4/5/6 Mark Qs Biological Molecules

Question 1

Describe the chemical reactions involved in the conversion of polymers to monomers and monomers to polymers. (5) Give two named examples of polymers and their associated monomers to illustrate your answer.

Answer 1

1. A condensation reaction joins monomers together and forms a (chemical) bond and releases water;
2. A hydrolysis reaction breaks a (chemical) bond between monomers and uses water;
3. A suitable example of polymers and the monomers from which they are made;
   3 and 4. Polymers must contain many monomers.
   3 and 4: suitable examples include: amino acid and polypeptide, protein, enzyme, antibody or specific example nucleotide and polynucleotide, DNA or RNA
   Alpha glucose and starch/glycogen
   Beta glucose and cellulose.
   If neither specific carbohydrate example is given, allow monosaccharide/glucose and polysaccharide.
   3 and 4. Reject (once) reference to triglycerides.
4. A second suitable example of polymers and the monomers from which they are made;
5. Reference to a correct bond within a named polymer; Reject reference to ester bond.

Question 2

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

Answer 2

1. Structure is determined by (relative) position of amino acid/R group/interactions; Accept for ‘interactions’, hydrogen bonds / disulfide bridges / ionic bonds / hydrophobichydrophilic interactions
2. Primary structure is sequence/order of amino acids;
3. Secondary structure formed by hydrogen bonding (between amino acids); Accept alpha helix/β-pleated sheet for ‘secondary structure’
4. Tertiary structure formed by interactions (between R groups); Accept for ‘interactions’, hydrogen bonds / disulfide bridges / ionic bonds / hydrophobic hydrophilic interactions
5. Creates active site in enzymes OR Creates complementary/specific shapes in antibodies/carrier proteins/receptor (molecules);
6. Quaternary structure contains >1 polypeptide chain OR Quaternary structure formed by interactions/bonds between polypeptides; Accept for ‘intereactions’, hydrogen bonds/ disulfide bridges/ionic bonds/hydrophobichydrophilic interactions Accept prosthetic (group)

Question 3

Describe the structure of DNA. (5)

Answer 3

1. Polymer of nucleotides; Accept ‘Polynucleotide’ Accept for ‘phosphate’. phosphoric acid
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 4

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

Answer 4

1. and 2. Accept for 2 marks correct names of three components adenine, ribose/pentose, three phosphates;; Accept for 1 mark, correct name of two components Accept for 1 mark, ADP and phosphate/Pi Ignore adenosine Accept suitably labelled diagram
2. Condensation (reaction); Ignore phosphodiester
3. ATP synthase; Reject ATPase

Question 5

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

Answer 5

1. A metabolite in condensation/hydrolysis/ photosynthesis/respiration;
2. A solvent so (metabolic) reactions can occur OR A solvent so allowing transport of substances;
3. High (specific) heat capacity so buffers changes in temperature; For ‘buffer’ accept ‘resist’.
4. Large latent heat of vaporisation so provides a cooling effect (through evaporation); Reject latent heat of evaporation
5. Cohesion (between water molecules) so supports columns of water (in plants); For ‘columns of water’ accept ‘transpiration stream’. Do not credit ‘transpiration’ alone but accept description of ‘stream’. For ‘columns of water’ accept ‘cohesion-tension (theory)’.
6. Cohesion (between water molecules) so produces surface tension supporting (small) organisms; For cohesion accept hydrogen bonding Ignore reference to pH. Allow other suitable properties but must have a valid explanation. For example * ice floating so maintaining aquatic habitat beneath * water transparent so allowing light penetration for photosynthesis

Question 6

Describe the roles of iron ions, sodium ions, and phosphate ions in cells. (5)

Answer 6

Must have MP1 for 5 max 3 max for sodium and 3 max for phosphate
Iron ions
1. Haemoglobin binds/associates with oxygen OR Haemoglobin transports/loads oxygen; Ignore reference to 2+ or 3+ in Fe2+ or Fe3+
Sodium ions
1. Co-transport of glucose/amino acids (into cells);
2. (Because) sodium moved out by active transport/Na – K pump;
3. Creates a sodium concentration/diffusion gradient;
4. Affects osmosis/water potential;
Phosphate ions
1. Affects osmosis/water potential; Accept 5. OR 6. – not both
2. Joins nucleotides/in phosphodiester bond/in backbone of DNA/RNA/in nucleotides;
3. Used in/to produce ATP; Reject ‘energy produced’
4. Phosphorylates other compounds (usually) making them more reactive;
5. Hydrophilic/water soluble part of phospholipid bilayer/membrane; Accept for 1 mark, Sodium ions cause water reabsorption in kidneys OR Sodium ions establish resting potential (in neurones) OR Sodium ion diffusion creates action potential

