Chapter 16 Carbohydrates Questions

What is the chemical difference between a sugar phosphate and a sugar involved in a glycosidic bond?

In sugar phosphate, an ester bond is formed between one of the sugar hydroxyls and phosphoric acid. A glycosidic bond is an acetal, which can be hydrolyzed to regenerate the two original sugar hydorxyls

Two sugars are epimers of each other. Is it possible to convert one to the other without break covalent bonds

No. Converting a sugar to an epimer requires inversion of configuation at a chiral center.

Name two differences between sucrose and lactose. Name two similarities.

Similarities: sucrose and lactose are both disaccharides, and both contain glucose. Differences: sucrose contains fructose, whereas lactose contains galactose. sucrose has an alpha,beta(1-->2) glycosidic linkage, whereas lactose has a beta(1-->4) glycosidic linkage

Does the cyclization of sugars introduce a new chiral center?

Yes

pyranose

a cyclic sugar that contains a six membered ring

glycosidic bond

acetal linkage that joins two sugars

Some athletes eat diets high in carbohydrates before an event. Suggest a biochemical basis for this practice.

athletes try to increase their stores of glycogen before an event. the most direct way to increwase the amount of this polymer of glucose is to eat carbs

Glycogen is highly branched. What advantage, if any, does this provide an animal?

because of branching, the glycogen molecules gives rise to a number of available glucose molecules at a time when it is being hydrolyzed to provide energy. a linear molecule could produce only one available glucose at a time

How does glycogen differ from starch in structure and function?

both are vehicles of energy storage. glycogen in animasla dn starch in plants. mainly different in degree of chain branching

What are glycoproteins? What are some of their biochemical roles.

carbs that are covalently bonded to protein. role in eukaryotic cell membranes, frequenctly as recognition sites for external molecules. antibodies are glycoproteins

What are the main differences between the cell walls of plants and those of bacteria?

cell walls of plants contain cellulose, whereas those in bacteria consist mainly of polysaccharides with peptide crosslinks

How would you expect the active site of a cellulase to differ from the active site of an enzyme that degrades starch?

cellulase needs an active site that can recognize glucose residues joined in a beta glycosidic linkage and hydrolyze that linkage. an enzyme that degrades starch ahas the same requirements with regard to glucos residued joined in an alpha glycosidic linkage

What is the main structural difference between cellulose and starch?

cellulose and starch are polymers of glucose. in cellulose, the monomers are joined by beta glycosidic linkages. whereas starch is joined by alpha glycosidic linkage

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?

chitin has mechanical strength. individual polymer strands are cross linked by hydrogen bonding, accounting for stregnth. cellulose is similar

How does chitin differ from cellulose in structure and in function?

chitin is a polymer of N-acetyl-beta-D-glucosamine, whereas cellulose is a polymer of D-glucose. Both polymers play a structural role, but chitin occurs in the exoskeletons of invertebrates and cellulose primarily in plants

oligosaccharide

compound formed by the linkage of several simple sugars by glycosidic bonds

glycoprotein

covalent bonding of sugars to protein

Would you expect cross-linking to play a role in the structure of polysaccharides? If so, how would the cross-links be formed?

cross linking plays a roles where mechanism stregnth is an issue

furanose

cyclic sugar that contains a five-membered ring similar to that in furna

What is the difference between an enantiomer and a diastereomer

enantiomers are nonsuperimposable mirror image stereoisomers. diastereomers are nonsuperimposable non mirror image stereoisomers

What is the benefit of fiber in the diet?

fiber binds to many toxic substances in the gut and decreases the transit time of ingested food in the digestive tract, so that harmful compounds such as carcinogens are removed from the body more quickly than would be for the case witha low-fiber diet

How many possible epimers of D-glucose exist

four

How many chiral centers are there in the open-chain form of glucose? in the cyclic form?

four in open chain form;five in cyclic form - hemiacetal formation

What is the main structural difference between glycogen and starch?

glycogen exists as a highly branched polymer. Starch can have both a linear and a branched form, which is not as highly branched as that of glycogen

Blood samples for research or medical tests sometimes have heparin added. Why is this done?

heparin is an anticoagulant. it presence prevents blood clotting

How does the presence of alph bonds versus beta bonds influence the digestibility of glucose polymers by humans? There are two effects.

humans lack the enzyme to hydrolyze cellulose. in addition, the fibrous structure of cellulose make is insoluble to digest.

Why is it advantageous that polysaccharides can have branched chains? How do they achieve this structural feature?

it is necessary to release residues from the ends as quickly as possible. achieve by having 1,4 and 1,6 glycosidic linkages

Why are furanoses and pyranoses the most common cyclic forms of sugars

most stable and most readily formed

Could bacterial cell walls consist largely of protein? Why or why ot?

no. polysaccharides are easily formed and confer considerable mechanicalstrength. they are likely to play a large role

How do the cell walls of bacteria differ form those of plants?

plant cell wallls almost exclusovely of carbs, whereas bacterial cell walls contains peptides

Polysaccharide

polymer of simple sugars

Define the term reducing sugar.

reducing sugar has a free aldehyde group. the aldehyde is easily oxidized, thus reducing the oxidizing agent.

What is the metabolic basis for the observation that many adults cannot ingest large quantities of milk without developing gastric difficulties?

some lack the enzyme that degrades lactose. the enzyme in other cases, the enzyme isomerizes galactose to glucose for further metabolic breakdown

ketose

sugar that contains a keton egroup

aldose

sugar that contains an aldehyde group

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?

the concentration of reducing groups is too small to detect

No animal can digest cellulose. Reconcile this statement with the fact that many animals are herbivores that depend heavily on cellulose as a food source.

the digestive tract of these animals contains bacteria that have the enzyme to hydrolyze cellulose

How do the sites of cleavage of starch differ from one another when the cleavage reaction is catalyzed by alpha-amylase and beta-amylase?

the enzyme beta-amylase is an exoglycosidase, degrading polysaccharides from the ends. the enzyme alpha amylase is an endoglycosidase, cleaving interal glycosidic bonds

Compare the information in the sequence of monomers in a polysaccharide with that in the sequence of amino acid residue sin a protein.

the seuqences of monomers in a polysaccharide is not genetically coded, and, in this sense, it does not contain information

Briefly indicate the role of glycoproteins as antigenic determinants for blood groups

the sugar portion of blood-group glycoproteins are the source of antigenic difference

Explain how the minor structural difference btween alpha and bet-glucose is realted to the differences in sutrcutre and function in the polymers formed from these two monomers.

Differences in structure: celluloe consists of linear fibers, starch has a coil form. Difference in function: cellulose has a structural role, but starch is used for energy storage

Advertisements for food supplement to be taken by athletes claimed that the energy bars contained the two best precursors of glycogen. What were they?

GLucose and Fructose