[3.1.2] Carbohydrates

Question 1

Lactulose is a disaccharide formed from one molecule of galactose and one molecule of fructose.
Other than both being disaccharides, give one similarity and one difference between the structures of
lactulose and lactose. (2)

Answer 1

Similarity 1. Both contain galactose / a glycosidic bond; Ignore references to hydrolysis and / or
condensation

Difference 2. Lactulose contains fructose, whereas lactose contains glucose; [Ignore alpha / beta prefix for glucose]

Question 2

Explain the difference in the structure of the starch molecule and the cellulose
molecule shown in the diagram above. (2)

Answer 2

1. Starch formed from a-glucose but cellulose formed from B-glucose;
2. Position of hydrogen and hydroxyl groups on carbon atom 1 inverted.

Question 3

Starch molecules and cellulose molecules have different functions in plant cells. Each molecule is adapted for its function.
Explain one way in which starch molecules are adapted for their function in plant cells. (2)

Answer 3

1. Insoluble; 2. Don’t affect water potential;
   OR
2. Helical; Accept form spirals 4. Compact;
   OR
3. Large molecule; 6. Cannot leave cell.

Question 4

Explain how cellulose molecules are

adapted for their function in plant cells. (3)

Answer 4

1. Long and straight chains;
2. Become linked together by many hydrogen
   bonds to form fibrils;
3. Provide strength (to cell wall)

Question 5

Describe how the structures of starch and cellulose

molecules are related to their functions. (5)

Answer 5

Starch:
1. Large (molecule) so cannot leave cell/cross
cell-surface membrane/ osmotically inactive
2. Branched so glucose is (easily) released for
respiration;
3. Helical/ spiral shape so compact;

Cellulose:
1. Long, straight/unbranched chair
of B glucose;
2. Joined by hydrogen bonding;
3. To form (micro/macro)fibrils;

Question 6

Identify the structures found in a glucose molecule

Answer 6

1. Hydroxyl (-OH) group
2. Aldehyde (-CH2OH) group
3. A “pyramide ring” - a loop containing 5 carbons and 1 oxygen atom

Question 7

Identify the structural difference between alpha and beta glucose

Answer 7

1. Alpha glucose has both of its OH group “pointed downwards”
2. Beta glucose has one OH group facing down and another OH group elevated upwards

Question 8

Describe two differences between the structure of cellulose and the structure of starch (2)

Answer 8

1. Starch contains 1-4 and 1-6 glycosidic bonds, Cellulose only has 1-4 glycosidic bonds
2. Starch is branched, Cellulose is unbranched
3. Starch is coiled and compact, Cellulose is straight
4. Starch doesn’t form (micro/macro) fibrils, Cellulose forms (micro/macro) fibrils

Question 9

Describe the structure of glycogen

Answer 9

1. Polysaccharide if alpha glucose
2. Joined by glycosidic bonds
   OR
3. Branched Structure

Question 10

Describe how glycogen acts as a source of energy (2)

Answer 10

1. Glycogen can be hydrolysed to form glucose
2. Glucose is used in respiration for the formation of ATP (which releases energy)

IGNORE MARKS IF MENTIONED:

1. Broken Down
2. Energy Produced

Question 11

A precipitate is produced in a positive result for reducing sugar in a Benedict’s test. A precipitate is solid matter suspended in solution. A student carried out the Benedict’s test.

Suggest a method, other than using a colorimeter, that this student could use to measure the quantity of reducing sugar in a solution. (2)

Answer 11

1. Filter and dry the precipitate

2. Find the mass of the precipitate

Question 12

Use of a colorimeter in this investigation would improve the repeatability of the student’s results.
Give one reason why.

Answer 12

1.   Quantitative
OR
(Colour change is) subjective;
Accept: accurate/precise
2.   Standardises (the) method;

Question 13

What is a monomer (1)

Answer 13

) (a monomer is a smaller / repeating) unit / molecule from which larger molecules / polymers are made;
Reject atoms / elements / ’building blocks’ for units / molecules
Ignore examples

Question 14

Following digestion and absorption of food, the undigested remains are processed to form faeces in the parts of the intestine below the ileum.
The faeces of people with constipation are dry and hard. Constipation can be treated by drinking lactulose. Lactulose is soluble, but is not digested or absorbed in the human intestine.
Use your knowledge of water potential to suggest why lactulose can be used to help people suffering from constipation. (2)

Answer 14

1. (Lactulose) lowers the water potential of faeces / intestine / contents of the intestine;
   Accept Ψ for water potential
2. Water retained / enters (due to osmosis) and softens the faeces;
   Accept desc