Cell Membrane & Transport

Q1. [3 marks — 1 each]

• (a) Selective permeability: the property of a membrane that allows some substances to pass through while restricting others. (controls what enters/leaves the cell)
• (b) Isotonic solution: a solution with the same solute concentration (water potential) as the cell, so there is no net movement of water.
• (c) Plasmolysis: the shrinking of the protoplast (cytoplasm/cell membrane) away from the cell wall in a plant cell placed in a hypertonic solution, due to water loss by osmosis.

Q2. [4 marks — 1 each]

• (a) FALSE — the bilayer is fluid/flexible, not rigid; components move.
• (b) TRUE.
• (c) FALSE — carbohydrates attach to the outer/external surface (glycoproteins/glycolipids).
• (d) TRUE. (Correcting a false statement is required for the mark.)

Q3. [4 marks]

• Phospholipids are amphipathic: they have a hydrophilic (phosphate) head and hydrophobic (fatty acid) tails. [2]
• In water, the hydrophilic heads face outward toward the water on both sides, while the hydrophobic tails point inward, away from water. [1]
• This forms a bilayer, the most thermodynamically stable arrangement, shielding the tails from water. [1]

Q4. [4 marks]

• Integral proteins: embedded within/span the bilayer; often transmembrane; can only be removed by disrupting the membrane. Function example: transport (channel/carrier) or receptor. [2]
• Peripheral proteins: loosely bound to the surface (often to integral proteins or head groups); easily removed. Function example: enzymatic activity or cell signalling/structural support. [2]

Q5. [4 marks — 2 each]

• Fluidity buffer / regulation: cholesterol reduces membrane fluidity at high temperatures (restricts phospholipid movement) and prevents solidification/packing at low temperatures — maintaining stable fluidity.
• Reduces permeability: cholesterol fills gaps between phospholipids, decreasing the passage of small water-soluble molecules and ions, and provides mechanical stability.

Q6. [6 marks — 1 each]

• (i) No ATP required
• (ii) Yes ATP required
• (iii) Down the concentration gradient (high → low)
• (iv) Against the concentration gradient (low → high)
• (v) e.g. simple/facilitated diffusion or osmosis
• (vi) e.g. sodium–potassium pump (or any named pump)

Q7. [4 marks — 1 each; process 1 mark]

• Cell A (distilled water = hypotonic): water enters by osmosis; cell swells and bursts → lysis (haemolysis). [1.5]
• Cell B (isotonic): no net water movement; cell remains normal/unchanged. [1]
• Cell C (hypertonic 5% saline): water leaves by osmosis; cell shrinks/shrivels → crenation. [1.5]

Q8. [4 marks]

• Simple diffusion: movement of molecules directly through the phospholipid bilayer, down the concentration gradient, no protein needed (e.g. O₂, CO₂). [1]
• Facilitated diffusion: passive movement down the gradient but through membrane proteins (for polar/charged molecules). [1]
• Channel proteins: form a hydrophilic pore/pathway; ions/water pass through (may be gated). [1]
• Carrier proteins: bind the specific molecule and change shape to move it across. [1]

Q9. [4 marks]

• The Na⁺/K⁺ pump is a carrier protein that uses ATP (primary active transport). [1]
• It pumps 3 Na⁺ ions OUT of the cell [1] and 2 K⁺ ions IN to the cell [1] per ATP hydrolysed.
• Both ions are moved against their concentration gradients, maintaining the resting membrane potential/ion balance. [1]

Q10. [3 marks — 1 each]

• Phagocytosis: "cell eating" — engulfing of large solid particles (e.g. bacteria) into a vesicle.
• Pinocytosis: "cell drinking" — engulfing of extracellular fluid/dissolved substances into small vesicles.
• Exocytosis: vesicle fuses with the plasma membrane to release/secrete contents out of the cell.

[
  {"claim": "Na/K pump moves net 1 positive charge out per cycle (3 Na out - 2 K in)", "code": "na_out=3; k_in=2; net_charge=na_out-k_in; result=(net_charge==1)"},
  {"claim": "Total marks sum to 40", "code": "marks=[3,4,4,4,4,6,4,4,4,3]; result=(sum(marks)==40)"},
  {"claim": "Na/K pump total ions moved per cycle is 5", "code": "result=(3+2==5)"},
  {"claim": "Isotonic saline for RBC is 0.9 percent, less than 5 percent hypertonic", "code": "iso=0.9; hyper=5.0; result=(iso<hyper)"}
]