Ch 4 - Carbohydrate Structure and Function

Question 1

How are carbohydrates organized?

Answer 1

by their number of carbon atoms and functional groups

Question 2

What are 3 carbon sugars called? 4 carbon?

Answer 2

trioses

tetroses

Question 3

What are sugars with aldehydes as their most oxidized groups? ketones?

Answer 3

a: aldoses
k: ketoses

Question 4

What is the nomenclature of all sugars based on?

Answer 4

the D and L form of glyceraldehyde
- sugars with the highest numbered chiral carbon with the OH group on the right are D sugars
- those with the OH on the left are L sugars
D and L forms of the same sugar are enantiomers

Question 5

What are diastereomers?

Answer 5

• nonsuperimposable configurations of molecules with similar connectivity
• differe at at least one - but not all - chiral carbons

Question 6

What are epimers?

Answer 6

subtype of diastereomers that differ at exactly one chiral carbon
- have different chemical and physical properties and their optical activities have no relation to each other

Question 7

What are anomers?

Answer 7

subtype of epimers that differ at the anomeric carbon

Question 8

What does cyclization describe?

Answer 8

the ring formation of carbohydrates from their straight-chain forms

Question 9

What conformation can that anomeric carbon take when rings form?

Answer 9

• alpha anomers: have the OH on the anomeric carbon trans to the free -CH2OH group
• beta anomers: have the OH on the anomeric carbon cis to the free -CH2OH group

Question 10

What is the anomeric carbon?

Answer 10

• the new chiral center formed in right closure

- was the carbon containing the carbonyl in the strain-chain form

Question 11

What do the Haworth projections provide?

Answer 11

a good way to represent 3D structure

Question 12

How do cyclic compounds under mutarotation?

Answer 12

they shift from one anomeric form to another with the straight-chain form as an intermediate (alpha to beta)

Question 13

What are monosaccharides?

Answer 13

• single carbohydrate units, with glucose as the most commonly observed monomer

Question 14

What 3 reactions can monosaccharides undergo?

Answer 14

• oxidation-reduction
• esterification: sugars reacting with carboxylic acids and their derivatives to form esters
• glycoside formation: basis for building complex carbohydrates and requires the anomeric carbon to link to another sugar

Question 15

What can aldoses be oxidized and reduced to?

Answer 15

• ox: aldonic acids

- red: alditols

Question 16

What are sugars that can be oxidized and how can they be detected?

Answer 16

• reducing agents (sugars)

- can be detected by reacting with Tollens’ or Benedict’s reagents

Question 17

What are sugars with an - replacing an -OH called?

Answer 17

deoxy sugars

Question 18

What is phosphorylation?

Answer 18

phosphate ester is formed by transferring a phosphate group from ATP onto a sugar

Question 19

How are disaccharides formed?

Answer 19

as a result of glycosidic bonding between 2 monosaccharides subunits

Question 20

How are polysaccharides formed?

Answer 20

by repeated monosaccharides or polysaccharide glycosidic bonding

Question 21

What are 3 common disaccharides?

Answer 21

• sucrose (glucose-beta-1,2-fructose)
• lactose (galactose-beta-1,4-glucose)
• maltose (glucose-alpha-1,4-glucose)

Question 22

What is cellulose?

Answer 22

the main structural component for plant cell walls and is a main source of fiber in the human diet

Question 23

What is the difference between starches (amylose and amylopectin) and glycogen?

Answer 23

• starch: function as a main energy storage form for plants

- glycogen: functions a main energy storage for animals

Question 24

How many stereoisomers does glucose have when in a straight chain formation?

Answer 24

glucose has 4 chiral centers
stereoisomers = 2^n where n is the number of chiral centers
2^4 = 16

Question 25

When do aldonic acids form?

Answer 25

- after the aldehyde group on a reducing sugar reduces another compound, becoming oxidized in the process

Question 26

What happens when ketose sugars undergo tautomerization?

Answer 26

- tautomerization is a rearrangement of bonds | - ketose sugars do it to undergo keto-enol shift forming an aldose, which then allows them to act as reducing sugars

Question 27

What happens when glucose reacts with methanol under acid catalysis?

Answer 27

the hemiacetal is converted to an acetal via replacement of the anomeric hydroxyl group with an alkoxy group

Question 28

Which enzyme cleaves polysaccharide chains and yields maltose exclusively?

Answer 28

beta maltose

Question 29

Where does alpha maltose cleave and what does it yield?

Answer 29

cleaves amylose anywhere along the chain to yield short polysaccharides

Question 30

What do debranching enzymes remove?

Answer 30

oligosaccharides from a branch in glycogen or starches

Question 31

What does glycogen phosphorylase yield?

Answer 31

glucose 1-phosphate

Question 32

Why is the alpha-anomer of D glucose less likely to form than the beta-anomer?

Answer 32

the beta-anomer undergoes less electron repulsion - the hydroxyl group on the anomeric carbon of the beta-anomer is equatorial, thereby creating less non bonded strain than the alpha-anomer, which has the hydroxyl group of the anomeric carbon in axial position

Question 33

Which polysaccharides share all of their glycosidic linkage types in common?

Answer 33

- glycogen and amylopectin are the only polysaccharide forms that demonstrate branching structure, making them the most similar in terms of linkage - both use alpha-1,4 and alpha-1,6 linkages

Question 34

Why is maltose digestible by humans compared to cellobiose?

Answer 34

because the beta-glycosidic linkage in cellobiose cannot be cleaved in the human body

Question 35

What are the cyclic forms of monosaccharides?

Answer 35

hemiacetals and hemiketals, depending on whether they are aldoses or ketoses

Question 36

What is the name of a 5-carbon sugar with an aldehyde group? 6 carbon sugar with a ketone group?

Answer 36

aldopentose | ketohexose

Question 37

Explain the relationship between the carbonyl carbon, anomeric carbon, and the alpha and beta forms of a sugar molecule?

Answer 37

- during hemiacetal and hemiketal formation, the carbonyl carbon becomes chiral and is termed the anomeric carbon - the orientation of the OH substituent on this carbon determines if the sugar molecule is the alpha or beta anomer

Question 38

What is the difference between esterification and glycoside formation?

Answer 38

- esterification is the reaction by which a hydroxyl group reacts with either a carboxylic acid or its derivative to form an ester - glycosidic formation refers to the reaction between an alcohol and a hemiacetal (or hemiketal) group on a sugar to yield an alkoxy group

Question 39

What purpose do Tollens' reagent and Benedict's reagent serve? How do they differ from each other?

Answer 39

- Tollens' and Benedict's reagents are used to detect the presence of reducing sugars - Tollens' is reduced to produce a silvery mirror which aldehydes are present whereas Benedict's is indicated by a reddish precipitate of Cu2O

Question 40

From a metabolic standpoint, does it make sense for carbohydrates to get oxidized or reduced? What is the purpose of this process?

Answer 40

- it makes sense for carbohydrates to become oxidized while reducing other groups - this is the case because aerobic metabolism requires reduced forms of electron carriers to facilitate processes such as oxidative phosphorylation - because carbohydrates are a primary energy source, they are oxidized

Question 41

Which of the 2 forms of starch is more soluble and why?

Answer 41

- amylopectin is more soluble in solution than amylose because of its highly branched structure that decreases intermolecular bonding between polysaccharide polymers and increases interaction with the surrounding solution

Question 42

Regarding glycogen and amylopectin, which of these polymers should experience a higher rate of enzyme activity from enzymes that cleave side branches and why?

Answer 42

glycogen has a higher rate because it contains significantly more branching than amylopectin