1. 1
   Researchers made an artificial membrane consisting only of a phospholipid bilayer, with no proteins, and measured how quickly different substances crossed it by simple diffusion. From fastest to slowest, the order was: oxygen, carbon dioxide, water, glycerol, glucose, chloride ions, sodium ions. Water crossed about a billion times faster than sodium ions. When the experiment was repeated with membranes from red blood cells, water crossed them about ten times faster than it crossed the artificial bilayer.

   (a)

   Which properties of the substances best explain the order in which they crossed the artificial bilayer?

   [1 mark]

   • ATheir size and whether they are hydrophobic or hydrophilic
   • BTheir affinity for the channel proteins in the bilayer
   • CThe amount of ATP each one requires
   • DWhether they bind to glycoproteins on the surface

   (b)

   Why do sodium ions cross the artificial bilayer most slowly?

   [1 mark]

   • AThey are larger than glucose molecules
   • BThey are charged, so they cannot easily pass through the hydrophobic hydrocarbon core
   • CThey are pumped back out as soon as they enter
   • DThey move only by osmosis

   (c)

   Suggest why water crosses red blood cell membranes faster than it crosses the artificial bilayer.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Red blood cell membranes were isolated and treated in two ways. A concentrated salt solution removed one group of proteins while leaving the phospholipid bilayer intact. A second group of proteins could be removed only by a detergent that breaks up the phospholipid bilayer. Some proteins in the second group had carbohydrate chains attached. A person's ABO blood group depends on which carbohydrate chains are attached to glycoproteins and glycolipids in these membranes.

   (a)

   Which proteins were removed by the salt solution?

   [1 mark]

   • AIntegral proteins embedded in both layers of the bilayer
   • BChannel proteins passing through the hydrocarbon core
   • CGlycoproteins that span the membrane
   • DPeripheral proteins attached to one surface of the bilayer

   (b)

   On which side of the membrane are the carbohydrate chains of glycoproteins and glycolipids located?

   [1 mark]

   • AWithin the hydrophobic core
   • BThe cytoplasmic side only
   • CThe extracellular side only
   • DEqually on both sides

   (c)

   Outline the roles of glycoproteins and glycolipids, referring to ABO blood groups.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Frog egg cells (oocytes) naturally have membranes with very low permeability to water. In a classic experiment, one group of oocytes was injected with mRNA coding for CHIP28, a protein found in red blood cell membranes, so that the oocytes made this protein and inserted it into their membranes; a control group was injected with water only. Both groups were moved from a solution with the same solute concentration as their cytoplasm into a solution diluted to one third of that concentration. The oocytes making CHIP28 swelled rapidly and burst within about five minutes, while the control oocytes swelled only slightly over an hour and did not burst. Adding mercury ions, which bind to CHIP28 and block it, greatly slowed the swelling of the oocytes making CHIP28.

   (a)

   Explain why the oocytes making CHIP28 swelled when placed in the diluted solution.

   [3 marks]

   (b)

   Evaluate the evidence from this experiment that CHIP28 is an aquaporin (water channel).

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Barley roots were placed in an aerated solution containing 0.5 mmol dm⁻³ potassium ions (K⁺). After six hours, the concentration of K⁺ in the root cells was 60 mmol dm⁻³. When nitrogen gas was bubbled through the solution instead of air, the rate of K⁺ uptake fell by about 90%. Adding a respiratory poison that stops ATP production had the same effect. The membranes of the root cells also contain potassium channel proteins, which, when open, allow K⁺ to diffuse out of the cells.

   (a)

   Using the data, explain how barley root cells absorb potassium ions.

   [6 marks]

   (b)

   Discuss how the plasma membrane of a root cell achieves selective permeability, comparing simple diffusion, facilitated diffusion through channel proteins and active transport by pump proteins.

   [6 marks]

   Total for question 4: 12 marks

End of questions