Carbohydrates

Question 1

Functions of carbohydrates

Answer 1

energy source
energy storage
structural component
immune function
intercellular communication

Question 2

General formula of carbohydrates

Answer 2

(CH2O)n [hydrate of carbon)

Question 3

How many sugar units are in monosaccharides, disaccharides, oligosaccharides and polysaccharides?

Answer 3

monosaccharides = 1
disaccharides = 2
oligosaccharides = 3-10
polysaccharides = 11->100

Question 4

What are the generic names of monosaccharides and how are they classified

Answer 4

classified by number of carbon atoms
3 = triose
4 = tetrose
5 = pentose
6 = hexose
7 = heptose
9 = nonose

Question 5

What is an aldose?

Answer 5

aldehyde functional group present
-ose
eg. glucose
RCOH

Question 6

What is a ketose?

Answer 6

ketone functional group present
-ulose
eg. fructose (should really be called fructulose)
R1COR2

Question 7

Define isomer

Answer 7

compounds that have the same chemical formula but different structures

Question 8

Define epimer

Answer 8

carbohydrate isomers that differ in configuration around only one specific carbon atom

Question 9

Define enantiomer

Answer 9

pairs of structures that are mirror images of each other

non-superimposable mirror images

Question 10

What are D and L designations?

Answer 10

based on configuration of single asymmetric carbon atom in glyceraldehyde
L-glyceraldehyde = hydroxyl group below carbon backbone plane
D-glyceraldehyde = hydroxyl group above carbon backbone plane
for sugars with more than one chiral centre, D + L refers to the farthest asymmetric carbon atom from the carbonyl group
most naturally occurring sugars = D isomers

Question 11

What is a pyranose ring?

Answer 11

5 carbon atoms and one oxygen atom

Question 12

What is a furanose ring?

Answer 12

4 carbon atoms and one oxygen atom

Question 13

What is the different between beta and alpha glucose anomers

Answer 13

cyclisation of monosaccharides creates an anomeric carbon atom
beta anomer = hydrogen below plane of ring on carbon 1
alpha anomer = hydrogen above plane of ring on carbon 1
alpha and beta glucose anomers are in equilibrium and can interconvert

Question 14

Describe sucrose

Answer 14

disaccharide of glucose + fructose

alpha 1-2 beta glycosidic bond between glucose + fructose

Question 15

Describe maltose

Answer 15

disaccharide of glucose + glucose
alpha 1-4 glycosidic bond between glucoses
major degradation product of starch

Question 16

Describe lactose

Answer 16

disaccharide of glucose and galactose
beta 1-4 glycosidic bond between galactose and glucose
in mammalian milk

Question 17

Describe cellulose

Answer 17

structural polysaccharide in plants
beta 1-4 glycosidic bonds between glucose molecules
major constituent of plant cell walls
humans cannot digest as no enzymes to break bonds

Question 18

Describe starch

Answer 18

storage polysaccharide in plants
alpha 1-4 glycosidic bonds between glucose molecules
amylose = unbranched + only alpha 1-4 linkages
amylopectin = branched (every 20-30 residues) + mainly alpha 1-4 linkages, branches = alpha 1-6 linkages

Question 19

Describe glycogen

Answer 19

storage polysaccharide in animals
branched (every 8-10 residues)
mainly alpha 1-4 linkages, branches = alpha 1-6 linkages

Question 20

What are the 3 sites of digestion of carbohydrates?

Answer 20

mouth = salivary alpha amylase hydrolysing alpha 1-4 bonds, yields maltose + dextrin

small intestine = pancreatic alpha amylase, more maltose + dextrin

upper jejunum = brush border membrane-associated oligosaccharidases + disaccharidases of intestinal mucosal cells, yields glucose, fructose + galactose

Question 21

How are different carbohydrates absorbed?

Answer 21

monosaccharides are absorbed by intestinal mucosal cells:

• glucose + galactose = sodium-dependent cotransporter-1 (SGLT-1)
• fructose = sodium-independent monosaccharide transporter (GLUT-5)

all 3 monosaccharides are transported from the intestinal cells into the portal circulation by GLUT-2

Question 22

What can deficiencies in carbohydrate degradation cause?

Answer 22

lactose intolerance

sucrose/isomaltose intolerance

Question 23

Symptoms of sugar intolerance

Answer 23

undigested carbohydrates pass into large intestine = osmotic diarrhoea
bacterial fermentation of carbohydrate produces large volumes of carbon dioxide and hydrogen = abdominal cramps, diarrhoea, flatulence

Question 24

How are patients with sugar intolerance managed?

Answer 24

withholding dietary sugar

enzyme replacement therapy

Question 25

What is galactosaemia?

Answer 25

genetic disorder that affects ability to metabolise galactose
leads to galactose accumulation (oxidised and reduced to toxic metabolites)
treatment = elimination of lactose and galactose from diet