A Level H1 Biology Practice Paper 4

TuitionGoWhere Practice Paper - Biology H1 A-Level

Answer Key and Marking Scheme

Subject: Biology H1
Paper: Practice Paper (Version 4 of 5)
Topic Focus: Cells & Biomolecules

Section A: Structured Questions

1. (a) A: Hydrophilic head (phosphate group) [1]; B: Hydrophobic tail (fatty acid) [1]. (b) Heads are polar/hydrophilic and attract water [1]; Tails are non-polar/hydrophobic and repel water [1]; Therefore, heads face outward towards aqueous environment and tails face inward [1]. (Max 2) (c) Any two: Regulates membrane fluidity [1]; Stabilizes the membrane at high temperatures [1]; Prevents packing of phospholipids at low temperatures [1]; Reduces permeability to small water-soluble molecules [1].

2. (a) Increased kinetic energy of enzyme and substrate molecules [1]; More frequent collisions between enzyme and substrate [1]; More enzyme-substrate complexes formed [1]. (Max 2) (b) High temperature breaks hydrogen/ionic bonds holding the tertiary structure [1]; Active site changes shape/denatures [1]; Substrate no longer fits the active site [1]. (Max 2) (c) pH 3 is acidic/away from optimum pH [1]; Changes the charge of R-groups/ionic bonds in the enzyme [1]; Alters the shape of the active site/denatures enzyme [1]. (Max 2)

3. (a) C: Outer membrane [1]; D: Cristae (inner membrane) [1]. (b) Reduced NAD (NADH) [1]; Reduced FAD (FADH₂) [1]. (c) Increases surface area [1]; For attachment of electron transport chain proteins/ATP synthase [1]; Allows for more ATP production [1]. (Max 2)

4. (a) Water molecules are polar/dipolar [1]; Form hydrogen bonds with polar/charged solutes [1]; Surrounding solute molecules keeps them in solution [1]. (Max 2) (b) Water can absorb/lose large amounts of heat energy with little change in temperature [1]; Helps maintain stable internal body temperature/homeostasis [1]; Prevents overheating of cells [1]. (Max 2)

5. (a) E: Glycerol [1]; F: Fatty acids [1]. (b) High energy content per gram (more than carbohydrates) [1]; Insoluble in water (does not affect water potential/osmosis) [1]; Compact storage [1]. (Max 2) (c) Emulsion test [1]; Add ethanol and shake, then add water [1]; White cloudy emulsion indicates lipids [1]. (Max 2)

6. (a) Any two: Active transport requires energy (ATP) / Facilitated diffusion does not [1]; Active transport moves against concentration gradient / Facilitated diffusion moves down concentration gradient [1]; Active transport uses carrier proteins only / Facilitated diffusion uses channel or carrier proteins [1]. (b) Concentration of mineral ions is higher in root hair cells than in soil [1]; Ions must move against concentration gradient [1]; Requires active transport to accumulate ions [1]. (Max 2)

7. (a) The sequence of amino acids in the polypeptide chain [1]. (b) The sequence of amino acids (primary structure) determines the position of R-groups [1]; R-groups interact via hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions [1]; These interactions cause the polypeptide to fold into a specific 3D shape (tertiary structure) [1].

8. (a) Metaphase [1]. (b) Chromosomes align at the equator/center of the cell [1]; Spindle fibers attach to the centromeres of chromosomes [1]. (c) Produces genetically identical cells [1]; Essential for growth, repair, and asexual reproduction [1].

9. (a) Any two: DNA is double-stranded / RNA is single-stranded [1]; DNA contains thymine / RNA contains uracil [1]; DNA contains deoxyribose sugar / RNA contains ribose sugar [1]. (b) Carries genetic code from DNA in nucleus to ribosomes in cytoplasm [1]; Template for translation/protein synthesis [1].

10. (a) The minimum amount of energy required for a reaction to occur [1]. (b) Enzyme binds to substrate to form enzyme-substrate complex [1]; Strains bonds in substrate or holds substrates in correct orientation [1]; Lowers the energy barrier for the reaction to proceed [1]. (Max 2)

Section B: Data Interpretation and Application

11. (a) 0.3 mol dm⁻³ [1]; This is the isotonic point where there is no net movement of water/no change in mass [1]. (b) Water potential of 0.1 mol dm⁻³ solution is higher (less negative) than potato cells [1]; Water enters cells by osmosis [1]; Through partially permeable membrane [1]; Down water potential gradient [1]. (Max 3) (c) Mass would decrease further/plasmolysis occurs [1]; Water potential of solution is much lower than cells [1]; Water leaves cells by osmosis [1]. (Max 2)

12. (a) Globular shape is compact/soluble in blood [1]; Contains heme group with iron to bind oxygen [1]; Quaternary structure (4 subunits) allows for cooperative binding [1]. (Max 3) (b) Change in amino acid changes primary structure [1]; Alters R-group interactions/tertiary structure [1]; Hemoglobin becomes insoluble/forms fibers/distorts red blood cell shape [1]; Reduced oxygen carrying capacity [1]. (Max 3)

13. (a) Phospholipids and proteins can move laterally within the layer [1]; Due to weak hydrophobic interactions between tails [1]. (b) Proteins are embedded in the bilayer like tiles in a mosaic [1]; Different types of proteins (channel, carrier, receptor) are scattered throughout [1]. (c) Small, non-polar/lipid-soluble substances (e.g., O₂, CO₂) can pass through the phospholipid bilayer by simple diffusion [1]; Large, polar, or charged substances (e.g., ions, glucose) cannot pass through the hydrophobic core [1]; Require channel/carrier proteins to facilitate movement [1].

