1. 1
   When phospholipids are mixed with water they spontaneously form liposomes: small hollow spheres whose wall is a single phospholipid bilayer enclosing a droplet of water. Liposomes containing no proteins were loaded with a water-soluble anticancer drug; over 24 hours less than 5% of the drug leaked out. When the same liposomes were placed in water with a high concentration of dissolved oxygen, the oxygen concentration inside them quickly reached the same level as outside.

   (a)

   Why do phospholipids spontaneously form a bilayer in water?

   [1 mark]

   • AThey are amphipathic: hydrophilic heads face the water and hydrophobic tails face each other
   • BTheir heads are hydrophobic and are repelled by water
   • CCovalent bonds form between neighbouring phospholipids
   • DTheir hydrocarbon tails form hydrogen bonds with water

   (b)

   By which process did oxygen enter the liposomes?

   [1 mark]

   • AFacilitated diffusion through channel proteins
   • BSimple diffusion between the phospholipids
   • CActive transport by pump proteins
   • DOsmosis

   (c)

   Explain why the water-soluble drug remained trapped inside the liposomes.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Goldfish were kept for several weeks at either 5 °C or 25 °C, and the fatty acids in the phospholipids of their gill cell membranes were analysed. In fish kept at 5 °C, 62% of the fatty acids were unsaturated; in fish kept at 25 °C, 44% were unsaturated. In a separate study, the cholesterol content of the plasma membranes of cultured mammalian cells was reduced experimentally. These membranes became leaky at 40 °C and became rigid at a higher temperature than normal when cooled.

   (a)

   Which statement explains the difference in fatty acid composition between the two groups of fish?

   [1 mark]

   • ASaturated fatty acids have lower melting points, so they keep membranes fluid at 5 °C
   • BUnsaturated fatty acids make membranes stronger at higher temperatures
   • CUnsaturated fatty acids have lower melting points, so they keep membranes fluid at 5 °C
   • DUnsaturated fatty acids have straight chains that pack closely together

   (b)

   Where are cholesterol molecules positioned in animal cell membranes?

   [1 mark]

   • AOn the extracellular surface, attached to glycoproteins
   • BInside the pores of channel proteins
   • COnly on the cytoplasmic surface, attached to peripheral proteins
   • DBetween phospholipids in the hydrophobic region, with the hydroxyl group near the phosphate heads

   (c)

   Explain the effects of reducing the cholesterol content of the membranes.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Cells lining the small intestine absorb glucose from the gut lumen, even when the glucose concentration in the lumen is lower than inside the cells. Strips of intestine were bathed in glucose solutions and glucose uptake into the lining cells was measured. With a normal solution containing sodium ions, uptake was high. When the sodium ions in the lumen solution were replaced with potassium ions, uptake fell to about 5% of normal. When ouabain, a drug that inhibits sodium–potassium pumps in the membrane facing the blood, was added to the normal solution, glucose uptake fell gradually over 30 minutes to about 10% of normal, while the sodium ion concentration inside the cells rose.

   (a)

   Explain the role of sodium–potassium pumps in the absorption of glucose by these cells.

   [3 marks]

   (b)

   Analyse the data to show that glucose enters these cells by indirect active transport through sodium-dependent glucose cotransporters.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   At a synapse between a motor neuron and a muscle fibre, acetylcholine is stored in vesicles in the neuron terminal. When an action potential arrives, the vesicles fuse with the plasma membrane and release acetylcholine, which diffuses across the gap and binds to nicotinic acetylcholine receptors in the muscle fibre membrane. α-bungarotoxin, a snake venom toxin, binds irreversibly to these receptors and causes paralysis. Along the neuron, voltage-gated sodium and potassium channels open and close in sequence as an action potential passes. After release, the neuron takes membrane back from its surface by forming new vesicles, which are refilled with acetylcholine.

   (a)

   Explain how gated ion channels allow signals to pass along the neuron and on to the muscle fibre, and suggest why α-bungarotoxin causes paralysis.

   [6 marks]

   (b)

   Discuss the importance of membrane fluidity in the release of acetylcholine and the recycling of membrane at the synapse, referring to other examples of endocytosis and exocytosis.

   [6 marks]

   Total for question 4: 12 marks

End of questions