Carbohydrates

Question 1

Describe the structure of a starch molecule and explain how starch is
adapted for its function in cells. [6]

Answer 1

1. a polysaccharide made from α-glucose;
2. joined by condensation/removing molecule of water/glycosidic bond;
3. Mainly 1 : 4 glycosidic bonds;
4. Some 1-6 glycosidic bonds form branches;
5. Chains are helical so they are compact;

6 starch is insoluble so it doesn’t affect osmotic pressure / water potential;

7. Starch is a large molecule so it cannot leave the cell

Question 2

Describe the structure of a glycogen molecule and explain how glycogen
is adapted for its function in cells. [6]

Answer 2

1. A polysaccharide made from α-glucose;
2. joined by condensation/removing molecule of water/glycosidic bond;
3. Mainly 1 : 4 glycosidic bonds;
4. Lots of 1-6 glycosidic bonds to form many branches;
5. Chains are coiled so they are compact;

6 glycogen is insoluble so it doesn’t affect osmotic pressure;

7 highly branched allows it to be hydrolysed to release glucose for respiration

Question 3

Describe the structure of a cellulose molecule and explain how cellulose
is adapted for its function in cells. [6]

Answer 3

1. A polysaccharide made from β-glucose that forms long, straight chains;
2. joined by condensation/removing molecule of water;
3. Forms a 1 : 4 glycosidic bond;
4. “flipping over” of alternate molecules;
5. MANY hydrogen bonds link chains together forming fibrlis;
6. cellulose makes cell walls strong/cellulose fibres are strong;
7. can resist turgor pressure/osmotic pressure/pulling forces;
8. bond difficult to break;
9. resists digestion/action of microorganisms/enzymes;

Question 4

Describe how the structures of starch and cellulose molecules
are related to their functions. [6]

Answer 4

Starch (max 3)
Helical/ spiral shape so compact;

Large (molecule)/insoluble so osmotically inactive;

Branched so glucose is (easily) released for respiration;

Large (molecule) so cannot leave cell/cross cell-surface membrane;

Cellulose (max 3)

Long, straight/unbranched chains of β glucose;

Joined by many hydrogen bonding;
To form (micro/macro)fibrils;

Provides rigidity/strength;