Chapter 5 Questions (end of chapter)

Carbohydrates are energy-storage molecules, so minimizing their consumption may reduce total energy intake. Lack of available carbohydrate also forces the body to use fats for energy, reducing the amount of fat that is stored.

A weight-loss program for humans that emphasized minimal consumption of carbohydrates was popular in some countries in the early 2000s. What was the logic behind this diet? (Note: This diet plan caused controversy and is not endorsed by some physicians and researchers).

Starch and glycogen both consist of glucose monomers joined by α-1,4-glycosidic linkages, and both function as storage carbohydrates. Starch is a mixture of unbranched and branched polysaccharides - called amylose and amylopectin, respectively. All glycogen polysaccharides are branched.

Compare and contract the structures and functions of starch and glycogen. How are these molecules similar? How are they different?

Carbohydrates are ideal for displaying the identity of the cell because they are so diverse structurally. This diversity enables them to serve as very specific identity tags for cells.

Explain how the structure of carbohydrates supports their function in displaying the identity of a cell.

d; lactose is a disaccharide of glucose and galactose, which can be cleaved by enzymes expressed in the human gut to release galactose.

Galactosemia is a potentially fatal disease that occurs in humans who lack the enzyme that converts galactose to glucose. To treat this disease, physicians exclude the monosaccharide galactose from the diet. Which of the following would you also predict to be excluded from the diet? a) maltose b) starch c) mannose d) lactose

Because (1) no mechanism is known for the prebiotic polymerization of sugars; (2) no catalytic carbohydrates have been discovered that can perform polymerization reactions; and (3) sugar residues in a polysaccharide are not capable of complementary base pairing.

Give three reasons why researchers have concluded that polysaccharides were unlikely to play a large role in the origin of life.

Amylase breaks down the starch in the cracker into glucose monomers, which stimulate the sweet receptors in your tongue.

If you hold a salty cracker in your mouth long enough, it will begin to taste sweet. What is responsible for this change in taste?

When bacteria contacts lysozyme, the peptidoglycan in their cell walls begins to degrade, leading to the death of the bacteria. Lysozyme therefore helps protect humans against bacterial infections.

Lysozyme, an enzyme found in human saliva, tears, and other secretions, catalyzes, the hydrolysis of the β-1, 4-glycosidic linkages in peptidoglycan. Predict the effect of this enzyme on bacteria, and explain the role its activity plays in human health.

a

What are the primary functions of carbohydrates in cells? a) energy storage, cell identity, structure, and building blocks for synthesis b) catalysis, structure, and energy storage c) information storage and catalysis d) source of carbon, information storage, and energy storage

Monosaccharides can differ from one another in three ways: (1) the location of their carbonyl group; (2) the number of carbon atoms they contain; and (3) the orientations of their hydroxyl groups.

What are three ways monosaccharides differ from one another?

c

What holds cellulose molecules together in bundles large enough to form fibers? a) the cell wall b) peptide bonds c) hydrogen bonds d) hydrophobic interactions between different residues in the cellulose helix

The electrons in the C=O bonds of carbon dioxide molecules are held tightly by the highly electronegative oxygen atoms, so they have low potential energy. The electrons in the C-C and C-H bonds of carbohydrates are shared equally, so they have much higher potential energy.

What is responsible for the difference in potential energy between carbohydrates and carbon dioxide?

d

What is the difference between a monosaccharide, an oligosaccharide, and a polysaccharide? a) the number of carbon atoms in the molecule b) the type of glycosidic linkage between monomers c) the spatial arrangement of the various hydroxyl residues in the molecule d) the number of monomers in the molecule

When you compare the glucose monomers in an α-1,4-glycosidic linkage versus in a β-1,4-glycosidic linkage, the linkages are located on opposite sides of the plane of the glucose rings, and the glucose monomers are linked in the same orientation versus having energy other glucose flipped in orientation. β-1,4-glycosidic linkages are much more likely to form linear fibers and sheets, so they resist degradation.

What is the difference between linking glucose molecules with α-1, 4-glycosidic linkages versus β-1, 4-glycosidic linkages? What are the consequences?

a

What type of bond is formed between two sugars in a disaccharide? a) glycosidic linkage b) phosphodiester bond c) peptide bond d) hydrogen bond

a; lactose is a disaccharide formed from a β-1,4-glycosidic linkage, so if two glucose molecules were linked with this bond, they would resemble units of cellulose and not be digested by human infants or adults.

What would most likely occur if the galactose in lactose were replaced with glucose? a) It would not be digested by human infants or adults. b) It would be digested by most adult humans. c) It would be digested by human infants, but not adults. d) It would be digested by human adults, but not infants.

c

Which of the differences listed here could be found in the same monosaccharide? a) different orientation of a hydroxyl in the linear form b) different number of carbons c) different orientation of a hydroxyl in the ring form d) different position of the carbonyl group in the linear form