carbohydrates

Question 1

a biochemical test for reducing sugar produces a negative result with raffinose solution.
describe a biochemical test to show that raffinose solution contains a non-reducing sugar. (3)

Answer 1

1. heat with (dilute HCL) acid and neutralise (sodium hydrogen carbonate solution);
2. heat with benedict’s solution;
3. red precipitate colour

Question 2

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 2

1. filter and dry the precipitate;
2. find mass/weight;

Question 3

use of colorimeter in an investigation would improve the repeatability of the student’s result.
give one reason why. (1)

Answer 3

1. quantitative
   OR
   colour change is subjective;
2. standardises the method;

Question 4

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

Answer 4

similarity
1. both contain galactose/glycosidic bond;

difference
2. lactulose contains fructose, whereas lactose contains glucose;

Question 5

glycogen and cellulose are both carbohydrates.
describe two differences between the structure of a cellulose molecule and a glycogen molecule. (2)

Answer 5

1. cellulose is made of b-glucose monomers and glycogen is made of a-glucose monomers;
2. cellulose molecule has straight chain and glycogen is branched;
3. cellulose molecule has straight chain and glycogen is coiled;
4. glycogen has 1,4 and 1,6 glycosidic bonds and cellulose only has 1,4 glycosidic bonds;

Question 6

starch is a carbohydrate often stored in plant cells.
describe and explain two features of starch that make it a good storage molecule. (2)

Answer 6

1. insoluble in water, so doesn’t affect water potential;
2. branched/coiled a-helix, so makes molecule compact
   OR
   branched/coiled a-helix so can fit may molecules in small area;
3. polymer of a-glucose so provides glucose for respiration;
4. branched/more ends for fast breakdown/enzyme action;
5. large molecule, co can’t cross the cell membrane;

Question 7

test for starch. (1)

Answer 7

iodine/potassium iodide;

Question 8

describe the structure of glycogen. (2)

Answer 8

1. polysaccharide of a-glucose;
   OR
   polymer of a-glucose;
2. joined by glycosidic bonds
   OR
   branched structure;

Question 9

suggest how glycogen acts as a source of energy.
do not include transport across membranes in your answer. (2)

Answer 9

1. hydrolysed to glucose;
2. glucose used in respiration;

Question 10

name the monomers from which a maltose molecule is made. (1)

Answer 10

glucose and glucose;

Question 11

name the type of chemical bond that joins the two monomers to form maltose. (1)

Answer 11

glycosidic (a-1,4)

Question 12

explain how you would use the graph to determine the maltose concentration with a light absorbance of 0.45 arbitrary units. (2)

Answer 12

1. line of best fit drawn;
2. read off value at 0.45;

Question 13

explain the difference in the structure of the starch molecule and the cellulose molecule shown in the diagram above. (1)

Answer 13

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 14

explain one way in which starch molecules are adapted for their function in plant cells. (2)

Answer 14

1. insoluble;
2. doesn’t affect water potential;
   OR
3. helical;
4. compact;
   OR
5. large molecule;
6. cannot leave cell;

Question 15

explain how cellulose molecules are adapted for their function in plant cells. (3)

Answer 15

1. long and straight chains;
2. become linked together by many hydrogen bonds to form (micro)fibrils;
3. provide strength to cell wall;