A Level H2 Biology Practice Paper 4

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

Subject: Biology H2
Paper: Practice Paper (Version 4 of 5) - Answer Key
Topic: Cells & Biomolecules

Section A: Structured Questions

1. (a) C: Channel protein / Transport protein [1] E: Cholesterol [1]

(b)

• Phospholipids are amphipathic / have a hydrophilic head and hydrophobic tails. [1]
• In an aqueous environment, the hydrophilic heads face outward towards the water. [1]
• The hydrophobic tails face inward, away from water, forming a bilayer. [1]

(c)

• Acts as a receptor / antigen for cell recognition / cell signalling. [1]

2. (a)

• Rate = 1 / Time
• Rate = 1 / 45
• Answer: 0.0222 s⁻¹ [1]

(b)

• At 40°C, the enzyme is near its optimum temperature; kinetic energy is high, leading to frequent successful collisions. [1]
• At 60°C, the high temperature breaks hydrogen bonds / ionic bonds holding the tertiary structure. [1]
• The active site changes shape / denatures. [1]
• Substrate can no longer bind to the active site / enzyme-substrate complexes cannot form. [1]

(c)

• To maintain a constant pH / prevent pH from changing, which could affect enzyme activity. [1]

3. (a)

• Lane 3 [1]

(b)

• Heterozygous individuals have both the $Hb^A$ and $Hb^S$ alleles. [1]
• These alleles code for haemoglobin proteins with different charges / masses, causing them to migrate to different positions on the gel. [1]

(c)

• Proteins are placed in a gel matrix. [1]
• An electric field / potential difference is applied. [1]
• Proteins migrate based on their charge and size / molecular mass. [1]

4. (a)

• Krebs cycle / Link reaction (Accept either, though Krebs produces more reduced coenzymes). [1]

(b)

• Oxygen acts as the final electron acceptor. [1]
• It combines with electrons and protons to form water. [1]

(c)

• Cyanide blocks the electron transport chain. [1]
• Electrons cannot flow, so protons are not pumped into the intermembrane space. [1]
• No proton gradient is established, so ATP synthase cannot produce ATP via chemiosmosis. [1]

5. (a)

• Phospholipids have a phosphate group / two fatty acid tails, whereas triglycerides have three fatty acid tails and no phosphate group. [1]

(b)

• Phospholipids are amphipathic (hydrophilic head, hydrophobic tail), allowing them to form a bilayer in water. [1]
• Triglycerides are non-polar / hydrophobic. [1]
• Triglycerides would form droplets rather than a membrane barrier. [1]

(c)

• High energy content per gram (due to many C-H bonds). [1]
• Insoluble in water, so they do not affect water potential / osmotic balance of cells. [1]

Section B: Data Interpretation and Application

6. (a)

• A group of genes controlled by a single promoter / operator region. [1]

(b)

• A repressor protein is bound to the operator region. [1]
• This prevents RNA polymerase from binding to the promoter. [1]
• Therefore, transcription of the structural genes cannot occur. [1]

(c)

• It saves energy / resources. [1]
• Enzymes are only produced when the substrate (lactose) is present, preventing waste. [1]

7. (a)

• As glucose concentration increases, the rate of uptake increases. [1]
• Up to a certain point, after which the rate levels off / reaches a maximum. [1]

(b)

• Glucose enters via facilitated diffusion using carrier proteins. [1]
• At high concentrations, all carrier proteins are saturated / occupied. [1]
• The rate is limited by the number of carrier proteins available. [1]

(c)

• Sketch: Curve starts at origin, rises less steeply than original, and plateaus at the same maximum rate (Vmax) but at a higher concentration. [2]
  • Marking note: 1 mark for lower initial slope, 1 mark for same Vmax.

8. (a)

• Each new DNA molecule consists of one original (parental) strand and one newly synthesized strand. [1]

(b)

• DNA Helicase: Breaks hydrogen bonds between base pairs; unzips the double helix. [2]
• DNA Polymerase: Joins nucleotides together; forms phosphodiester bonds; adds nucleotides in the 5' to 3' direction. [2]

(c)

• To ensure that each daughter cell receives a complete set of genetic information. [1]
• To maintain the chromosome number / genetic identity. [1]

9. (a)

• Peptide bond. [1]

(b)

• Condensation reaction. [1]
• Between the amine group of one amino acid and the carboxyl group of another. [1]
• A molecule of water is released. [1]

(c)

• The sequence of amino acids (primary structure) determines the interactions between R-groups. [1]
• These interactions (hydrogen bonds, ionic bonds, disulfide bridges) cause folding into secondary and tertiary structures. [1]
• The specific 3D shape determines the protein's function. [1]

10. (a)

• Water molecules are polar (dipole). [1]
• Positive ends attract negative ions/parts of molecules; negative ends attract positive ions/parts. [1]
• This surrounds solute molecules, keeping them in solution. [1]

(b)

• Importance: High specific heat capacity. [1]
• Explanation: Buffers temperature changes, providing a stable environment for enzymes. [1]
  • Alternative: High latent heat of vaporisation -> Cooling effect via sweating.

Section C: Extended Response

11. Compare and contrast prokaryotic and eukaryotic cells. [8]

• Genetic Material:
  • Prokaryotes: DNA is circular, naked (no histones), located in the nucleoid region. [1]
  • Eukaryotes: DNA is linear, associated with histone proteins, enclosed within a nuclear envelope. [1]
• Organelles:
  • Prokaryotes: Lack membrane-bound organelles (e.g., no mitochondria, Golgi, ER). [1]
  • Eukaryotes: Possess membrane-bound organelles (mitochondria, ER, Golgi, lysosomes). [1]
• Ribosomes:
  • Prokaryotes: Have 70S ribosomes. [1]
  • Eukaryotes: Have 80S ribosomes (and 70S in mitochondria/chloroplasts). [1]
• Cell Wall:
  • Prokaryotes: Cell wall made of peptidoglycan (murein). [1]
  • Eukaryotes: Plant cell walls made of cellulose; animal cells lack cell walls. [1]
• Size:
  • Prokaryotes are generally smaller (1-10 µm). [1]
  • Eukaryotes are generally larger (10-100 µm). [1]
  • (Award max 8 marks. Points must be comparative.)

12. Discuss the importance of enzymes in metabolic pathways. [10]

• Activation Energy:
  • Enzymes lower the activation energy required for reactions. [1]
  • This allows metabolic reactions to occur rapidly at body temperature. [1]
  • Without enzymes, reactions would be too slow to sustain life. [1]
• Specificity:
  • Enzymes are specific due to the complementary shape of the active site (lock and key / induced fit). [1]
  • This ensures that metabolic pathways are controlled and specific products are formed. [1]
  • Prevents unwanted side reactions. [1]
• Control/Regulation:
  • Enzyme activity can be regulated (e.g., by inhibitors, pH, temperature, cofactors). [1]
  • This allows cells to respond to changes in environment or metabolic demand. [1]
  • Example: End-product inhibition prevents waste of resources. [1]
• Environmental Factors:
  • Temperature: Increases kinetic energy up to optimum; denaturation above optimum. [1]
  • pH: Affects ionization of R-groups and active site shape; each enzyme has an optimum pH. [1]
  • Substrate concentration: Rate increases until saturation (Vmax). [1]
  • (Award marks for clear explanation and linkage to metabolic importance. Max 10 marks.)