Biological Molecules : EXAM MARK SCHEME ASNWERE

Question 1

Describe how the structure of glycogen is related to its function.

Answer 1

• it has a helix and branched structure and so is compact
• Branched so more ends for faster hydrolysis;
• polymer of Glucose so provides respiratory substrate for energy (release);
• Insoluble so does not affect water potential/osmosis

alternatives :
- Polymer of glucose so easily hydrolysed

Question 2

Describe and explain how the structure of cellulose relates to its function (4)

Answer 2

• cellulose is made up of long , straight , unbranched chains of beta glucose
• which interact to form microfibrils
• the chains of b glucose are joined together by hydrogen bonds
• which provide strength in cell walls

Question 3

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 3

similarity :
Both contain galactose / a glycosidic bond
Difference :
Lactulose contains fructose whereas lactose contains glucose

Question 4

Glycogen and cellulose are both carbohydrates.
Describe 4 differences between the structure of a cellulose molecule and a glycogen molecule.

Answer 4

• Cellulose is made up of beta glucose no mere and glue even is made up of alpha glucose monomers
• cellulose molecule has straight chains and glycogen is branched
• cellulose molecule has straight chain and glycogen is coiled.
• glycogen has 1,4 and 1,6 glycosides bonds and cellulose only has 1,4- glycosidic bonds

Question 5

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

Answer 5

Any two from:
1. Insoluble (in water), so doesn’t affect water potential;
2. Branched / coiled / (a-)helix, so makes molecule compact;
3. Polymer of (a-)glucose so provides glucose for respiration;
4. Branched / more ends for fast breakdown / enzyme action;
5. Large (molecule), so can’t cross the cell membrane

Question 6

Describe the structure of glycogen (2)

Answer 6

polysaccharide of a Alpha glucose joined by glycosidic bonds with a branched structure.

Question 7

Suggest how glycogen acts as a source or energy.

Answer 7

• hydrolysed to glucose
• glucose is than used in respiration

Question 8

Explain how cellulose molecules are adapted to their function in plant cells.

Question 9

Describe how a triglyceride molecule is formed ( 3)

Answer 9

1. Made up of One glycerol and three fatty acids
2. Condensation reaction and removal of three molecules of water
3. Ester bonds formed

Question 10

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

Answer 10

1. Condensation reaction
2. Between a glycerol and a fatty acid

Question 11

Describe how you would test a liquid sample for the presence of lipids and how you would recognise a positive result

Answer 11

• add ethanol to sample
• shake and mix
• then add water
• positive result : milky white emulsion

Question 12

Describe the induced fit model and how an enzyme acts as a catalyst

Answer 12

• Substrate binds to the active site
• active site undergoes a conformational change so distorts bonds in the substrate to make it complementary to substrate
• Reducing activation energy

Question 13

Explain how competitive inhibitor reduce the rate of an enzyme catalysed reaction (3)

Answer 13

1. Inhibitor is a similar shape to substrate
2. This binds to the active site
3. Thus preventing an enzyme substrate complex from Forminf

Question 14

Describe how the structure of a protein depends on the amino acid it contains (5)

Answer 14

• Structure is determined by (relative) position of amino acid interactions I.e. hydrogen bonds
  2. Primary structure is sequence/order of amino acids;
  3. Secondary structure is formed by hydrogen bonding (between amino
  acids); leading to a alpha helix/3-pleated sheet’
  4. Tertiary structure is formed by interactions I.e differing bonds between R groups
  5. Creates specific proteins e.g. enzyme active site, antibodies
  6. quaternary structure contains more than 1 polypeptide chain

Question 15

When bread becomes stale, the structure of some of the starch is
changed. This changed starch is called retrograded starch.
Scientists have suggested retrograded starch is a competitive inhibitor of
amylase in the small intestine.
Assuming the scientists are correct, suggest how eating stale bread could
help to reduce weight gain.

Answer 15

1. Less hydrolysis of starch;
2. (To) maltose;
3. (So) less absorption (of glucose)
   OR
   (So) more egested (starch/carbohydrate);

Question 16

Explain how the active site of an enzyme causes a high rate of reaction.(3)

Answer 16

1. Lowers activation energy;
2. Induced fit causes active site (of enzyme) to change shape;
3. (So) enzyme-substrate complex causes bonds to form/break;

Question 17

Describe a biochemical test to confirm the presence of protein in a solution (2)

Answer 17

1. Add biuret (reagent);
2. (Positive result) purple/lilac/violet /mauve;

Question 18

Dipeptides may differ in the type of amino acids they contain.
Describe two other ways in which all dipeptides are similar and one way in
which they might differ.

