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A Level H2 Biology Practice Paper 2

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TuitionGoWhere Exam Practice (AI) - Biology H2 A-Level

Marking Scheme & Answer Key (Version 2)

Subject: Biology H2
Paper: Practice Paper (Version 2 of 5)
Total Marks: 60

Section A: Structured Questions

1. Membrane Transport (a) Facilitated diffusion. [1] (b)

  • Movement is down the concentration gradient (from high to low concentration); [1]
  • Therefore, no additional energy (ATP) is required to move the solute. [1] (c)
  • The specific shape/tertiary structure of the binding site; [1]
  • Accept: Complementary shape to solute X.

2. Enzyme Activity (a)

  • As temperature increases from 20°C to 40°C, the rate of reaction increases; [1]
  • This is due to increased kinetic energy of molecules, leading to more frequent successful collisions between enzyme and substrate. [1] (b)
  • At 70°C, the enzyme is denatured; [1]
  • High heat breaks the hydrogen bonds (and ionic bonds) holding the tertiary structure together; [1]
  • The active site changes shape and is no longer complementary to the substrate, so no enzyme-substrate complexes can form. [1]

3. Lipids (a) Glycerol and fatty acids. [1] (b)

  • High energy content: Triglycerides have a high ratio of carbon-hydrogen bonds, which release much energy when oxidised (more than twice the energy per gram compared to carbohydrates); [2]
  • Hydrophobic/Insoluble: They do not affect the water potential of cells/osmotic balance, allowing them to be stored in large quantities without causing water to enter cells by osmosis. [2]
    • Note: Award 1 mark for each property explained. Max 4 marks.

4. Mitochondria (a)

  • The inner membrane contains the electron transport chain (ETC) and ATP synthase; [1]
  • It is impermeable to protons (H+H^+), allowing a proton gradient to be established across it; [1]
  • Protons flow back into the matrix through ATP synthase, driving the synthesis of ATP from ADP and Pi (chemiosmosis). [1] (b)
  • Cyanide blocks the transfer of electrons to oxygen (the final electron acceptor); [1]
  • This stops the flow of electrons through the ETC, preventing the pumping of protons and the establishment of the proton gradient required for ATP synthesis. [1]

5. Gel Electrophoresis (a) Individuals B and D. [1] (b)

  • DNA fragments are negatively charged (due to phosphate groups); [1]
  • When an electric field/potential difference is applied, DNA moves towards the positive anode; [1]
  • Smaller fragments move faster/further through the gel matrix than larger fragments, separating them by size. [1]

Section B: Long Structured Questions

6. The lac Operon (a) A cluster of structural genes controlled by a single promoter/operator region. [1] (b) (i)

  • The regulator gene (lacI) is constitutively expressed to produce the repressor protein; [1]
  • In the absence of lactose, the repressor protein is active. [1] (ii)
  • The active repressor protein binds to the operator region; [1]
  • This physically blocks RNA polymerase from binding to the promoter/transcribing the structural genes (lacZ, lacY, lacA). [1] (c)
  • It prevents the wasteful synthesis of enzymes (beta-galactosidase, permease) when the substrate (lactose) is not available; [1]
  • This conserves energy and resources (amino acids/ATP) for the cell; [1]
  • Allows the bacterium to respond rapidly to changes in environmental nutrient availability. [1]

7. Protein Structure and Misfolding (a)

  • Secondary structure: Local folding of the polypeptide chain into alpha-helices or beta-pleated sheets, stabilised by hydrogen bonds between the backbone atoms (C=O and N-H groups); [2]
  • Tertiary structure: The overall 3D shape of the entire polypeptide chain, stabilised by interactions between R-groups (side chains), including hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions. [2] (b)
  • Changes in pH alter the concentration of H+H^+ ions; [1]
  • This affects the ionisation of charged R-groups (e.g., carboxyl and amino groups); [1]
  • Disruption of ionic bonds and hydrogen bonds causes the tertiary structure to unfold/denature. [1] (c)
  • In correctly folded proteins, hydrophobic R-groups are buried in the core; [1]
  • Misfolding exposes these hydrophobic regions to the aqueous cytoplasm; [1]
  • Hydrophobic interactions cause these exposed regions to clump together with other misfolded proteins to avoid water, leading to aggregation. [1]

8. Cell Division (a) (i) Mitosis: 2 daughter cells; Meiosis: 4 daughter cells. [1] (ii)

  • Mitosis: Daughter cells are genetically identical to the parent cell (clones); [1]
  • Meiosis: Daughter cells are genetically unique/different from the parent and each other (haploid). [1] (b)
  • Crossing over involves the exchange of genetic material between non-sister chromatids of homologous chromosomes; [1]
  • This creates new combinations of alleles on the chromatids (recombinants); [1]
  • Increasing genetic variation in the gametes/offspring. [1] (c)
  • Tumour suppressor genes normally inhibit cell division or promote apoptosis (cell death) if DNA is damaged; [1]
  • A mutation in p53 can lead to a non-functional protein; [1]
  • Cells with damaged DNA are not stopped from dividing or destroyed, leading to uncontrolled mitosis and tumour formation. [1]

9. Properties of Water (a)

  • Oxygen is more electronegative than hydrogen, creating a partial negative charge (δ−\delta-) on oxygen and partial positive charge (δ+\delta+) on hydrogen; [1]
  • The δ+\delta+ hydrogen of one molecule is attracted to the δ−\delta- oxygen of another, forming a hydrogen bond. [1] (b) (i)
  • Water can absorb a large amount of heat energy with only a small rise in temperature; [1]
  • This helps organisms maintain a stable internal body temperature (homeostasis) despite environmental fluctuations. [1] (ii)
  • Water is a polar solvent and dissolves many polar/ionic substances (e.g., glucose, salts, amino acids); [1]
  • This allows metabolic reactions to occur in solution and facilitates the transport of nutrients and waste products in blood/plasma. [1]

10. Cell Cycle (a)

  • DNA replication occurs; [1]
  • Each chromosome is replicated to form two sister chromatids joined at the centromere. [1] (b)
  • Checkpoints ensure that the cell only proceeds to the next stage if specific conditions are met (e.g., DNA is undamaged, chromosomes are attached to spindle fibres); [1]
  • This prevents the propagation of mutations or chromosomal abnormalities to daughter cells. [1] (c)
  • Microtubules form the spindle fibres required to separate chromosomes during mitosis; [1]
  • Inhibiting microtubule formation prevents chromosome segregation, arresting the cell in metaphase and triggering apoptosis or stopping division. [1]