Chapter 16: Carbohydrates

Food for Thought

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Courtesy Digital Stock/Corbis Images

Figure 16.1: The color produced by an iodine solution distinguishes the starch of a potato from the cellulose of an apple.

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Courtesy Ken Karp

A 5% solution of glucose, a carbohydrate also known as dextrose and blood sugar, supplies the body with energy as it flows intravenously into the bloodstream.

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Courtesy Yoav Levy/Phototake

Adding a little concentrated sulfuric acid to powdered sucrose produces heat, steam, and a black, brittle solid that resembles badly charred wood.

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Courtesy OPC, Inc.

Figure 16.2: A tetrahedral carbon bonded to four different groups: W, X. Y, and Z.

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Figure 16.3: Glyceraldehyde stereochemistry.

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A left hand is a mirror image of a right hand.

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Courtesy Andy Washnik

Figure 16.4: The two enantiomers of glyceraldehyde. The two molecules are nonsuperposable mirror images of each other.

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Figure 16.5a: This is a enantiomer held in the right hand of figure 16.4.

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Figure 16.5b: Here the molecule is rotated so that the top and bottom carbons lie behind or below the central, chiral carbon and the H – and – OH substituents lie in front of or above the chiral carbon.

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Figure 16.5c: The Fischer projection of the enantiomer of glyceraldehyde held in the right hand of Figure 16.4.

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Figure 16.5d: The Fischer projection of the enantiomer of glyceraldehyde held in the left hand of Figure 16.4.

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Figure 16.6: Superposition and nonsuperposition of Fischer projections.

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Figure 16.7: An apple bobbing in water.

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Figure 16.8: Unpolarized light.

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Figure 16.9: Plane-polarized light.

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Figure 16.10: Reflected glare is plane-polarized light.

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Figure 16.11: Polarizing sunglasses versus glare.

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Figure 16.12: The effect of polarizing lenses on unpolarized light.

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Courtesy Andy Washnik

Figure 16.13: The essentials of a polarimeter.

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Figure 16.14: The Fischer projection of the naturally occurring enantiomer of glucose.

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Figure 16.15: The Fischer projection of the naturally occurring enantiomer of fructose.

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Honey is largely invert sugar, a mixture of glucose and fructose.

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Courtesy Dutch Gold Honey

Figure 16.16: Formation of the cyclist structure of glucose.

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Figure 16.17: The two glucose rings.

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Figure 16.18: Two glucose molecules combine to form cellobiose and water.

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Figure 16.19: Beta-glucose and the chain of the cellulose molecule.

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Figure 16.20: Two glucose molecules combine to form maltose and water.

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Figure 16.21: Alpha-glucose and the chain of the starch molecule.

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Two polysaccharides of food: the starch of potatoes and the cellulose of cabbage.

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Courtesy Ken Karp

Fruits and vegetables provide us with plenty of fiber.

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Courtesy PhotoDisc Inc./Getty Images

Figure 16.22: Galatose, glucose, and lactose.

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Milk and milk products, such as cheese and ice cream, are rich sources of lactose.

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Courtesy PhotoDisc Inc./Getty Images

Figure 16.23: The lock-and-key analogy of enzyme action. Each key fits only its own lock.

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Animation

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Blood makes up about 8% of the body weight of a healthy adult. How many grams of glucose circulate through the body of a typical 110 pound person?

QUESTION

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The carbohydrate sucrose (table sugar) has the molecular formula C12H22O11. Rewrite this molecular formula as it would appear if sucrose were a true hydrate of carbon.

QUESTION

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Give the chemical names of two: (a) monosaccharides; (b) disaccharides; (c) polysaccharides.

QUESTION

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Suppose that the carbon of CWXYZ were square planar rather than tetrahedral. That is, suppose all five of the atoms lay in the same plane, with the carbon at the center and each of the substituent atoms at one of the corners of the square. Would a molecule with this stereochemistry be chiral? Explain.

QUESTION

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Suppose you had a pair of polarizing sunglasses with perfectly circular lenses. One of the circular lenses suddenly popped out and rolled a bit on the ground. You want to repair the glasses by snapping the lens back in the frame but you’re not sure what orientation to use. How would you go about determining the proper orientation of the lens before reinserting it into the frame?

QUESTION

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Knowing that the two enantiomers of a chiral compound rotate polarized light to exactly the same extent, but in opposite directions, what effect do you think a mixture of equal amounts of the two enantiomers of a chiral compound would have on plane polarized light? Mixtures of this kind are called racemates. Is a racemate optically active? What term used in Section 16.8 can be applied to a racemate to describe its effect on polarized light?

QUESTION

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You want to convert a portion of invert sugar into an optically inactive substance by increasing the amount of one of its monosaccharide components. Would you add naturally occurring glucose or fructose to the invert sugar? If you added just the right amount of the selected monosaccharide to achieve optical inactivity, would the resulting mixture be a racemate? Explain.

QUESTION

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How many chiral carbons are there in a molecule of the noncyclic form of glucose? How many are in a molecule of the cyclic form? What’s the reason for this difference?

QUESTION

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In what single way does the molecular structure of maltose differ from the molecular structure of cellobiose?

QUESTION

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What monosaccharide is produced by the action of both cellobiase and maltase on polysaccharides?

QUESTION

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Why might a baby who is born lactase-deficient not be able to obtain sufficient nourishment through nursing alone, without supplemental nutrition?

QUESTION

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Suppose that you added a few drops of a solution of an oxidizing agent, one that gives a positive test with a starch-iodide solution, to a solution of potassium iodide and a soluble form of cellulose. Based on the results of the opening demonstration, what do you think you would see?

QUESTION