biological molecules past paper questions

Question 1

explain how cellulose molecules are adapted for their function (3) (function is structural molecule in plants)

Answer 1

-long straight chains (1)
-become linked together by many hydrogen bonds to form microfibrils (2)
-which provides strength and rigidity to cell wall(3) as well as opposing fluid in a fluid filled cell, preventing further water entering the cell and causing the cell to burst.

Question 2

explain the difference in structure of the starch molecule and the cellulose molecule? (2)

Answer 2

-starch is made of a glucose and cellulose is made of b glucose (1)
-the position of the hydroxyl and hydrogen groups on carbon 1 are inverted (2) because they are like that on a and b glucose

Question 3

explain one way in which starch molecules are adapted for their function in plant cells? (2) (function is energy storage in plants)

Answer 3

-helical (1) which makes molecule more compact (2) so more glucose can be stored in a smaller space

Question 4

describe the chemical reaction involved in the conversion of polymers to monomers and monomers to polymers

give two examples of polymers and their associated monomers in your answer (5)

Answer 4

• a condensation reaction joins monomers together and forms a bond and releases water (1)
  -a hydrolysis reaction breaks a chemical bond between monomers and uses water (2)
  -e.g. condensation reaction to make starch from many alpha glucose molecules ; hydroxyl group on one molecule reacts with hydroxyl group on another, forming a glycosidic link and water removed (3)
  -e.g. hydrolysis reaction to break down the polymer of cellulose into beta glucose monomers(4)

1 mark available for getting bonds right

Question 5

what is a monomer (1)

Answer 5

a single chemical unit from which polymers are made

Question 6

describe a biochemical test to show that raffinose solution contains a non-reducing sugar (3)

Answer 6

-heat with acid and neutralise (1)
-heat with benedicts (2)
-red precipitate if non reducing sugars present (3)

Question 7

describe two differences between the structure of a cellulose molecule and a glycogen molecule ? (2)

Answer 7

-cellulose made up of b glucose and glycogen made of a glucose (1)
-cellulose molecule has a straight straight chains and glycogen alpha helix (2)

Question 8

describe the structure of glycogen (2)

Answer 8

polymer of a glucose (1)
joined by glycosidic bonds (2)

Question 9

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

Answer 9

-hydrolysed to glucose (1)
-glucose used in respiration (2)

Question 10

describe how you would test for the presence of a lipid in a liquid sample of food (2)

Answer 10

-add ethanol then add water and shake (1)
-white milky emulsion if positive test (2)

Question 11

describe how a triglyceride molecule is formed (3)

Answer 11

-one glycerol and three fatty acids (1)
-condensation reactions and removal of three molecules of water (2)
-ester bonds formed (3)

Question 12

describe how an ester bond is formed in a phospholipid molecule (2)

Answer 12

-condensation reaction
-between glycerol and fatty acid

Question 13

describe how an ATP molecule is formed from its component molecules (4)

Answer 13

-adenine, ribose and three phosphates (1)
-joined by condensation reaction (2)
-using enzyme ATP synthase (3)

Question 14

what are the two products of ATP hydrolysis (2)

Answer 14

-ADP
-Pi

Question 15

give two ways in which ATP is a suitable energy source for cells to use (2)

Answer 15

-releases energy instantaneously (1)
-phosphorylates other compounds to make them more reactive (2)

Question 16

describe how ATP is resynthesized in cells (2)

Answer 16

-from ADP and phosphate condensation reaction (1)
-by ATP synthase (2)

Question 17

give two ways in which the hydrolysis of ATP is used in cells (2)

Answer 17

-to provide energy for other reactions (2)
-to add phosphate to other substances (phosphorylate them) and make them more reactive (2)

Question 18

explain five properties that make water important for organisms (5)

Answer 18

• a metabolite in condensation/hydrolysis
• a solvent so allowing transport of substances
  -high specific heat capacity so buffers changes in temperature
  -high latent heat of vaporisation so large cooling effect
  -cohesion between water molecules so supports columns of water in plants

Question 19

give two properties of water that are important in the cytoplasm and explain its importance (4)

Answer 19

-good solvent (1) and reactions occur faster in solution (2)
-reactive (3) so reactant/ product in hydrolysis/ condensation reactions (4)

Question 20

Describe the biochemical tests you would use to confirm the presence of lipid,
non-reducing sugar and amylase in a sample. (5)

Answer 20

Lipids;
-add ethanol, shake, add water, shake
-positive test is a milky white emulsion formed

Non-reducing sugar;
-do benedict’s test and stays blue/ negative
-boil with acid and then neutralise with alkali
-heat with benedict’s and red precipitate will form

Amylase;
-add biuret’s and turns purple
-add starch and then test for presence of reducing sugar