A Level H2 Biology Practice Paper 2

TuitionGoWhere Practice Paper — Answer Key (Version 2)

Subject: Biology H2
Level: A-Level
Paper: Practice Paper (Cells & Biomolecules)
Total Marks: 60

Section A (16 marks)

Q1 [3]

• All living organisms are composed of cells. [1]
• The cell is the smallest unit of life. [1]
• Cells arise from pre-existing cells. [1]
  Teaching note: These three statements are the core of cell theory. A common mistake is omitting "pre-existing cells" (spontaneous generation is false).

Q2 [1]

• Glycosidic bond.
  Teaching note: Formed by condensation reaction; in starch it is α-1,4 and α-1,6 (branches).

Q3 [1]

• Bacterial cell lacks membrane-bound organelles (e.g., no nucleus) OR has circular DNA / 70S ribosomes / peptidoglycan wall. (Any one.)
  Marking: 1 mark for a valid structural difference.

Q4 [2]

• Glycolipids [1]
• Cholesterol [1] (or glycoproteins are proteins, not lipids; accept only lipids)
  Teaching note: Fluid mosaic model includes phospholipids, cholesterol, glycolipids, proteins, glycoproteins.

Q5 [2]

• The induced-fit model states that the enzyme's active site changes shape slightly to better fit the substrate upon binding.
  Marking: 2 marks for mentioning conformational change + substrate fit. (1 mark if only "enzyme fits substrate" without change.)

Q6 [1]

• Tertiary (and also quaternary, but primary target is tertiary).
  Teaching note: Disulfide bridges form between cysteine residues in tertiary folding.

Q7 [1]

• Endocytosis (or phagocytosis/pinocytosis if specified large particles → phagocytosis; accept endocytosis).

Q8 [1]

• Reduces membrane fluidity at high temperature / maintains stability / reduces permeability to very small water-soluble molecules. (Any one.)

Section B (24 marks)

Q9 [2]

• Organelle G = Golgi body (Golgi apparatus). [1]
• Function: modifies, sorts, and packages proteins/lipids from ER for secretion or delivery. [1]
  From image: G labelled near stacked membranes.

Q10 [4]

• Type: Competitive inhibition. [1]
• Vmax unchanged (~35 µmol min⁻¹) for both curves. [1]
• Km increased: curve B needs higher substrate concentration to reach half Vmax. [1]
• Explanation: inhibitor X resembles substrate, competes for active site; more substrate overcomes inhibition. [1]
  Teaching note: On graph, both plateaus equal → Vmax same; B shifted right → apparent Km higher.

Q11 [2]

• Distance moved = 4.2 cm – 0 cm = 4.2 cm.
• Time = 30 min.
• Rate = 4.2 / 30 = 0.14 cm min⁻¹. [2]
  Marking: 1 for correct subtraction, 1 for rate with unit.

Q12 [3]

• Phosphate head is polar/hydrophilic. [1]
• Fatty acid tails are non-polar/hydrophobic. [1]
• In water, phospholipids arrange into bilayer with heads outward, tails inward, forming membrane. [1]
  From Fig: head labelled polar, tails non-polar.

Q13 [2]

• Δ[O₂] = 8.0 – 4.8 = 3.2 mg dm⁻³ over 10 min.
• Mean rate = 3.2 / 10 = 0.32 mg dm⁻³ min⁻¹. [2]
  Marking: 1 for Δ, 1 for rate + unit.

Q14 [2]

• Bacteriophage is not cellular (acellular particle). [1]
• It cannot reproduce independently; requires host cell. [1]
  Teaching note: Challenges "all life is cellular" and "cells from cells" because virus needs host.

Section C (20 marks)

Q15 [4]

• Starch: α-glucose, α-1,4 and α-1,6 glycosidic bonds; energy storage in plants. [1+1]
• Cellulose: β-glucose, β-1,4 bonds; structural in plant cell walls. [1+1]
• Glycogen: α-glucose, α-1,4 and α-1,6 (more branches); energy storage in animals. [1+1]
  Max 4 marks: allocate 2 for linkages, 2 for functions. Common mistake: confusing α/β.

Q16 [5]

• Primary: sequence of amino acids from gene; peptide bonds. [1]
• Secondary: α-helix/β-sheet from H-bonds between backbone. [1]
• Tertiary: 3D folding by H-bonds, ionic, hydrophobic, disulfide. [2]
• Quaternary: multiple polypeptides held by same bonds as tertiary. [1]
  Teaching note: Bonds listed per level; mark per correct level+bond.

Q17 [4]

• Facilitated diffusion: down gradient, no ATP, carrier/channel protein. [2]
• Active transport: against gradient, ATP needed, pump protein. [2]
  Example: GLUT1 vs Na⁺/K⁺ ATPase. Marking: 2 each for mechanism contrast.

Q18 [4]

• Totipotency: can form all cell types + extraembryonic. Example: zygote. [1+1]
• Pluripotency: all cell types of body. Example: embryonic stem cell. [1+1]
• Multipotency: limited lineages. Example: blood stem cell (lymphoid/myeloid). [1+1]
  Max 4: 2 def, 2 examples.

Q19 [3]

• Quaternary = 4 subunits (2α 2β). [1]
• Sigmoidal curve shows cooperativity: binding O₂ to one subunit increases affinity of others. [1]
• Supports efficient loading at lungs (high pO₂) and unloading at tissues (low pO₂). [1]
  From Fig: S-shape visible.

Q20 [4]

• At 60 °C, protein denatures: tertiary/quaternary bonds (H, ionic, hydrophobic, disulfide) break. [2]
• Active site destroyed; return to 37 °C does not refold correctly (irreversible). [1]
• Thus activity not recovered. [1]
  Teaching note: High temp beyond optimum causes permanent denaturation.