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MCAT Biochem > Ch 4 - Carbohydrate Structure and Function > Flashcards

Ch 4 - Carbohydrate Structure and Function Flashcards

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MCAT flashcards
1
Q

How are carbohydrates organized?

A

by their number of carbon atoms and functional groups

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2
Q

What are 3 carbon sugars called? 4 carbon?

A

trioses

tetroses

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3
Q

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

A

a: aldoses
k: ketoses

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4
Q

What is the nomenclature of all sugars based on?

A

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

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5
Q

What are diastereomers?

A
  • nonsuperimposable configurations of molecules with similar connectivity
  • differe at at least one - but not all - chiral carbons
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6
Q

What are epimers?

A

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

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7
Q

What are anomers?

A

subtype of epimers that differ at the anomeric carbon

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8
Q

What does cyclization describe?

A

the ring formation of carbohydrates from their straight-chain forms

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9
Q

What conformation can that anomeric carbon take when rings form?

A
  • 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
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10
Q

What is the anomeric carbon?

A
  • the new chiral center formed in right closure

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

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11
Q

What do the Haworth projections provide?

A

a good way to represent 3D structure

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12
Q

How do cyclic compounds under mutarotation?

A

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

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13
Q

What are monosaccharides?

A
  • single carbohydrate units, with glucose as the most commonly observed monomer
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14
Q

What 3 reactions can monosaccharides undergo?

A
  • 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
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15
Q

What can aldoses be oxidized and reduced to?

A
  • ox: aldonic acids

- red: alditols

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16
Q

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

A
  • reducing agents (sugars)

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

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17
Q

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

A

deoxy sugars

18
Q

What is phosphorylation?

A

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

19
Q

How are disaccharides formed?

A

as a result of glycosidic bonding between 2 monosaccharides subunits

20
Q

How are polysaccharides formed?

A

by repeated monosaccharides or polysaccharide glycosidic bonding

21
Q

What are 3 common disaccharides?

A
  • sucrose (glucose-beta-1,2-fructose)
  • lactose (galactose-beta-1,4-glucose)
  • maltose (glucose-alpha-1,4-glucose)
22
Q

What is cellulose?

A

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

23
Q

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

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

- glycogen: functions a main energy storage for animals

24
Q

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

A

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

25
Q

When do aldonic acids form?

A
  • after the aldehyde group on a reducing sugar reduces another compound, becoming oxidized in the process
26
Q

What happens when ketose sugars undergo tautomerization?

A
  • 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

27
Q

What happens when glucose reacts with methanol under acid catalysis?

A

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

28
Q

Which enzyme cleaves polysaccharide chains and yields maltose exclusively?

A

beta maltose

29
Q

Where does alpha maltose cleave and what does it yield?

A

cleaves amylose anywhere along the chain to yield short polysaccharides

30
Q

What do debranching enzymes remove?

A

oligosaccharides from a branch in glycogen or starches

31
Q

What does glycogen phosphorylase yield?

A

glucose 1-phosphate

32
Q

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

A

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

33
Q

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

A
  • 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
34
Q

Why is maltose digestible by humans compared to cellobiose?

A

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

35
Q

What are the cyclic forms of monosaccharides?

A

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

36
Q

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

A

aldopentose

ketohexose

37
Q

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

A
  • 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
38
Q

What is the difference between esterification and glycoside formation?

A
  • 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
39
Q

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

A
  • 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
40
Q

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

A
  • 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
41
Q

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

A
  • 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
42
Q

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

A

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

MCAT Biochem (12 decks)

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