Membranes and membrane transport: exam questions

Answer: C – oxygen is small and non-polar, so it passes between the phospholipids; ions and polar molecules do not readily enter the hydrophobic core [1]

Answer: B – pumps use energy from ATP to move particles against a concentration gradient; both processes are selective and both use integral proteins [1]

• phospholipid bilayer drawn with two layers, heads facing outwards and tails inwards [1]
• hydrophilic regions and hydrophobic region correctly labelled [1]
• integral protein shown embedded in / spanning the bilayer [1]
• peripheral protein shown attached to one surface only [1]
• glycoprotein with carbohydrate chain shown on the extracellular side only [1]
• cholesterol shown within the hydrophobic region, between the hydrocarbon tails [1]
• Labels must point clearly to the structure; do not accept proteins drawn on top of the bilayer as integral
• max 4

• (a) (1.32 − 1.00) ÷ 1.00 × 100 = 32 % [1]
• (b) aquaporin mRNA is translated, so aquaporins are inserted into the plasma membrane [1]
• (b) aquaporins increase the permeability of the membrane to water [1]
• (b) solute concentration inside the cell is higher than in the dilute solution, so water enters by osmosis [1]
• (b) water enters faster, so volume increases faster / cells burst [1]
• (b) max 3
• (c) water can still cross the bilayer slowly by diffusing between phospholipids / a few aquaporins are present OWTTE [1]

• phospholipids form a bilayer with a hydrophobic (hydrocarbon) core [1]
• core has low permeability to ions / polar molecules / large molecules, so it acts as a barrier [1]
• small non-polar molecules such as O₂ / CO₂ cross by simple diffusion between phospholipids [1]
• simple diffusion is not selective / depends only on size and hydrophilic or hydrophobic properties [1]
• channel proteins allow facilitated diffusion of specific ions [1]
• channels can be gated / opened or closed to control permeability [1]
• aquaporins allow rapid movement of water by osmosis [1]
• pump proteins use energy from ATP to move specific particles against a concentration gradient [1]
• the proteins present determine which particles cross, giving selective permeability [1]
• max 6

• pump uses energy from ATP / is active transport [1]
• ATP phosphorylates the pump, causing a change in shape [1]
• three Na⁺ pumped out for every two K⁺ pumped in [1]
• net movement of positive charge out of the cell, so the inside becomes negative relative to the outside [1]
• builds concentration gradients: Na⁺ high outside, K⁺ high inside [1]
• contributes to a resting potential of about −70 mV [1]
• membrane is more permeable to K⁺ than Na⁺, so K⁺ diffusing out makes the inside more negative [1]
• max 4

• sodium–potassium pumps actively pump Na⁺ out of the cell using ATP [1]
• this keeps the Na⁺ concentration inside the cell low / creates a Na⁺ gradient into the cell [1]
• cotransporter in the membrane facing the lumen moves Na⁺ and glucose into the cell together [1]
• Na⁺ moves down its concentration gradient [1]
• glucose moves against its concentration gradient [1]
• indirect active transport: the cotransporter does not use ATP directly [1]
• glucose then leaves the cell into the blood by facilitated diffusion [1]
• the same mechanism reabsorbs glucose in the proximal convoluted tubule of the nephron [1]
• max 5

Answer: B – it is a neurotransmitter-gated ion channel; the sodium and potassium channels are the voltage-gated examples [1]