Answer 18

similarities:
Amine/NH2 (group at end);
Carboxyl/COOH (group at end);
Two R groups;
All contain C and H and N and O;
differences:
Variable/different R group(s);

Question 19

Describe how a non-competitive inhibitor can reduce the rate of an
enzyme-controlled reaction. (3)

Answer 19

• Attaches to the enzyme at a site other than the active site ( the allosteric site)
• Changes (shape of) the active site
  OR
  Changes tertiary structure (of enzyme);
  3. (So active site and substrate) no longer complementary so
  no substrate can bind.

Question 20

Describe how you would test a liquid sample for the presence of lipid and
how you would recognise a positive result.

Answer 20

• Add ethanol/alcohol and shake/mix then pour into/add water;
• White/milky (emulsion)

Question 21

Describe how a triglyceride molecule is formed.

Answer 21

• Condensation (reaction)
• Between of glycerol and fatty acid;

Question 22

Name the type of bond between:
complementary base pairs ______
adjacent nucleotides in a DNA strand (2)

Answer 22

1. Hydrogen (bonds);
2. Phosphodiester (bonds);

Question 23

Describe the structure of DNA. (5)

Answer 23

1. Polymer of nucleotides;
2. Each nucleotide formed from deoxyribose, a phosphate (group) and
   an organic/nitrogenous base;
3. Phosphodiester bonds (between nucleotides);
4. Double helix/2 strands held by hydrogen bonds;
5. (Hydrogen bonds/pairing) between adenine, thymine and
   cytosine, guanine;

Question 24

Describe how a phosphodiester bond is formed between two nucleotides
within a DNA molecule. (2)

Answer 24

1. Condensation (reaction)/loss of water;
2. (Between) phosphate and deoxyribose;
3. (Catalysed by) DNA polymerase

Question 25

Name the protein associated with DNA in a chromosome. (1)

Answer 25

Histone

Question 26

In the process of semi-conservative DNA replication, the two strands within
a DNA molecule are separated. Each then acts as a template.
Describe how the separation of strands occurs. (2)

Answer 26

DNA helicase;
2. Breaks hydrogen bonds between base pairs/ AT and
GC/complementary bases

Question 27

Name the two scientists who proposed models of the chemical structure of
DNA and of DNA replication. (1)

Answer 27

Watson and Crick

Question 28

A scientist replicated DNA in a test tube. To do this, he mixed an enzyme with
identical single-stranded DNA fragments and a solution containing DNA
nucleotides.
(b) Name the enzyme used in this DNA replication.

Answer 28

DNA polymerase

Question 29

Use your knowledge of semi-conservative replication of DNA to suggest:
1. the role of the single-stranded DNA fragments ____________________ and the role of the DNA nucleotides (3)

Answer 29

Role of single-stranded DNA fragments
1. Template;
2. Determines order of nucleotides/bases;
Role of DNA nucleotides
3. Forms complementary pairs / A – T, G - C
OR
Forms complementary (DNA) strand;

Question 30

Give two features of DNA and explain how each one is important in the
semi-conservative replication of DNA. (2)

Answer 30

• Weak / easily broken hydrogen bonds between bases allow two
  strands to separate / unzip;
  may appear in the same feature
  2. Two strands, so both can act as templates;
  3. Complementary base pairing allows accurate replication;

Question 31

Describe the role of two named enzymes in the process of semiconservative
replication of DNA. (3)

Answer 31

1. (DNA) helicase causes breaking of hydrogen/H bonds (between DNA
   strands);
2. DNA polymerase joins the (DNA) nucleotides
3. Forming phosphodiester bonds;

Question 32

Describe the function of each of these enzymes:
- DNA Helicase
-DNA polymerase

Question 33

Adenosine triphosphate (ATP) is a nucleotide derivative. Contrast the structures of ATP and a nucleotide found in DNA to give two differences. (2)

Answer 33

ATP has ribose and DNA nucleotide has
deoxyribose;
2. ATP has 3 phosphate (groups) and DNA
nucleotide has 1 phosphate (group);
3. ATP – base always adenine and in DNA
nucleotide base can be different / varies;