14. (a) Percentage of Adenine equals percentage of Thymine [1]. (b) A pairs with T via hydrogen bonds in double helix [1]; Therefore, amount of A must equal amount of T [1]. (c) Cannot be determined / Not applicable [1]; In single-stranded DNA, base pairing rules (Chargaff's rules) do not apply [1]; OR if assuming double-stranded logic was mistakenly applied: 30% (but correct answer is that it's unpredictable without more data). Accept: "Cannot be calculated because it is single-stranded and bases do not pair." [2]

15. (a) At low substrate concentration, active sites are empty/available [1]; Increasing substrate concentration increases frequency of collisions/ES complex formation [1]; Rate increases proportionally [1]. (b) Sketch: Curve rises more slowly initially but reaches same Vmax [1]; Explanation: Inhibitor competes with substrate for active site [1]; At high substrate concentration, substrate outcompetes inhibitor [1]; Vmax is unchanged [1]. (Max 3)

Section C: Extended Response

16. Marking Guidance:

• Solvent: Polar nature allows dissolution of ions/polar molecules for metabolic reactions/transport [1].
• High Specific Heat Capacity: Buffers temperature changes, maintaining stable environment for enzymes [1].
• High Latent Heat of Vaporization: Cooling effect via sweating/transpiration [1].
• Cohesion/Adhesion: Transpiration stream in plants/column of water in xylem [1].
• Metabolite: Reactant in hydrolysis/photosynthesis [1].
• Incompressibility: Hydrostatic skeleton/turgor pressure in plants [1]. (1 mark per valid point explained, up to 6 marks.)

17. Marking Guidance:

• Starch (Amylose/Amylopectin): Alpha-glucose, helical/branched, compact storage in plants, insoluble [1].
• Glycogen: Alpha-glucose, highly branched, compact storage in animals/fungi, insoluble, rapid hydrolysis [1].
• Cellulose: Beta-glucose, straight chains, hydrogen bonds between chains form microfibrils, structural support in plant cell walls, high tensile strength [1].
• Comparison: Starch/Glycogen for storage vs Cellulose for structure [1]; Alpha vs Beta linkages [1]; Solubility/branching differences [1]. (1 mark per valid comparison/contrast, up to 6 marks.)

18. Marking Guidance:

• Phospholipid Bilayer: Hydrophobic core prevents passage of water-soluble/charged substances [1]; Allows passage of small non-polar molecules [1].
• Proteins: Channel proteins allow facilitated diffusion of ions/polar molecules [1]; Carrier proteins allow active transport/facilitated diffusion [1]; Specificity of proteins controls what enters/leaves [1].
• Cholesterol: Regulates fluidity and permeability [1].
• Glycoproteins/Glycolipids: Cell recognition/signaling [1].
• Selective Permeability: Membrane controls internal environment/homeostasis [1]. (1 mark per valid point explained, up to 6 marks.)

19. Marking Guidance:

• Glycolysis: Cytoplasm [1]; Glucose to Pyruvate [1]; Net gain 2 ATP, Reduced NAD [1].
• Link Reaction: Mitochondrial Matrix [1]; Pyruvate to Acetyl-CoA [1]; CO₂ released, Reduced NAD formed [1].
• Krebs Cycle: Mitochondrial Matrix [1]; Acetyl-CoA enters cycle [1]; CO₂ released, Reduced NAD/FAD, ATP formed [1].
• Oxidative Phosphorylation: Inner Mitochondrial Membrane/Cristae [1]; Electron Transport Chain pumps protons [1]; Chemiosmosis/ATP Synthase produces ATP [1]; Oxygen is final electron acceptor forming water [1]. (1 mark per valid point, up to 6 marks.)

20. Marking Guidance:

• Catalysis: Lower activation energy, speed up metabolic reactions [1].
• Specificity: Active site shape matches substrate [1].
• Metabolic Pathways: Series of enzyme-catalyzed reactions [1]; Product of one is substrate for next [1].
• Regulation (Inhibition): Competitive/Non-competitive inhibitors [1]; End-product inhibition (negative feedback) [1]; Prevents waste of resources [1].
• Regulation (Environmental): pH/Temperature affects enzyme activity [1].
• Compartmentalization: Enzymes in specific organelles (e.g., lysosomes) [1]. (1 mark per valid point explained, up to 6 marks.)