The Simple Carbohydrates

The following list of the six sugars most important in nutrition symbolizes them as hexagons and pentagons of different colors. Three are single sugars or monosaccharides:

  • Glucose. ^^ • Fructose. ^^ • Galactose. ^^ Three are double sugars or disaccharides:
  • Maltose (glucose + glucose).
  • Sucrose (glucose + fructose).
  • Lactose (glucose + galactose).

Monosaccharides

The three monosaccharides important in nutrition all have the same numbers and kinds of atoms, but in different arrangements. These chemical differences account for the differing sweetness of the monosaccharides. A pinch of purified glucose on the tongue gives only a mild sweet flavor, and galactose hardly tastes sweet at all, but fructose is as intensely sweet as honey and, in fact, is the sugar primarily responsible for honey's sweetness.

• Glucose • Chemically, glucose is a larger and more complicated molecule than ethyl alcohol, but it obeys the same rules of chemistry: each carbon atom has four bonds; each oxygen, two bonds; and each hydrogen, one bond. Figure 4-2 illustrates the chemical structure of a glucose molecule.

The diagram of a glucose molecule shows all the relationships between the atoms and proves simple on examination, but chemists have adopted even simpler ways to depict chemical structures. Figure 4-3 shows that a chemical struc-

HO OH H OH

The carbons at the corners are not shown, and the formula CH2OH stands for the structure in Figure 4-2.

Now the single hydrogens are not shown, but lines still extend upward or downward from the ring to show where they belong.

Another way to look at glucose is to notice that its six carbon atoms are all connected.

In this and other illustrations throughout this book, glucose is represented as a blue hexagon.

CH2OH

CH2OH

Numbering Carbon Atoms Glucose
Glucose

OH Fructose

Figure 4-4

Two Monosaccharides: Glucose and Fructose

Can you see the similarities? If you learned the rules in Figure 4-3, you will be able to "see" 6 carbons (numbered), 12 hydrogens (those shown plus one at the end of each single line), and 6 oxygens in both these compounds.

ture can combine or omit a number of letters without losing the information it conveys.

The significance of glucose to nutrition is tremendous. Glucose is one of the two sugars in every disaccharide and is the unit from which the polysaccharides are made almost exclusively. One of these polysaccharides, starch, is the chief energy food of the world's people; another, glycogen, is a major storage form of energy in the body. Glucose reappears frequently throughout this chapter and all those that follow.

  • Fructose • Fructose is the sweetest of the sugars. Curiously, fructose has exactly the same chemical formula as glucose—C6H12O6—but its structure differs (see Figure 4-4). The arrangement of the atoms in fructose stimulates the taste buds on the tongue to produce the sweet sensation. Fructose occurs naturally in fruits and honey; food manufacturers also use it in products sweetened with high-fructose corn syrup (HFCS), a corn product used as an additive.
  • Galactose • Seldom occurring free in nature, galactose binds with another monosaccharide to form the sugar in milk. Galactose has the same numbers and kinds of atoms as glucose and fructose in yet another arrangement. Figure 4-5 shows galactose beside a molecule of glucose for comparison.

Disaccharides

The disaccharides are pairs of the three sugars just discussed. Glucose occurs in all three; the second member of the pair is either fructose, galactose, or another glucose. These carbohydrates and all the other energy nutrients are put together and taken apart by similar chemical reactions.

• Condensation • To make a disaccharide, a chemical reaction known as condensation links two monosaccharides together (see Figure 4-6 on p. 94). A hydroxyl (OH) group from one monosaccharide and a hydrogen atom (H) from the other combine to create a molecule of water (H2O). The two originally separate monosaccharides link together with a single oxygen (O).

fructose (FRUK-tose or FROOK-tose): a monosaccharide; sometimes known as fruit sugar or levulose, fructose is found abundantly in fruits, honey, and saps.

  • fruct = fruit
  • fe = fructose galactose (ga-LAK-tose): a monosaccharide; part of the disaccharide lactose. • ^^ = galactose disaccharide (dye-SACK-uh-ride): a pair of monosaccharides linked together. See Appendix C for the chemical structures of the disaccharides. • di = two condensation: a chemical reaction in which two reactants combine to yield a larger product.

