Cell membranes and passive transportEdexcel A-Level Biology B: Subtopic test

1. 1
   A liver cell takes up and releases many substances across its cell surface membrane, including testosterone (a lipid-soluble steroid hormone), glucose, sodium ions and amino acids. The membrane is described by the fluid mosaic model.

   (a)

   Which of the substances would cross the phospholipid bilayer most easily by simple diffusion?

   [1 mark]

   • ATestosterone
   • BGlucose
   • CSodium ions
   • DAmino acids

   (b)

   Why is the cell surface membrane described as a fluid mosaic?

   [1 mark]

   • AIt is a rigid structure with proteins fixed in position
   • BIt is a single layer of phospholipids with proteins on the outside only
   • CPhospholipids and proteins can move within the layer, and the proteins are scattered through it
   • DIt contains water-filled channels formed only by phospholipids

   (c)

   Explain why sodium ions cannot cross the membrane by simple diffusion, and state how they can cross it by a passive process.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student investigated the effect of temperature on the permeability of beetroot cell membranes. She cut discs of beetroot of identical size, rinsed them, and placed one in each tube of distilled water at 20, 30, 40, 50 and 60 °C for 20 minutes. She then measured the absorbance of the water in each tube with a colorimeter, as a measure of the red pigment, betalain, that had leaked out. The absorbance readings were 0.05, 0.08, 0.15, 0.62 and 1.10.

   (a)

   Which is the best explanation for the high absorbance at 60 °C?

   [1 mark]

   • AThe cells made more betalain at the higher temperature
   • BProteins in the membranes were denatured and the bilayer became more fluid, so more betalain leaked out
   • CWater entered the cells and diluted the pigment
   • DThe beetroot discs absorbed the pigment from the water

   (b)

   Which step makes sure the comparison between the temperatures is valid?

   [1 mark]

   • AUsing a different volume of distilled water at each temperature
   • BLeaving each tube for a different length of time
   • CCutting the discs for each temperature from a different beetroot
   • DCutting every disc from the same beetroot to the same size, and rinsing them before use

   (c)

   Explain the large increase in absorbance between 40 °C and 50 °C.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Glucose enters red blood cells through carrier proteins in the cell surface membrane. A scientist measured the rate of glucose uptake at different external glucose concentrations. The rate rose in proportion to the concentration up to 5 mmol dm⁻³, but at higher concentrations it levelled off at a maximum. The uptake was not changed when a respiratory inhibitor was added to the cells.

   (a)

   Explain why the rate of glucose uptake levels off at higher glucose concentrations.

   [3 marks]

   (b)

   Explain why the respiratory inhibitor did not change the rate of glucose uptake, and why glucose cannot enter the cell by simple diffusion.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student cut cylinders of potato tissue of equal length and mass and placed one in each of five sucrose solutions (0.0, 0.2, 0.4, 0.6 and 0.8 mol dm⁻³) for 60 minutes. She then blotted and reweighed them. The percentage changes in mass were +10.2, +3.1, −4.2, −11.0 and −16.5. The water potentials of the five solutions are 0, −550, −1130, −1760 and −2450 kPa. Water potential ψ = P + π, where P is pressure potential and π is osmotic (solute) potential.

   (a)

   Explain the changes in mass in the 0.0 and 0.8 mol dm⁻³ solutions, and describe how the results can be used to estimate the water potential of the potato cells.

   [6 marks]

   (b)

   A potato cell with a water potential of −800 kPa is placed in pure water. When it reaches equilibrium its osmotic potential has changed to −650 kPa. Calculate the pressure potential of the cell, and explain why the cell does not burst. Suggest one way the student could improve the reliability of her estimate.

   [6 marks]

   Total for question 4: 12 marks

End of questions