monosaccharides

Question 1

what are carbohydrates?

Answer 1

C, H, O compounds -> major source of energy (intake from diet, mammals synthesise them from MW metabolites at cost of energy)

Question 2

how are carbohydrates synthesised?

Answer 2

plants synthesise sugars from CO2 and energy in form of radiation
- synthesis is reduction, respiration is oxidation

Question 3

what are monosaccharides?

Answer 3

simplest carbohydrates

• carbon chain, aldehyde or ketone, hydroxyls on each carbon atoms except carbonyl
• classified according to number of carbon atoms present (3 triose, 4 tetrose, 5 pentone, 6 hexose)
• most have many chiral centres

Question 4

describe the reactions of monosaccharides

Answer 4

• react as aldehydes/ketones: reduction, oxidation, aldol reaction, reaction with amines, formation of acetals and emiacetals
• react as alcohols: oxidation, water elimination, formation of esters

Question 5

isomers of monosaccharides

Answer 5

isomers: glucose, galactose and fructose (Chiral carbon furthest from ketone/aldehyde determines the D or L monosaccharide)

Question 6

D-glucose

Answer 6

most common hexose (also known as dextrose and blood sugar in body)

• building blocks in disaccharides sucrose and lactose
• building blocks polysaccharides such as cellulose and glycogen

Question 7

D-galactose

Answer 7

from disaccharide lactose found in milk

• cellular membranes of brain and nervous system
• galactosemia: enzyme needed to convert D-galactose in D-glucose is missing
• galactose accumulates in blood and tissue, can lead to cataracts, mental retardation, failure to thrive and liver disease

Question 8

D-fructose

Answer 8

sweetest of carbohydrates

• obtained through hydrolysis products of sucrose
• High-fructose corn syrup is a sweetner produced by enzyme breakdown sucrose to glucose and fructose

Question 9

D-ribose

Answer 9

component of RNA

- reduced form (2-deoxy-D-ribose) is part of skeleton of DNA

Question 10

describe the cyclic structures of monosaccharides

Answer 10

Haworth structures:

• most stable forms of pentose/hexose sugars are five or six atom rings hemiacetals
• produced form reaction of carbonyl group and hydroxyl group in the same molecule
• sp2 to sp3 carbon (becomes chiral carbon)
• happens with both aldose and ketones

Question 11

anomers of cyclic monosaccharides

Answer 11

isomers with different chirality at the atom that in open chain forms carbonyl

• if OH group is below plane at carbon 1 -> alpha glucose
• if OH group is above the plane at carbon 1 -> beta glucose

Question 12

what is mutarotation?

Answer 12

when placed in solution cyclic structures open and close (mutual rotation)

• α-D-glucose converts to beta-D-glucose and vice versa (Beta glucose in solution is more stable therefore more is found in solution)
• little amount of open chain allow for the reaction to continuously establish equilibrium

Question 13

what happens in fructose mutarotation?

Answer 13

5 membered ring, hydroxyl position determine the anomer

Question 14

oxidation of monosaccharides

Answer 14

small amounts of open chain in cyclic forms in acqueous solutions allow oxidation to corresponding -ionic acid

Question 15

reduction in monosaccharides

Answer 15

carbonyl group can be reduced to the corresponding alcohol

Question 16

what is an alditol?

Answer 16

sweeteners with less calories