Chapter 8 Sugar: The Simplest of Carbohydrates 137 When scientists create diagrams of any ring-shaped organic compound, they simplify the drawing. They use the junction of lines to represent the location of a carbon atom. They draw lines where hydrogen atoms are located, but they do not write the letter H. In the following example, the diagram on the left shows glucose as a ring with all the atoms represented by letters. The diagram on the right is the shorthand version of the same molecule. C — O C — C OH OH OH H OHH — C C H — H H — C — H OH — H CH2OH C — O OH OH C C C — C OH OH All monosaccharides occur most frequently in a ring structure that contains five carbon atoms and one oxygen atom. They differ in the way the hydrogen and oxygen atoms are arranged around the ring. The difference in position affects the characteristics of the sugar and how it will respond in food preparation. To understand the importance of the arrange- ment of the atoms, look at two basic forms of glucose: α–D-glucose and β–D-glucose. As you can see from the following diagram, the alpha (α) glucose and beta (β) glucose have only one difference. That is the position of the -OH group on the right side of the ring. This one change in position determines whether the body is able to digest the sugar. Alpha-glucose is the basic energy source for humans. Beta-glucose is the main component of dietary fiber that provides bulk for the digestive track, but it provides no nutritive value because people cannot digest it. CH2OH OH OH C C CH2OH OH OH C C C — C OH OH C — C OH OH C — O C — O α - D - glucose β - D - glucose So far, the simple sugars described have a basic formula of C6H12O6. There are also some sugars that contain only five carbon atoms. These are riboses. The body uses them as the basic building blocks for ribonucleic acid (RNA). RNA carries the genetic code from DNA and is used for making protein. Disaccharides Sugars found in nature do not normally occur as monosaccharides. The sugar molecule’s structure enables it to readily combine with other sugars to form chains. A disaccharide is two joined monosaccharides. Most sugars consumed in the world are disaccharides. Sucrose, or table sugar, is a disaccharide that contains one glucose molecule and one fructose molecule. Other disaccharides found in the food supply are maltose and lactose. Maltose is commonly found in malted grains. It is made of two glucose molecules. This disaccharide is the least sweet. When in powder form, it is tan rather than clear or white like sucrose. Lactose is the sugar found in milk. It is composed of one glucose molecule and one galactose molecule. In pure form, it is white and contributes some of the color you associate with milk. When two monosaccharides join, a hydroxyl group from one and a hydrogen atom from the other separate to form water. See 8-4. This process of molecules joining and releasing water is reversible. Hydrolysis occurs when a large molecule, such as sugar, is divided into smaller parts by adding water. If you hydrolyze a molecule of sucrose, you will get one molecule of fructose and one molecule of glucose. The body uses this hydrolysis process to digest the disaccharides in food. For hydrolysis to occur, water must be present. This is why sweetened drinks are not as thirst quenching as plain water. Part of the water is used to digest the sugar and is not available for other functions. Three conditions can trigger hydrolysis. One condition is the presence of an enzyme to set off the reaction. A second condition is the addition of an acid. A third condition is the addition of heat. Digestion of sugar involves the addition of the enzyme sucrase. This enzyme is present in saliva. Thorough chewing of food enables the enzyme to be mixed into the food so the enzyme can work quickly. Lactose-free milk is produced by adding an enzyme to hydrolyze the sugar. Each type of sugar requires a different enzyme for hydrolysis to occur. You can determine the names of the enzymes by simply changing the -ose ending of the sugar to -ase. For instance, sucrase hydrolyzes sucrose and lactase hydrolyzes lactose. Sucrase is also known as invertase. This is because the fructose and glucose mixture that results from the hydrolysis of sucrose is sometimes called invert sugar. Copyright Goodheart-Willcox Co., Inc.