MCAT Biology Ch14: Carbohydrates

Question 1

carbohydrates are compounds that contain

Answer 1

C, H, and O in form of polyhydroxylated aldehydes or ketones, Cn(H2O)n

Question 2

monosaccharide

Answer 2

single carbohydrate (simple sugar)

Question 3

disaccharide

Answer 3

two sugars

Question 4

oligosaccharides

Answer 4

short carb chain

Question 5

polysacs

Answer 5

long carb chains

Question 6

monosaccharide

Answer 6

• simplest units
• # of carbons they possess; num. prefix and suffix -ose (ex: triose, tetrose); begins w/ C close to carbonyl (c-1)
• simplest: glyceraldehyde
• polyhydroxlyated aldehyde (aldose)
• simplest ketone sugar (ketose): dihydroyacetone; ketone will receive lowest possible number; ketone group C-2 on MCAT
• every carbon (except carbonyl) carry -OH

Question 7

aldose

Answer 7

aldehyde sugar

Question 8

ketose

Answer 8

simplest ketone sugar

Question 9

stereochemistry of monosacs

Answer 9

• used glyceraldehyde –> D(+)and L(-) config
• all other monosacs assigned to D and L in relationship to glyceraldehyde
• highest # chiral center (farthest from carbonyl) right is D and left is L
• same sugars, diff. optical fam (D and L–> enantiomers
• nonidentical (nonmirror image) w/in same fam (ketose/aldoses, same # C) –> diastereomers
• diastereomers differ only at one chiral center –> epimers

Question 10

glyceraldehyde

Answer 10

http://upload.wikimedia.org/wikipedia/commons/a/a1/D-glyceraldehyde-2D-Fischer.png

Question 11

tetroses

Answer 11

http://www.scientificpsychic.com/fitness/erythrose.gif

Question 12

trioses

Answer 12

http://www.fao.org/docrep/field/003/AB470E/AB470E24.gif

Question 13

dihydroxyacetone

Answer 13

http://upload.wikimedia.org/wikipedia/commons/2/28/Dihydroxyacetone.png

Question 14

D-fructose

Answer 14

http://web.pdx.edu/~wamserc/CH331F97/Egifs/E2_3a.gif

Question 15

D-glucose

Answer 15

http://www.uspto.gov/web/patents/classification/uspc536/c536s1-11-2.gif

Question 16

D-galactose

Answer 16

http://upload.wikimedia.org/wikipedia/commons/5/58/D-galactose.png

Question 17

D-mannose

Answer 17

http://1.bp.blogspot.com/-YH0ZTmm6VX0/T5gNNDJhm2I/AAAAAAAABF4/ptwRkYrNdYw/s200/2+D-mannose.png

Question 18

ring properties

Answer 18

• hydroxyl group (Nu) and carbonyl (electrophile) –> intramolecular rxn –> cyclic hemiacetal (from aldoses) or hemiketals (ketoses)
• ring strain –> only cyclic in sol is pyranose (6) or furanose (5)
• oxygen becomes member of ring
• c-1 becomes chiral (anomeric carbon), compound known as anomers
• when w/ water –> spon open and reform
• single bond between C1 and C2 rotate freely –> alpha or beta (mutarotation)

Question 19

pyranose

Answer 19

• 6 membered ring formed from intramolecular Nu acyl substitution of hydroxyl group (Nu) and carbonyl (electrophile)
• chairlike config, min steric hindrance
• Fisher Projection –> Haworth (right –> down, left –> up)
• from 6 carbon aldose or seven carbon ketose
• OH is either alpha (trans) or beta (cis)

Question 20

furanose

Answer 20

5 membered ring formed from intramolecular reaction of hydroxyl group (Nu) and carbonyl (electrophile)

-from 5 carbon aldose or 6 carbon ketose

Question 21

anomer

Answer 21

cyclic stereoisomer that differ about new chiral carbon

Question 22

anomeric carbon

Answer 22

• carbon that’s chiral in ring structure of sugar

- attached to two oxygen (O in ring and OH)

Question 23

alpha

Answer 23

OH is down, trans to CH2OH substituent

Question 24

beta

Answer 24

OH is up, cis to CH2OH substituent

Question 25

mutarotation

Answer 25

- a or b config after ring exposed to water - occurs more rapidly when catalyzed w/ acid or base - mix both in eq. concentration - process of one anomer changing into the other anomer by opening and reclosing - alpha --> less favored due to OH axial --> steric strain

Question 26

anomerization

Answer 26

forming one another or another from the straight-chain sugar

Question 27

Monosac rxns

Answer 27

- ester formation, oxidation of monosacs, glycosidic rxn | - since OH, same rxns as simple alcohols

Question 28

Monosac rxns: ester formation

Answer 28

- monosacs --> esters - using acid anhydride and a base (CH3CO)2O - all hydroxyls are esterified

Question 29

Monosac rxns: oxdation of monosacs

Answer 29

- switching between anomeric config, hemiacetal rings spend short period of time in open-chain aldehyde form - aldehydes can be oxidized to carb acids (oxidized aldoses called aldonic acids); reducing agents

Question 30

aldonic acids

Answer 30

oxidized aldoses

Question 31

reducing sugar

Answer 31

- hemiacetal ring - Tollen's (reducing Ag+ to metallic silver) and Benedict's (red ppt of Cu2O) reagent detect presence - positive - ketose also reducing sugar and is pos, but since can't be oxidized to carb acid, it isomerizes to aldoses via keto-enols shifts - more powerful oxidizing agent like dilute HNO3 w/ oxidize both aldehyde and primary alcohol to carb acid

Question 32

glycosidic rxns

Answer 32

hemiacetals + alcohols --> acetals - under acidic conditions - anomeric hydroxyl group transformed into alkoxyl group --> mix of alpha and beta acetal (w/ water as LG) - glycoside - forms glycosidic linkage (C-O) - Sn2, sugar as nucleophile

Question 33

glycoside

Answer 33

result of hemiacetals + alcohols --> acetals

Question 34

glycosidic linkage

Answer 34

resulting C-O from hemiacetals + alcohols --> acetals

Question 35

disacs

Answer 35

- two monosacs join, hydroxides act like alcohol - most common is glycosidic linkage, 1,4 linkage (a - maltose, b - cellobiose), there's also 1,6 and 1,2 - either alpha or beta, depending on orientation of OH group on anomeric carbon - often cleaved in presence aq. acid

Question 36

polysaccharides

Answer 36

- large chains of monosac linked by glycosidic bonds - three most important: cellulose, starch, and glycogen (diff function, same monosac, D-glu) - diff in config about anomeric carbon and position of glycosidic bonds -- bio diff.

Question 37

cellulose

Answer 37

- 1-4, B glycosidic bond - components of plants - not digestable by humans (fiber)

Question 38

starch

Answer 38

- digestible by humans - plants store energy - 1,4 alpha glycosidic bonds, although occasional 1,6 alpha glycosidic bonds off chain

Question 39

glycogen

Answer 39

- similar to starch, more 1,6 alpha glycosidic bonds (1 every 12 glu) - highly branched compound (dendrimer)