bios 352 ch. 7

What are carbohydrates?

Polyhydroxy aldehydes or ketones, or compounds that yield them upon hydrolysis.

What is the common empirical formula of many carbohydrates?

(CH2O)n

What are the three carbohydrate size classes?

Monosaccharides, oligosaccharides, and polysaccharides.

What is a monosaccharide?

A carbohydrate containing one sugar unit.

What is an oligosaccharide?

A short chain of monosaccharides connected by glycosidic bonds.

What is a polysaccharide?

A carbohydrate polymer containing more than 20 monosaccharide units.

What are three major functions of carbohydrates?

Energy storage, structural support, and cell-cell signaling.

Where do carbohydrates ultimately originate?

CO2 fixation by autotrophs, mainly photoautotrophs.

What is an aldose?

A monosaccharide containing an aldehyde functional group.

What is a ketose?

A monosaccharide containing a ketone functional group.

What is a triose?

A monosaccharide containing three carbon atoms.

What is a tetrose?

A monosaccharide containing four carbon atoms.

What is a pentose?

A monosaccharide containing five carbon atoms.

What is a hexose?

A monosaccharide containing six carbon atoms.

What type of sugar is D-glucose?

An aldohexose.

What type of sugar is D-fructose?

A ketohexose.

What is the only aldotriose?

Glyceraldehyde.

What is the only ketotriose?

Dihydroxyacetone.

Which common monosaccharide has no chiral center?

Dihydroxyacetone.

What is an enantiomer?

A stereoisomer that is a nonsuperimposable mirror image.

What are D- and L-glyceraldehyde?

Enantiomers with opposite configurations at their single chiral center.

What is a Fischer projection?

A drawing with horizontal bonds toward the viewer and vertical bonds away.

How is D/L configuration determined in a Fischer projection?

By the OH position on the chiral carbon farthest from the carbonyl.

What does an OH on the right of the reference carbon indicate?

D configuration.

What does an OH on the left of the reference carbon indicate?

L configuration.

How many stereoisomers are possible with n chiral centers?

2^n stereoisomers.

How many stereoisomers are possible for an aldohexose?

16, because it has four chiral centers.

Which sugar stereoisomers predominate in living organisms?

D-hexoses.

What is a diastereomer?

A stereoisomer that is not a mirror image of another.

What is an epimer?

A sugar differing from another at exactly one stereocenter.

At which carbon do D-glucose and D-mannose differ?

C-2.

At which carbon do D-glucose and D-galactose differ?

C-4.

What is ribose?

A standard five-carbon aldopentose.

What is glucose?

A standard six-carbon aldohexose.

What is fructose?

The ketose form corresponding to glucose.

What forms when an alcohol attacks an aldehyde?

A hemiacetal.

What forms when an alcohol attacks a ketone?

A hemiketal.

What is cyclization of a monosaccharide?

Intramolecular ring formation through reaction of a hydroxyl with the carbonyl.

What is the anomeric carbon?

The former carbonyl carbon that becomes a new stereocenter upon cyclization.

Which carbon is anomeric in glucose?

C-1.

Which carbon is anomeric in fructose?

C-2.

What is an alpha anomer?

The anomeric OH is opposite (trans) to the CH2OH group.

What is a beta anomer?

The anomeric OH is on the same side (cis) as the CH2OH group.

What is a pyranose?

A sugar containing a six-membered oxygen-containing ring.

What is a furanose?

A sugar containing a five-membered oxygen-containing ring.

In standard Fischer-to-Haworth conversion, where does a right-side substituent go?

Down.

In standard Fischer-to-Haworth conversion, where does a left-side substituent go?

Up.

What is mutarotation?

Interconversion of alpha and beta anomers through ring opening and reclosure.

Which D-glucose anomer is more abundant in aqueous solution?

Beta-D-glucose, approximately 65%.

Approximately how much alpha-D-glucose exists at equilibrium in water?

33%.

What is a reducing sugar?

A sugar with a free anomeric hemiacetal or hemiketal that can yield a reactive carbonyl.

Why are aldoses reducing sugars?

