Cell Membrane & Transport

Question 1 (10 marks)

(a) Most fluid at 40 °C — Vesicle A (no cholesterol). (3)

• At high temperature phospholipids move rapidly. (1)
• Cholesterol restrains/immobilises fatty acid tails, reducing fluidity at high temp. (1)
• With no cholesterol nothing dampens the motion → most fluid. Vesicle C (double cholesterol) is least fluid. (1)

(b) Best resists solidifying at 10 °C — Vesicle C (double cholesterol) / cholesterol-containing. (3)

• At low temperature tails pack tightly and gel/solidify. (1)
• Cholesterol sits between phospholipids and prevents tight packing, keeping the membrane fluid. (1)
• Highest cholesterol (C) gives greatest resistance to solidifying → maintains fluidity best. (1)

(c) Ranking of non-polar molecule entry at 40 °C: A > B > C. (4)

• Non-polar molecules cross by simple diffusion through the lipid bilayer. (1)
• Rate depends on fluidity/packing: more fluid, looser bilayer = faster diffusion. (1)
• A most fluid → fastest; C least fluid → slowest. (1)
• Order: A (fastest) > B > C (slowest). (1)

Question 2 (12 marks)

(a) (3)

• Pure water has the highest (least negative) water potential = 0 kPa. (1)
• Both cells have lower (more negative) internal water potential than pure water. (1)
• Net water moves into both cells by osmosis (high → low water potential). (1)

(b) Plant cell final state (3)

• Water enters, vacuole/protoplast swells and presses on the cell wall → turgid. (1)
• The rigid cellulose cell wall resists expansion, generating wall/turgor pressure. (1)
• This inward pressure stops further net water entry, so the cell does not burst. (1)

(c) Red blood cell (3)

• Water enters continuously; cell swells and bursts → lysis (haemolysis). (1)
• It has no cell wall to resist expansion. (1)
• So unlike the plant cell there is no counter-pressure to halt water entry → membrane ruptures. (1)

(d) Hypertonic solution (3)

• External solution more negative water potential → water leaves the cell by osmosis. (1)
• Protoplast shrinks and pulls away from the cell wall = plasmolysis. (1)
• Consequence: loss of turgor → cell/plant wilts. (1)

Question 3 (10 marks)

(a) (4)

• SGLT1 uses the Na⁺ concentration/electrochemical gradient (Na⁺ high outside → moving in). (1)
• Na⁺ moving down its gradient releases energy. (1)
• This energy drags glucose against its gradient (co-transport/symport). (1)
• This is secondary active transport. (1)

(b) (4)

• Normally the Na⁺/K⁺ pump exports Na⁺, keeping intracellular Na⁺ low (maintaining the gradient). (1)
• Block the pump → Na⁺ accumulates inside the cell. (1)
• The inward Na⁺ gradient collapses / dissipates. (1)
• Without the Na⁺ gradient's energy, SGLT1 can no longer drive glucose uphill → glucose uptake stops. (1)

(c) (2)

• SGLT1 does not directly use ATP. (1)
• It relies on the Na⁺ gradient, which is indirectly maintained by ATP-driven Na⁺/K⁺ pump — the ATP use is on the pump, not the transporter. (1)

Question 4 (10 marks)

(a) Phagocytosis (4)

• Membrane extends pseudopodia around the bacterium. (1)
• Pseudopodia fuse, enclosing it in a vesicle/phagosome. (1)
• Vesicle taken into cytoplasm. (1)
• Fuses with lysosome → enzymes digest the bacterium. (1)

(b) Receptor-mediated endocytosis (3)

• Specific LDL binds complementary receptors on the membrane. (1)
• Receptors cluster in a coated pit, which invaginates and pinches off as a vesicle. (1)
• Only molecules matching the receptors are taken in → high selectivity. (1)

(c) Exocytosis (3)

• Enzymes packaged into secretory vesicles (via Golgi). (1)
• Vesicle moves to and fuses with the plasma membrane. (1)
• Membrane opens, releasing contents to the exterior. (1)

Question 5 (8 marks)

(a) (3)

• Facilitated diffusion (facilitated transport). (1) (carrier-protein type, passive.)
• Involves carrier proteins (transport/membrane proteins). (1)
• Passive — no ATP; still down the gradient. (1)

(b) (3)

• Each carrier binds one solute and changes shape to move it. (1)
• At high concentration all carriers are occupied/saturated. (1)
• Rate limited by number of carriers → cannot increase further → plateau (Vmax). (1)

(c) (2)

• Simple diffusion has no carriers, so no saturation. (1)
• Rate rises linearly (proportionally) with concentration, no plateau. (1)

[
  {"claim": "Pure water potential is 0 kPa, the maximum value; cell solutions are negative and thus lower.", "code": "psi_pure = 0; psi_cell = -800; net_into_cell = psi_pure > psi_cell; result = (psi_pure == 0) and net_into_cell"},
  {"claim": "Non-polar diffusion rate order A>B>C follows fluidity: no cholesterol > normal > double at high temp.", "code": "fluidity = {'A':3,'B':2,'C':1}; order = sorted(fluidity, key=lambda k:-fluidity[k]); result = order == ['A','B','C']"},
  {"claim": "Blocking Na/K pump raises internal Na, collapsing the gradient needed by SGLT1 secondary active transport.", "code": "Na_out=140; Na_in_normal=15; Na_in_blocked=140; grad_normal=Na_out-Na_in_normal; grad_blocked=Na_out-Na_in_blocked; result = (grad_normal>0) and (grad_blocked==0)"},
  {"claim": "Facilitated diffusion saturates (plateau) whereas simple diffusion is linear (no plateau).", "code": "conc=[1,2,4,8]; simple=[2*c for c in conc]; Vmax=10; Km=2; facil=[Vmax*c/(Km+c) for c in conc]; linear = all(abs(simple[i]/conc[i]-2)<1e-9 for i in range(4)); saturates = facil[-1] < Vmax and facil[-1] > facil[0]; result = linear and saturates"}
]