A Level H1 Biology Practice Paper 3

TuitionGoWhere Practice Paper - Biology H1 A-Level

Answer Key & Marking Scheme - Version 3

Subject: Biology H1
Topic: Cells & Biomolecules

Section A: Structured Questions

1. Phospholipids

• (a) [2]
  • Head: Hydrophilic (1)
  • Tails: Hydrophobic (1)
• (b) [2]
  • Phospholipids form a bilayer (1).
  • Hydrophilic heads face outward towards the aqueous environment (cytoplasm/tissue fluid), and hydrophobic tails face inward, away from water (1).
• (c) [1]
  • The interior of the membrane is hydrophobic/non-polar (1).
  • Note: Accept "impermeable to charged/polar molecules".

2. Enzyme Kinetics

• (a) [2]
  • Rate of reaction increases as temperature increases (1).
  • More kinetic energy leads to more frequent successful collisions between enzyme and substrate (1).
• (b) [3]
  • High temperature breaks hydrogen bonds/ionic bonds holding the tertiary structure (1).
  • The enzyme denatures (1).
  • The active site changes shape and is no longer complementary to the substrate (1).
• (c) [1]
  • The enzyme is fully denatured (1).

3. Membrane Transport

• (a) [2]
  • Mechanism A: Active Transport (1)
  • Mechanism B: Facilitated Diffusion (1)
  • Note: Accept Simple Diffusion for B if diagram shows no protein, but typically B implies protein channel/carrier in these diagrams. If diagram shows movement against gradient for A and down for B via protein, this is the standard answer.
• (b) [1]
  • Mechanism A requires ATP/energy; Mechanism B does not (1).
• (c) [2]
  • Glucose is a large/polar molecule (1).
  • It cannot pass through the hydrophobic fatty acid tails of the phospholipid bilayer (1).

4. Haemoglobin

• (a) [2]
  • Consists of four polypeptide chains (1).
  • Each chain is associated with a haem group (1).
• (b) [2]
  • Specific 3D shape creates a binding site for oxygen (1).
  • Conformational change allows cooperative binding (loading/unloading) (1).
• (c) [3]
  • Change in amino acid sequence (primary structure) changes the R-group interactions (1).
  • This alters the folding/tertiary structure (1).
  • Haemoglobin becomes insoluble/fibrous and cannot carry oxygen effectively (1).

5. Water

• (a) [1]
  • The tendency of water molecules to move from one region to another (or measure of free energy of water molecules) (1).
• (b)
  • (i) [1] Into the cell (1).
  • (ii) [2] The cell becomes turgid (1). The vacuole expands and pushes the cytoplasm against the cell wall (1).
• (c) [2]
  • Water is polar, allowing it to dissolve polar/ionic substances (1).
  • This allows metabolic reactions to occur in solution/transport of nutrients (1).

6. Lipids

• (a) [2]
  • One glycerol molecule (1).
  • Three fatty acid chains (1).
• (b) [2]
  • Triglyceride has 3 fatty acids; Phospholipid has 2 fatty acids and 1 phosphate group (1).
  • Phospholipid has a hydrophilic head and hydrophobic tails; Triglyceride is entirely hydrophobic (1).
• (c) [2]
  • Triglycerides are insoluble in water, so they do not affect water potential/osmosis in cells (1).
  • They have a high energy-to-mass ratio (more C-H bonds) (1).

7. Mitochondria

• (a) [1]
  • Cristae (or Inner Membrane) (1).
• (b) [2]
  • Increases surface area (1).
  • Allows for more electron transport chain proteins/ATP synthase enzymes to be embedded (1).
• (c) [1]
  • Krebs Cycle (Link Reaction also occurs here, but Krebs is the main cycle) (1).

8. Nucleic Acids

• (a) [3]
  • Sugar: Deoxyribose (DNA) vs Ribose (RNA) (1).
  • Bases: Thymine (DNA) vs Uracil (RNA) (1).
  • Structure: Double stranded/helix (DNA) vs Single stranded (RNA) (1).
• (b) [2]
  • Carries genetic code from DNA in nucleus to ribosomes in cytoplasm (1).
  • Serves as a template for translation/protein synthesis (1).

9. Inhibition

• (a) [2]
  • Competitive inhibitor has similar shape to substrate (1).
  • Competes for the active site, reducing the number of enzyme-substrate complexes formed (1).
• (b) [2]
  • Increasing substrate concentration reduces the effect of inhibition (1).
  • Substrate outcompetes the inhibitor for the active site (1).
• (c) [1]
  • Non-competitive inhibitors bind to an allosteric site (not the active site) (1).

10. Collagen

• (a) [3]
  • Three polypeptide chains wound into a triple helix (1).
  • Held together by hydrogen bonds (1).
  • Cross-links between molecules provide strength (1).
• (b) [2]
  • Location: Tendons/Ligaments/Bone/Skin (1).
  • Function: Withstands pulling forces/tensile strength due to strong cross-links (1).

Section B: Free Response Questions

11. Properties of Water [8]

• Thermal Properties (High Specific Heat Capacity):

  • Water has a high specific heat capacity due to hydrogen bonds requiring energy to break (1).
  • This allows water to buffer temperature changes, maintaining stable internal environments for organisms/homeostasis (1).
  • High latent heat of vaporization allows for cooling via sweating/transpiration without excessive water loss (1).

• Solvent Properties:

  • Water is a polar molecule, making it an excellent solvent for ions and polar molecules (1).
  • Metabolic reactions occur in aqueous solution (cytoplasm/blood) (1).
  • Transport of nutrients (glucose, amino acids) and waste (urea, CO2) in blood/plasma (1).

• Cohesion and Adhesion:

  • Cohesion: Water molecules stick to each other via hydrogen bonds (1).
  • Adhesion: Water molecules stick to other surfaces (e.g., xylem walls) (1).
  • This creates a continuous column of water in plants, allowing transpiration pull to transport water to leaves against gravity (1).
  • Surface tension supports small organisms (e.g., pond skaters) (1).

• Marking Note: Award marks for clear explanation linking property to biological significance. Max 8 marks.

12. Polysaccharides: Starch, Glycogen, Cellulose [10]

• Starch (Plants):

  • Structure: Mixture of amylose (helical, unbranched) and amylopectin (branched). Made of $\alpha$-glucose (1).
  • Function: Energy storage in plants (1).
  • Relation: Compact helical shape allows storage in small space; insoluble so doesn't affect water potential; branches allow rapid hydrolysis/release of glucose (1).

• Glycogen (Animals):

  • Structure: Similar to amylopectin but more highly branched. Made of $\alpha$-glucose (1).
  • Function: Energy storage in animals (liver/muscle) (1).
  • Relation: Highly branched structure provides many ends for enzyme action, allowing rapid release of glucose for respiration during activity (1).

• Cellulose (Plants):

  • Structure: Straight, unbranched chains of $\beta$-glucose. Chains linked by hydrogen bonds to form microfibrils (1).
  • Function: Structural component of cell walls (1).
  • Relation: High tensile strength due to hydrogen bonding between parallel chains; prevents cell bursting under turgor pressure; provides support for plant (1).

• Comparison:

  • Starch and Glycogen are $\alpha$-glucose polymers (storage); Cellulose is $\beta$-glucose (structural) (1).
  • Starch/Glycogen are coiled/branched; Cellulose is straight/linear (1).

• Marking Note: Award marks for accurate structural descriptions and clear links to function. Comparison points should be explicit. Max 10 marks.