Prokaryotic vs Eukaryotic Cells

Question 1 (10 marks)

(a) Table — award 1 mark per correct comparison row (max 5):

Marks: 1 per correct paired row. Why: each row contrasts a defining structural feature.

(b) (5 marks)

• Membrane-bound organelles allow separation of incompatible reactions (1) and creation of distinct chemical microenvironments/pH (1).
• Internal membranes increase available surface area for reactions (1).
• Prokaryotes lack these internal membranes so all processes share one cytoplasm (1).
• Advantage: greater efficiency/specialisation, e.g. concentrated enzymes, protection of DNA in nucleus (1).

Question 2 (9 marks)

(a) (5 marks) — 1 mark per correctly labelled structure: nucleoid (region of circular DNA), plasmid (small separate DNA ring), capsule (outer slime layer), cell wall (rigid layer outside membrane), flagellum (external rotating filament).

(b) (4 marks)

• Plasmid: carries accessory genes e.g. antibiotic resistance (1); allows survival/gene transfer under selective pressure such as antibiotics (1).
• Capsule: protects against desiccation/phagocytosis and aids adhesion (1); helps evade host immune defences in hostile host environment (1).

Question 3 (8 marks)

(a) (5 marks)

• Cell wall polymer is peptidoglycan (murein) — sugar chains cross-linked by peptides (1); provides rigidity/prevents osmotic lysis (1).
• Gram-positive: thick peptidoglycan retains crystal violet → purple (1).
• Gram-negative: thin peptidoglycan + outer membrane, loses stain, counterstained pink (1).
• Gram stain therefore distinguishes wall thickness/architecture (1).

(b) (3 marks)

• Bacterial flagellum is a rigid helix rotated like a propeller by a proton-motive-force-driven motor at its base (1).
• It rotates rather than bends (1).
• Eukaryotic flagellum uses ATP-driven dynein sliding of microtubules (9+2) to bend/whip, not rotate (1).

Question 4 (8 marks)

Statement (implicit): mitochondria and chloroplasts arose from free-living prokaryotes engulfed by a host cell. (evidence: 2 marks each, up to 4 lines)

1. Own circular DNA — resembles bacterial genome → once independent organisms.
2. 70S ribosomes — bacterial-type, not host 80S → prokaryotic ancestry.
3. Double membrane — inner from original bacterium, outer from host engulfing vesicle → phagocytic origin.
4. Binary fission / self-replication — divide independently like bacteria → retain autonomy of former free-living cells. (Other valid: antibiotic sensitivity of their ribosomes.) Why: each observation matches a bacterial trait not otherwise expected in a host organelle.

Question 5 (10 marks)

(a) (3 marks) $A = 6L^2,\quad V = L^3,\quad \frac{A}{V} = \frac{6L^2}{L^3} = \frac{6}{L}$ 1 mark each.

(b) (3 marks)

• $L=2$: $A/V = 6/2 = 3\ \mu m^{-1}$ (1.5)
• $L=10$: $A/V = 6/10 = 0.6\ \mu m^{-1}$ (1.5)

(c) (2 marks) As $L$ increases, $A/V$ falls ($\propto 1/L$) (1); membrane area per unit volume of cytoplasm shrinks so exchange cannot keep pace with metabolic demand (1).

(d) (2 marks) Any two: microvilli/folds increase area; flattened or elongated shape; small cell size maintained by division. (1 each)

Question 6 (5 marks)

• Metabolic demand scales with volume ($\propto L^3$) (1).
• Rate of exchange scales with surface area ($\propto L^2$) (1).
• So supply/demand ratio $\propto 1/L$ falls as size increases (1).
• Diffusion distance from membrane to centre also increases with size, slowing transport (1).
• Therefore cells stay small to keep a high $A/V$ and short diffusion paths, remaining microscopic (1).

[
  {"claim":"A/V for cube equals 6/L", "code":"L=symbols('L',positive=True); A=6*L**2; V=L**3; result = simplify(A/V - 6/L)==0"},
  {"claim":"A/V at L=2 micron equals 3", "code":"result = Rational(6,2)==3"},
  {"claim":"A/V at L=10 micron equals 0.6", "code":"result = Rational(6,10)==Rational(3,5)"},
  {"claim":"A/V ratio at L=2 is five times that at L=10", "code":"result = (Rational(6,2))/(Rational(6,10))==5"}
]