Their cyclic forms can open to expose an aldehyde.

Why can ketoses test positive as reducing sugars?

They can interconvert with aldoses in basic solution.

What does Fehling's test detect?

Reducing sugars that reduce Cu2+ to Cu+.

What does Tollens' test detect?

Reducing sugars that reduce Ag+ to metallic Ag.

What is an O-glycosidic bond?

A linkage involving an anomeric carbon and an oxygen from another sugar's hydroxyl.

What does a 1→4 glycosidic bond mean?

C-1 of one sugar is joined to C-4 of another.

What is the reducing end of a disaccharide?

The sugar residue retaining a free anomeric carbon.

Why is a disaccharide nonreducing when both anomeric carbons are linked?

Neither sugar retains a free hemiacetal or hemiketal.

What is maltose?

A disaccharide containing two glucose residues joined by a 1→4 glycosidic bond.

What is lactose?

Milk sugar; a reducing disaccharide.

What is sucrose?

Table sugar; a nonreducing disaccharide.

What is trehalose?

A nonreducing disaccharide.

Which disaccharides are nonreducing in the lecture?

Sucrose and trehalose.

What is a homopolysaccharide?

A polymer composed of one type of monosaccharide.

What is a heteropolysaccharide?

A polymer containing more than one type of monosaccharide.

Why do polysaccharides lack a defined molecular weight?

Their synthesis does not use a template specifying exact chain length.

What is starch?

The main storage polysaccharide in plants.

What two glucose polymers make up starch?

Amylose and amylopectin.

What is amylose?

An unbranched glucose polymer with alpha(1→4) linkages.

What is amylopectin?

A branched glucose polymer with alpha(1→4) chains and alpha(1→6) branches.

How often does amylopectin branch?

Approximately every 24–30 glucose residues.

What is glycogen?

The main glucose storage polysaccharide in animals.

Where is glycogen mainly stored?

Liver and muscles.

What bonds form glycogen's main chains?

Alpha(1→4) glycosidic bonds.

What bonds form glycogen's branch points?

Alpha(1→6) glycosidic bonds.

How often does glycogen branch?

Approximately every 8–12 glucose residues.

Which is more highly branched, glycogen or amylopectin?

Glycogen.

Why are multiple nonreducing ends useful in glycogen?

Enzymes can process many chain ends simultaneously.

What are dextrans?

Branched glucose homopolysaccharides produced by bacteria and yeast.

What bonds form the main chains of dextrans?

Alpha(1→6) linkages.

What branch linkages are found in all dextrans?

Alpha(1→3) linkages.

What is cellulose?

A tough, water-insoluble, linear glucose polymer in plant cell walls.

What linkage is found in cellulose?

Beta(1→4) glucose linkages.

What stabilizes cellulose fibers?

Hydrogen bonds within and between glucose chains.

What is the most abundant polysaccharide in nature?

Cellulose.

Why can most animals not digest cellulose?

They lack enzymes that hydrolyze beta(1→4) glucose linkages.

What is cellulase?

An enzyme that hydrolyzes cellulose.

How do termites and ruminants use cellulose?

Symbiotic microorganisms provide cellulase.

What is chitin?

A structural polymer found in arthropod exoskeletons and fungal cell walls.

What is the monomer of chitin?

N-acetylglucosamine.

What glycosidic linkage is present in chitin?

Beta(1→4).

What is agar?

A mixture of heteropolysaccharides containing modified galactose units.

What is agar commonly used for in laboratories?

Gel surfaces for growing bacteria.

What is agarose commonly used for?

Gels for separating DNA by electrophoresis.

What are glycosaminoglycans (GAGs)?

Linear extracellular-matrix heteropolysaccharides of repeating disaccharides.

What amino sugars occur in glycosaminoglycans?

N-acetylglucosamine or N-acetylgalactosamine.

What negatively charged groups occur in glycosaminoglycans?

Uronic acid groups or sulfate esters.

Why do glycosaminoglycans form extended hydrated structures?

Their negative charges repel each other and associate with water.

Where are glycosaminoglycans found?

Animal extracellular matrices, including cartilage and tendons.