Carbohydrates

Question 1

Monosaccharide

Answer 1

a) Carbohydrate which cannot be hydrolysed to simpler carbohydrates
b) Carbonyl group (C=O)
c) Multiple hydroxyl groups (OH)

Question 2

Aldose (aldehyde sugar)

Answer 2

Carbonyl group at the beginning of the carbon skeleton

Question 3

Ketone (ketone sugar)

Answer 3

Carbonyl group nested within the carbon skeleton

Question 4

Difference between alpha and beta glucose

Answer 4

OH group attached to C1:
Below plane of the ring in alpha
Above plane of the ring in beta

Question 5

Significance of molecular structure of monosaccharides

Answer 5

1. Small in size and many hydroxyl groups that can form hydrogen bonds with water -> readily soluble in water -> can be transported easily
2. Linear form has free carbonyl group -> reducing ability -> reducing sugars
3. Pentose and hexoses can exist as rings-> stable building blocks for larger molecules
4. alpha and beta isomerism -> same chemical formula different structural molecules -> diversity

Question 6

Disaccharide

Answer 6

2 monosaccharides joined by glycosidic bond/linkage via condensation -> loss of 1 water molecule
Hydrolyse -> add one molecule of water

Question 7

Maltose

Answer 7

alpha-glucose + alpha-glucose -> alpha(1-4) glycosidic bond

Question 8

Sucrose

Answer 8

alpha-glucose + beta-fructose -> alpha(1-2) glycosidic bond

Question 9

Reducing sugars

Answer 9

All monosaccharides

Some disaccharides except sucrose

Question 10

Test for reducing sugars

Answer 10

Benedict’s Test
2cm3 test solution + equal volume of Benedict’s reagent -> shake -> immerse in boiling water bath
Green -> yellow -> orange -> brick red

Question 11

Test for non-reducing sugars

Answer 11

Acid hydrolysis first -> boil with dilute HCL (1min) -> cool + neutralise with sodium bicarbonate solution
Then Benedict’s Test

Question 12

Amylose

Answer 12

alpha-glucose monomers forming unbranched polymers
alpha(1-4) glycosidic bonds
Coiled into a helix

Question 13

Amylopectin

Answer 13

alpha-glucose monomers forming branched polymers
alpha(1-4) glycosidic bonds within a branch and alpha(1-6) glycosidic bonds at branch points (every 20-30)
Helix + helical side chains

Question 14

Test for starch

Answer 14

Iodine test

Triiodide ion complex fits into centre of amylose helix -> starch-iodine complex -> blue-black colouration

Question 15

Glycogen

Answer 15

Similar to amylopectin but more extensively branched

Branch points every 12 monomers

Question 16

How do the structures of amylose, amylopectin and glycogen determine their function as storage molecules?

Answer 16

1. Many glucose residues
   Large energy store -> hydrolysed to many monosaccharides
2. Insoluble in water -> will not affect water potential of cells
   Large molecule
   Intramolecular hydrogen bonding and projection of hydroxyl groups into core of helix formation -> relatively fewer hydroxyl groups available for hydrogen bonding with water
3. Comprise of helices
   Pack many subunits per unit volume
4. Branched
   Multiple hydrolytic enzymes can work at the same time -> increase energy generation per unit time

Question 17

Cellulose

Answer 17

beta glucose monomers
beta(1-4) glycosidic bonds
alternate monomers rotated 180 degrees wrt each other
Linear unbranched molecule with OH groups projecting out in both directions
Parallel chains held together by intermolecular hydrogen bonds -> cross-linking -> microfibril -> criss
crossing -> cell wall

Question 18

How do the structures of cellulose determine their function as structural molecules?

Answer 18

1. High tensile strength
   Adjacent beta glucose monomers rotated 180 degrees wrt each other -> linear -> OH groups bond extensively with adjacent chains lying parallel -> microfibrils
2. Insoluble in water
   Few OH groups available for H-bonding with water
3. Meshwork of microfibrils
   Porous structure -> freely permeable to water
   Strong and rigid and distributes stress in all directions to prevent plant cells from bursting due to osmotic stress
4. Cellulases not commonly available -> not readily hydrolysed