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Chapter Questions
Define the following terms: polysaccharide, furanose, pyranose, aldose, ketose, glycosidic bond, oligosaccharide, glycoprotein.
Name which, if any, of the following are epimers of D-glucose: D-mannose, D-galactose, D-ribose.
Name which, if any, of the following groups are not aldoseketose pairs: D-ribose and D-ribulose, D-glucose and D-fructose, D-glyceraldehyde and dihydroxyacetone.
What is the difference between an enantiomer and a d-iastereomer?
How many possible epimers of n-glucose exist?
Why are furanoses and pyranoses the most common cyclic forms of sugars?
How many chiral centers are there in the open-chain form of glucose? In the cyclic form?
Following are Fischer projections for a group of five-carbon sugars, all of which are aldopentoses. Identify the pairs that are enantiomers and the pairs that are epimers. (The sugars shown here are not all of the possible five-carbon sugars.)
The sugar alcohol often used in "sugarless" gums and candies is t-sorbitol. Much of this alcohol is prepared by reduction of $\mathrm{D}$ -glucose. Compare these two structures and explain how this can be.
Consider the structures of arabinose and ribose. Explain why nucleotide derivatives of arabinose, such as ara$\mathrm{C}$ and ara-A, are effective metabolic poisons.
Two sugars are epimers of each other. Is it possible to convert one to the other without breaking covalent bonds?
How does the cyclization of sugars introduce a new chiral center?
Convert the following Haworth projections to a Fischer projection. Name the monosaccharides you have drawn.
Convert each of the following chair conformations to an open-chain from and to a Fischer projection. Name the monosaccharides you have drawn.
Starting with a Fischer projection of D-fructose, write equations showing the formation of $\alpha$ -p-fructopyranose, $\alpha$ -D-fructofuranose, $\beta$ -D-fructopyranose, and $\beta$ -D-fructofuranose.
Starting with the open-chain form of D-ribose, write equations for the cyclization reactions that form the pyranose and the furanose forms.
What is unusual about the structure of $N$ -acetylmuramic acid (Figure 16.18 ) compared with the structures of other carbohydrates?
What is the chemical difference between a sugar phosphate and a sugar involved in a glycosidic bond?
Define the term reducing sugar
What are the structural differences between vitamin C and sugars? Do these structural differences play a role in the susceptibility of this vitamin to air oxidation?
Name two differences between sucrose and lactose. Name two similarities.
Draw a Haworth projection for the disaccharide gentibiose, given the following information:(a) It is a dimer of glucose.(b) The glycosidic linkage is $\beta(1 \rightarrow 6)$(c) The anomeric carbon not involved in the glycosidic linkage is in the $\alpha$ configuration.
What is the metabolic basis for the observation that many adults cannot ingest large quantities of milk without developing gastric difficulties?
Draw Haworth projection formulas for dimers of glucose with the following types of glycosidic linkages:(a) $A \beta(1 \rightarrow 4)$ linkage (both molecules of glucose in the $\beta$ form)(b) $\operatorname{An} \alpha, \alpha(1 \rightarrow 1)$ linkage(c) $\mathrm{A} \beta(1 \rightarrow 6)$ linkage (both molecules of glucose in the $\beta$ form)
A friend asks you why some parents at her child's school want a choice of beverages served at lunch, rather than milk alone. What do you tell your friend?
What are some of the main differences between the cell walls of plants and those of bacteria?
How does chitin differ from cellulose in structure and function?
How does glycogen differ from starch in structure and function?
What is the main structural difference between cellulose and starch?
What is the main structural difference between glycogen and starch?
How do the cell walls of bacteria differ from those of plants?
Pectin, which occurs in plant cell walls, exists in nature as a polymer of b-galacturonic acid methylated at carbon 6 of the monomer. Draw a Haworth projection for a repeating disaccharide unit of pectin with one methylated and one unmethylated monomer unit in $\alpha(1 \rightarrow 4)$ linkage.
Advertisements for a food supplement to be taken by athletes claimed that the energy bars contained the two best precursors of glycogen. What were they?
Explain how the minor structural difference between $\alpha$ - and $\beta$ -glucose is related to the differences in structure and function in the polymers formed from these twomonomers.
All naturally occurring polysaccharides have one terminal residue, which contains a free anomeric carbon. Why do these polysaccharides not give a positive chemical test for a reducing sugar?
An amylose chain is 5000 glucose units long. At how many places must it be cleaved to reduce the average length to 2500 units? To 1000 units? To 200 units? What percentage of the glycosidic links are hydrolyzed in each case? (Even partial hydrolysis can drastically alter the physical properties of polysaccharides and thus affect their structural role in organisms.)
Suppose that a polymer of glucose with alternating $\alpha(1 \rightarrow 4)$ and $\beta(1 \rightarrow 4)$ glycosidic linkages has just been discovered. Draw a Haworth projection for a repeating tetramer (two repeating dimers) of such a polysaccharide. Would you expect this polymer to have primarily a structural role or an energy-storage role in organisms? What sort of organisms, if any, could use this polysaccharide as a food source?
Glycogen is highly branched. What advantage, if any, does this provide an animal?
How does the presence of $\alpha$ -bonds versus $\beta$ bonds influence the digestibility of glucose polymers by humans? Hint: There are $t w o$ effects.
How do the sites of cleavage of starch differ from one another when the cleavage reaction is catalyzed by $\alpha$ amylase and $\beta$ -amylase?
What is the benefit of fiber in the diet?
How would you expect the active site of a cellulase to differ from the active site of an enzyme that degrades starch?
Would you expect cross-linking to play a role in the structure of polysaccharides? If so, how would the cross-links be formed?
Compare the information in the sequence of monomers in a polysaccharide with that in the sequence of amino acid residues in a protein.
Why is it advantageous that polysaccharides can have branched chains? How do they achieve this structural feature?
Why is the polysaccharide chitin a suitable material for the exoskeleton of invertebrates such as lobsters? What other sort of material can play a similar role?
Could bacterial cell walls consist largely of protein? Why or why not?
Some athletes eat diets high in carbohydrates before an event. Suggest a biochemical basis for this practice.
You are a teaching assistant in a general chemistry lab. The next experiment is to be an oxidation-reduction titration involving iodine. You get a starch indicator from the stockroom. Why do you need it?
Blood samples for research or medical tests sometimes have heparin added. Why is this done?
Based on what you know about glycosidic bonds, propose a scheme for formation of covalent bonds between the carbohydrate and protein portions of glycoproteins.
What are glycoproteins? What are some of their biochemical roles?
Briefly indicate the role of glycoproteins as antigenic determinants for blood groups.