Carbohydrates

Question 1

Describe how to test for sucrose?

Answer 1

1) Add equal volume of HCl to equal volume of sample
2) Mix and heat in boiling water bath
3) Neutralise the acid by adding Na2CO3
in excess
4) Extract 2cm3 and carry out Benedicts’ Test

Question 2

What are the similarities between Amylose and Amylopectin?

Answer 2

1) Made up of alpha glucose residues linked by glycosidic bonds

2) Compact shape

3) Unreactive due to few free anomeric OH groups available

Question 3

What are the differences between Amylose and Amylopectin?

Answer 3

1) Amylose is coiled, unbranched chain whereas amylopectin is highly branched chain

2) Amylose only joined by alpha (1,4) glycosidic bonds vs amylopectin joined by alpha (1,4) and alpha (1,6) glycosidic bonds

Question 4

State the structure to function of Starch

Answer 4

S1: Large molecule
P1: insoluble
F1: does not affect water potential within cells as it cannot diffuse in and out

S2: Amylose is helical while amylopectin is highly branched
P2: compact shape
F2: store large amount of energy with minimal shape

S3: Branching in amylopectin due to alpha (1,6) glycosidic bond
P3: large number of free ends available
F3: for hydrolysis by amylase when required

Question 5

State the structure to function of glycogen

Answer 5

S1: Large molecule
P1: Insoluble
F1: Does not affect water potential within cells as it cannot diffuse in and out of cell

S2: Glucose monomers bonded by alpha (1,4) glycosidic bonds
P2: helical shape formed by the bonds
F2: compact

S3: Highly branched due to alpha (1,6) glycosidic bonds
P3: large number of free ends
F3: for hydrolysis by glycogen phosphorylase

Question 6

How is the molecular structure of cellulose related to high tensile strength?

Answer 6

1) Alternate beta glucose are inverted 180 degree, forming long, unbranched straight chains

2) Extensive hydrogen bond between protruding OH groups of neighbouring chains form rigid cross links between chains

3) Cross linked chain associate to form microfibrils, which bundles to form macrofibril

4) Cellulose fibres are laid in different orientations in various layers

Question 7

State the structure to function of cellulose

Answer 7

S1: Large molecule
P1: insoluble
F1: good structural

S2: Alternate beta glucose inverted 180 linked by beta (1,6) glycosidic bonds
P2: form long cellulose chain
F2: high tensile strength

S3: Extensive hydrogen bond between cellulose chains form microfibrils, which bundle to form macrofibril
F3: high tensile strength