A Level H1 Biology Practice Paper 5

TuitionGoWhere Practice Paper – Biology H1 A-Level – Version 5

Answer Key and Marking Scheme

Total Marks: 47

Section A

1. (a) Mitochondrion / mitochondria [1]; reject ‘mitochondria’ if spelling incorrect but if consistent with plural accept.
   (b) Golgi body / Golgi apparatus / Golgi complex [1].
   (c) Modification / packaging / sorting of proteins (for secretion) [1]; accept formation of secretory vesicles / lysosomes.

2. Phospholipids form a bilayer [1]; hydrophilic phosphate‑containing heads face outward toward aqueous environment on both sides of membrane; hydrophobic fatty‑acid tails face inward away from water [1]. Award [1] for bilayer concept and [1] for orientation of heads and tails.

3. (a) Plasmolysis [1].
   (b) Sucrose solution has lower water potential than cell sap [1]; water moves out of vacuole by osmosis down water potential gradient / from higher to lower water potential [1].

4. (a) As temperature rises from 20 °C to 40 °C, kinetic energy of enzyme and substrate molecules increases [1]; more frequent successful collisions / more enzyme‑substrate complexes formed per unit time [1].
   (b) At 50 °C, the enzyme begins to denature; hydrogen / ionic / hydrophobic bonds that maintain tertiary structure break [1]; by 70 °C denaturation is complete – active site has lost its specific shape and no longer complementary to substrate; no more enzyme‑substrate complexes form / rate drops to zero [1].

5. Secondary structure refers to local folding of the polypeptide chain into α‑helices or β‑pleated sheets, held by hydrogen bonds between backbone –NH and –C=O groups [1]; tertiary structure is the overall 3‑D folding of the whole polypeptide [1], stabilized by hydrogen bonds, ionic bonds, hydrophobic interactions and disulfide bridges between R‑groups [1].

Section B

6. (a) Thymine is a base found only in DNA, not in RNA [1].
   (b) S phase (synthesis phase) [1].
   (c) During S phase, DNA replication occurs; new DNA molecules are synthesised using existing strands as templates [1]; thus radioactive thymine is incorporated into newly made DNA / nuclear radioactivity increases [1].

7. Simple diffusion involves movement of molecules down a concentration gradient, from high to low concentration, without the expenditure of metabolic energy (passive); substances move directly through the phospholipid bilayer or through channel proteins [1]. Active transport moves molecules against a concentration gradient, from low to high concentration, requiring energy (ATP) and specific carrier proteins [1]. Both processes transport substances across the cell membrane, but they differ in energy requirement, direction of movement and protein involvement [1].

8. (a) Mitochondria [1].
   (b) Mitochondria are the site of aerobic respiration [1]; they produce large quantities of ATP via oxidative phosphorylation, which is required for muscle contraction [1].

9. (a) Krebs cycle (citric acid cycle) [1].
   (b) Pyruvate can enter the matrix of the mitochondrion, where it is converted to acetyl‑CoA and then enters the Krebs cycle, producing carbon dioxide as a waste product [1]. Glucose cannot enter the Krebs cycle directly; glycolysis occurs in the cytoplasm and breaks down glucose to pyruvate, but isolated mitochondria lack the enzymes for glycolysis [1]; therefore, incubation with glucose alone does not yield pyruvate inside the mitochondrion and no CO₂ is produced [1].

10. (a) Malonate has a shape similar to the substrate (succinate) [1]; it competes for the active site of succinate dehydrogenase, occupying it temporarily and preventing the substrate from binding, thus reducing enzyme activity [1].
    (b) Increasing the concentration of the substrate (succinate) will out‑compete the inhibitor [1].

Section C

11. Glycogen is soluble in water / cell sap, so it can be readily mobilised / transported within the cell, while triglycerides are insoluble [1]; glycogen is broken down by enzymes that recognise glycosidic bonds, releasing glucose rapidly for respiration, while triglycerides require more extensive processing (e.g., lipolysis, then β‑oxidation) before use [1].

12. A DNA nucleotide is composed of a deoxyribose sugar [1], a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, or guanine) [1].

13. Water is a polar molecule with a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms [1]; the charged regions surround and separate the Na⁺ and Cl⁻ ions, forming hydration shells and keeping them in solution [1].

14. Any two from, e.g.:

    • channel proteins allow facilitated diffusion of ions / small polar molecules [1];
    • carrier proteins mediate active transport / facilitated diffusion [1];
    • receptors for hormones / neurotransmitters (signal transduction) [1];
    • cell‑cell recognition (glycoproteins) [1];
    • enzymatic activity [1];
    • attachment to cytoskeleton / extracellular matrix [1]. Award [1] for each valid, distinct function.

15. Exocytosis is the process by which vesicles containing materials fuse with the cell membrane [1]; the membrane of the vesicle becomes part of the cell membrane, releasing the vesicle contents to the outside of the cell [1].

16. Use catalase extracted from potato tissue [1]; add to hydrogen peroxide at different pH buffers (e.g., pH 4–9); measure the volume of oxygen gas produced per unit time using a gas syringe / measuring cylinder over water, or measure the time taken for a filter‑paper disc soaked in enzyme to rise in a test tube of peroxide [1]. Accept any appropriate method that quantifies rate.

17. Extreme pH changes disrupt the ionic and hydrogen bonds that maintain the enzyme’s tertiary structure [1]; the active site becomes permanently altered and no longer complementary to the substrate; the enzyme is denatured [1].

18. Protein from ribosomes on rough ER is transported to Golgi in vesicles [1]; in the Golgi, the protein is modified (e.g., glycosylation) and packaged into secretory vesicles that bud off and move to the cell membrane for exocytosis [1].

19. (a) Ψ of cell = Ψₛ = –500 kPa (since Ψₚ = 0); solution Ψ = –300 kPa [1].
    (b) Water moves from the solution (higher water potential) into the cell (lower water potential) [1].

20. (award up to 6 marks for a well‑structured discussion)

    • Phospholipid bilayer: hydrophobic core prevents free passage of large polar molecules and ions; only small, non‑polar molecules (O₂, CO₂) and water (via osmosis) diffuse freely [1].
    • Channel proteins: provide hydrophilic pores for facilitated diffusion of specific ions / small polar molecules down their concentration gradient [1].
    • Carrier proteins: undergo conformational change to transport molecules (e.g., glucose) by facilitated diffusion or active transport against gradient using ATP [1].
    • Active transport driven by ATP hydrolysis (e.g., Na⁺/K⁺ pump) [1].
    • Endocytosis / exocytosis: large particles or macromolecules are engulfed or secreted via vesicle formation requiring membrane flexibility [1].
    • Cholesterol in membrane stabilizes fluidity, affecting permeability; unsaturated fatty acids maintain fluidity for protein movement and transport [1].
      Mark according to quality of response linking structural features to selective permeability.