 The structure of glycogen. Glycogen is a polysaccharide composed of glucose subunits joined together to form a large, highly branched molecule.
■ Figure 2.15 Dehydration synthesis of disaccharides. The two disaccharides formed here are (a) maltose and (b) sucrose (table sugar). Notice that a molecule of water is produced as the disaccharides are formed.
Glucose + Glucose
■ Figure 2.16 The hydrolysis of starch. The polysaccharide is first hydrolyzed into (a) disaccharides (maltose) and then into (b) monosaccharides (glucose). Notice that as the covalent bond between the subunits breaks, a molecule of water is split. In this way, the hydrogen atom and hydroxyl group from the water are added to the ends of the released subunits.
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