Glucose

Figure 4-5

Two Monosaccharides: Glucose and Galactose

Notice the similarities and the difference.

Glucose

OH Galactose

Figure 4-6

Condensation of Two Monosaccharides to Form a Disaccharide

Condensation Monosaccharides

Glucose + glucose

An OH group from one glucose and an H atom from another glucose combine to create a

Maltose

The two glucose molecules bond together with a single O atom to form the disaccharide maltose.

Glucose + glucose

An OH group from one glucose and an H atom from another glucose combine to create a

Maltose

The two glucose molecules bond together with a single O atom to form the disaccharide maltose.

molecule of H2O

Reminder: A hydrolysis reaction splits a major reactant into two products, with H added to one and OH to the other (from water).

maltose (MAWL-tose): a disaccharide composed of two glucose units; sometimes known as malt sugar. • ^ = maltose sucrose (SUE-krose): a disaccharide composed of glucose and fructose; commonly known as table sugar, beet sugar, or cane sugar. Sucrose also occurs in many fruits and some vegetables and grains.

  • sucro = sugar
  • V^^ = sucrose lactose (LAK-tose): a disaccharide composed of glucose and galactose; commonly known as milk sugar.
  • lact = milk
  • lactose
  • Hydrolysis • To break a disaccharide in two, a chemical reaction known as hydrolysis occurs (see Figure 4-7). A molecule of water splits to provide the H and OH needed to complete the resulting monosaccharides. Hydrolysis reactions commonly occur during digestion.
  • Maltose • The disaccharide maltose consists of two glucose units. Maltose is produced whenever starch breaks down—as happens in plants when seeds germinate and in human beings during carbohydrate digestion. It also occurs during the fermentation process that yields alcohol. Maltose is only a minor constituent of a few foods.
  • Sucrose • Fructose and glucose together form sucrose. Because the fructose is in a position accessible to the taste receptors, sucrose tastes sweet, accounting for some of the natural sweetness of fruits, vegetables, and grains. To make table sugar, sucrose is refined from the juices of sugarcane and sugar beets, then granulated. Depending on the extent to which it is refined, the product becomes the familiar brown, white, and powdered sugars available at grocery stores.
  • Lactose • The combination of galactose and glucose makes the disaccharide lactose, the principal carbohydrate of milk. Known as milk sugar, lactose contributes about 5 percent of milk's weight. Depending on the milk's fat content, lactose contributes 30 to 50 percent of milk's energy.

IN SUMMARY

Six simple carbohydrates, or sugars, are important in nutrition. The three monosaccharides (glucose, fructose, and galactose) all have the same chemical formula (C6H12O6), but their structures differ. The three disaccharides (sucrose, lactose, and maltose) are pairs of monosaccharides.

Figure 4-7

Hydrolysis of a Disaccharide

Hydrolysis occurs during digestion.

Bond broken

Bond broken

Bond broken nu nu nu nu nu nu nu nu

OH OH

OH OH

Maltose -► Glucose + glucose

The disaccharide maltose splits into two glucose molecules with H added to one and OH to the other (from water).

The sugars derive primarily from plants, except for lactose and its component galactose, which come from milk and milk products. Two monosaccharides can be linked together by a condensation reaction to form a disaccharide and water. A disaccharide, in turn, can be broken into its two monosaccharides by a hydrolysis reaction using water.

Food Allergies

Food Allergies

Peanuts can leave you breathless. Cat dander can lead to itchy eyes, a stuffy nose, coughing and sneezing. And most of us have suffered through those seasonal allergies with horrible pollen counts. Learn more...

Get My Free Ebook


Responses

  • Nicole
    Why hydrogens are not shown in sugars?
    7 years ago
  • david
    Why monosaccharides important in nutrition?
    7 years ago
  • geneva
    How do two monosaccharides form a disaccharide?
    7 years ago
  • rachel
    Why is glucosesymbolized by hexagons?
    7 years ago
  • adalrico
    What are the important facts about simple carbohydrates?
    6 years ago
  • kristin
    What is the most important monosaccharide?
    4 months ago

Post a comment