Question 7

Messenger RNA (mRNA) is used during translation to form polypeptides. Describe how mRNA is produced in the nucleus of a cell. (6)

Answer 7

1. Helicase;
2. Breaks hydrogen bonds;
3. Only one DNA strand acts as template;
4. RNA nucleotides attracted to exposed bases;
5. (Attraction) according to base pairing rule;
6. RNA polymerase joins (RNA) nucleotides together;
7. Pre-mRNA spliced to remove introns.

Question 8

Describe the structure of proteins. (6)

Answer 8

1. Polymer of amino acids;
2. Joined by peptide bonds;
3. Formed by condensation;
4. Primary structure is order of amino acids;
5. Secondary structure is folding of polypeptide chain due to hydrogen bonding; Accept alpha helix / pleated sheet
6. Tertiary structure is 3-D folding due to hydrogen bonding and ionic / disulfide bonds;
7. Quaternary structure is two or more polypeptide chains.

Question 9

In humans, the enzyme maltase breaks down maltose to glucose. This takes place at normal body temperature. Explain why maltase: * only breaks down maltose * allows this reaction to take place at normal body temperature. (5)

Answer 9

1. Tertiary structure / 3D shape of enzyme (means); Accept references to active site
2. Active site complementary to maltose / substrate / maltose fits into active site / active site and substrate fit like a lock and key; Idea of shapes fitting together
3. Description of induced fit;
4. Enzyme is a catalyst / lowers activation energy / energy required for reaction; Accept “provides alternative pathway for the reaction at a lower energy level”
5. By forming enzyme-substrate complex; Accept idea that binding stresses the bonds so more easily broken Do not award point 5 simply for any reference to E-S complex

Question 10

Scientists have investigated the effects of competitive and non-competitive inhibitors of the enzyme maltase. Describe competitive and non-competitive inhibition of an enzyme. (6)

Answer 10

1. Inhibitors reduce binding of enzyme to substrate / prevent formation of ES complex;
   Max 3 if only one type of inhibition dealt with. Accept maltase and maltose as examples of enzyme and substrate (and others) Only once, for either inhibitor
   (Competitive inhibition),
2. Inhibitor similar shape (idea) to substrate;
3. (Binds) in to active site (of enzyme); Accept allows max rate of reaction to be reached / max product will eventually be formed Accept complementary to active site
4. (Inhibition) can be overcome by more substrate;
   (Non-competitive inhibition),
5. Inhibitor binds to site on enzyme other than active site;
6. Prevents formation of active site / changes (shape of) active site; Accept does not allow max rate of reaction to be reached / max product will not be formed
7. Cannot be overcome by adding more substrate;

Question 11

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

Answer 11

1. made from β-glucose;
2. joined by condensation / removing molecule of water / glycosidic bond;
3. 1 : 4 link specified or described;
4. “flipping over” of alternate molecules;
5. hydrogen bonds linking chains / long straight chains;
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; (allow maximum of 4 marks for structural features)

Question 12

Describe the roles of calcium ions and ATP in the contraction of a myofibril. (5)

Answer 12

1. Calcium ions diffuse into myofibrils from (sarcoplasmic) reticulum;
2. (Calcium ions) cause movement of tropomyosin (on actin);
3. (This movement causes) exposure of the binding sites on the actin;
4. Myosin heads attach to binding sites on actin;
5. Hydrolysis of ATP (on myosin heads) causes myosin heads to bend;
6. (Bending) pulling actin molecules;
7. Attachment of a new ATP molecule to each myosin head causes myosin heads to detach (from actin sites).

Question 13

The events that take place during interphase and mitosis lead to the production of two genetically identical cells. Explain how. (4)

Answer 13

1. DNA replicated; Reject: DNA replication in the wrong stage
2. (Involving) specific / accurate / complementary base-pairing; Accept: semi conservative replication
3. (Ref to) two identical / sister chromatids;
4. Each chromatid / moves / is separated to (opposite) poles / ends of cell. Reject: meiosis / homologous chromosomes / crossing over Note: sister chromatids move to opposite poles / ends = 2 marks for mp 3 and mp 4 Reject: events in wrong phase / stage