Level 3 — ProductionCell Membrane & Transport

Cell Membrane & Transport

⏱ 45 minutes50 marksprintable — key stays hidden on paper

Level: 3 (Production — from-scratch derivations, explain-out-loud reasoning) Time Limit: 45 minutes Total Marks: 50

Instructions: Answer all questions. Explain your reasoning fully; marks are awarded for the logic of each step, not just the final statement. Diagrams may be used where helpful.

Question 1 — Build the Model (10 marks)

From memory, construct an explanation of the fluid mosaic model of the cell membrane.

(a) Explain why the phospholipid bilayer arranges itself spontaneously the way it does when placed in an aqueous environment. Reason from molecular structure, not from a memorised label. (4)

(b) Distinguish integral and peripheral proteins by both their position and their interaction with the bilayer, and give one functional role for each. (4)

Question 2 — Cholesterol Reasoning (8 marks)

Cholesterol is described as a "fluidity buffer."

(a) Explain how cholesterol affects membrane fluidity at high temperature versus low temperature. (4)

(b) An organism living in Antarctic waters and one living in a hot spring both maintain functional membranes. Predict which would likely have more cholesterol (or an analogous molecule) relative to the other's baseline, and justify your prediction mechanistically. (4)

Question 3 — Water Potential Derivation (10 marks)

(a) State the water potential equation and define each term, including the sign convention for solute potential. (3)

(b) A plant cell has a solute potential of $\Psi_s = -0.8\text{ MPa}$ and a pressure potential of $\Psi_p = +0.3\text{ MPa}$ . Calculate its water potential $\Psi$ . (2)

(c) This cell is placed in a solution with $\Psi = -0.2\text{ MPa}$ . State the direction of net water movement and justify it using the values. (3)

(d) Predict what happens to $\Psi_p$ as water moves, and what final state the cell approaches. (2)

Question 4 — Transport Sorting & Explanation (8 marks)

For each scenario, name the transport mechanism and explain the reasoning that lets you classify it, referencing energy source and concentration gradient:

(a) Glucose moving into an intestinal cell driven by the $\text{Na}^+$ gradient. (3)

(b) $\text{O}_2$ crossing the membrane down its gradient without a protein. (2)

Question 5 — Tonicity Prediction (8 marks)

A red blood cell and a plant cell are each placed into pure distilled water.

(a) Define the tonicity of distilled water relative to these cells, and state the direction of osmosis. (2)

(b) Predict and explain the fate of the red blood cell, naming the process. (3)

(c) Predict and explain the fate of the plant cell, naming the process, and explain why its outcome differs from the animal cell. (3)

Question 6 — Sodium-Potassium Pump From Scratch (6 marks)

Reconstruct the sodium–potassium pump cycle from memory.

(a) State the stoichiometry (ions moved per ATP) and the direction each ion moves relative to its gradient. (3)

(b) Explain why this pump is classified as primary active transport and how it can subsequently power secondary active transport. (3)

Answer keyMark scheme & solutions

Question 1 (10 marks)

(a) Phospholipids are amphipathic: a hydrophilic phosphate head and two hydrophobic fatty-acid tails (1). In water the hydrophobic tails cannot form favourable interactions with water molecules (1); to minimise contact between tails and water, tails cluster inward and heads face outward toward the aqueous cytoplasm and extracellular fluid (1). This produces a spontaneous bilayer driven by the hydrophobic effect (entropy of water) (1).

(b)

Integral proteins: embedded within/spanning the bilayer, interacting with the hydrophobic core; often transmembrane; require detergent to remove (1). Role: channels/carriers/transport (1).
Peripheral proteins: on membrane surface, bound to head groups or to integral proteins by ionic/H-bonds; easily removed (1). Role: signalling, enzymatic, or cytoskeletal anchoring (1).

(c) "Fluid" = components (lipids, proteins) move laterally within the plane (1). "Mosaic" = scattered, varied proteins embedded like tiles in the lipid background (1).

Question 2 (8 marks)

(a) At high temperature, phospholipids move rapidly; cholesterol wedges between them and restrains movement, reducing excessive fluidity (2). At low temperature, cholesterol prevents tight packing of tails, keeping the membrane from solidifying/gelling, maintaining fluidity (2).

(b) The hot-spring organism has more cholesterol/analogous stabiliser at baseline comparison — reasoning: at high temperature membranes tend to become too fluid/leaky, so more cholesterol is needed to restrain movement and preserve integrity (2). (Accept: Antarctic organism needs fewer rigidifying sterols and more unsaturated lipids to stay fluid in cold — full credit for coherent mechanistic justification either way.) Mechanistic justification: cholesterol dampens fluidity, so a hot environment requires that dampening more (2).

Note: award full marks for consistent mechanistic reasoning; the hot-spring answer is the expected primary response.

Question 3 (10 marks)

(a) $\Psi = \Psi_s + \Psi_p$ (1). $\Psi_s$ = solute potential, always negative (solutes lower water potential) (1); $\Psi_p$ = pressure potential, usually positive in turgid cells (1).

(b) $\Psi = -0.8 + 0.3 = -0.5\text{ MPa}$ (2).

(c) Cell $\Psi = -0.5$ MPa; solution $\Psi = -0.2$ MPa. Water moves from higher $\Psi$ to lower $\Psi$ (1), i.e. from solution ( $-0.2$ ) into the cell ( $-0.5$ ) (2), because water flows down a water-potential gradient.

(d) As water enters, the cell swells, $\Psi_p$ increases (1). It approaches equilibrium/turgor where cell $\Psi$ = solution $\Psi$ (net water movement stops) (1).

Question 4 (8 marks)

(a) Secondary active transport (co-transport / symport) (1). Glucose moves against its gradient (1) powered indirectly by the Na⁺ electrochemical gradient (established by ATP elsewhere), not ATP directly at this protein (1).

(b) Simple diffusion (1). Nonpolar O₂ crosses the bilayer directly, down its concentration gradient, no protein or energy (1).

(c) Primary active transport (1). K⁺ moves against its gradient (1) using ATP hydrolysis directly at the pump (1).

Question 5 (8 marks)

(a) Distilled water is hypotonic to both cells (1); osmosis moves water into the cells (higher $\Psi$ outside → lower $\Psi$ inside) (1).

(b) RBC: water enters, cell swells, has no cell wall, so it bursts — lysis / haemolysis (2 for outcome+process, 1 for reasoning re: no wall).

(c) Plant cell: water enters, but the rigid cell wall resists expansion, building pressure potential; cell becomes turgid (does not burst) (2). Differs because the wall provides mechanical resistance/back-pressure that stops net influx and prevents lysis (1).

Question 6 (6 marks)

(a) 3 Na⁺ out, 2 K⁺ in per ATP (2). Both moved against their concentration gradients (1).

(b) Primary: uses ATP directly as energy source (1). It builds a steep Na⁺ gradient (1); other transporters (e.g. Na⁺-glucose symport) then use the potential energy stored in that Na⁺ gradient — secondary active transport — no direct ATP (1).

[
  {"claim":"Water potential of cell = Psi_s + Psi_p = -0.5 MPa","code":"Psi_s=-0.8; Psi_p=0.3; Psi=Psi_s+Psi_p; result = (Psi == -0.5)"},
  {"claim":"Cell Psi (-0.5) is lower than solution Psi (-0.2), so water enters cell","code":"cell=-0.5; sol=-0.2; result = (cell < sol)"},
  {"claim":"Na/K pump moves net one positive charge out per cycle (3 out, 2 in)","code":"na_out=3; k_in=2; net_charge_out=na_out - k_in; result = (net_charge_out == 1)"}
]