Cell membrane structureAQA A-Level Biology: Subtopic test

1. 1
   A technician cut equal-sized discs from a beetroot with a cork borer and then rinsed them in distilled water before starting an experiment on membrane permeability.

   (a)

   Why were the discs rinsed in distilled water before the experiment?

   [1 mark]

   • ATo make the discs absorb more water
   • BTo bring the discs to the temperature of the laboratory
   • CTo denature the proteins in the cell-surface membranes
   • DTo remove pigment released from cells damaged when the discs were cut

   (b)

   The technician then heated some discs to 70 °C. Which membrane component is most likely to be denatured, making the membrane more permeable?

   [1 mark]

   • ACholesterol
   • BPhospholipid
   • CProtein
   • DCarbohydrate chain

   (c)

   The technician placed other discs in ethanol. Explain why the solution around these discs became redder than the solution around discs in water.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Phospholipid molecules have a hydrophilic head containing a phosphate group and two hydrophobic fatty acid tails. A cell-surface membrane has an aqueous solution on both sides: tissue fluid outside the cell and cytoplasm inside it.

   (a)

   How are the phospholipids arranged in the cell-surface membrane?

   [1 mark]

   • AThe hydrophobic tails face outwards towards the water on both sides
   • BThe hydrophilic heads face outwards towards the water on both sides, with the tails forming a hydrophobic core
   • CThe molecules form a single layer with heads and tails mixed at random
   • DThe heads face the cytoplasm and the tails face the tissue fluid

   (b)

   What is the effect of cholesterol in a cell-surface membrane?

   [1 mark]

   • AIt restricts the movement of the other molecules in the membrane
   • BIt increases the movement of phospholipids so the membrane is more fluid
   • CIt forms channels through which ions cross the membrane
   • DIt carries carbohydrate chains on the outer surface

   (c)

   Explain why sodium ions cannot easily diffuse through the middle of the membrane.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The cell-surface membrane of a liver cell contains phospholipids, cholesterol, proteins and short carbohydrate chains. In 1972 Singer and Nicolson proposed the fluid-mosaic model of membrane structure.

   (a)

   Describe the arrangement of the phospholipids, proteins and carbohydrates in this membrane.

   [3 marks]

   (b)

   Explain why the structure of the membrane is described as a fluid mosaic, and explain the effect of cholesterol on this structure.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Beetroot cells contain a red pigment in their vacuoles. The pigment leaks out if the cell membranes are damaged or made more permeable, and its concentration in a surrounding solution can be measured with a colorimeter.

   (a)

   Describe how you would investigate the effect of ethanol concentration on the permeability of beetroot cell membranes, and explain how your results would show the effect.

   [6 marks]

   (b)

   In an investigation of temperature, the absorbance of the solution was 0.05 at 20 °C, 0.07 at 40 °C, 0.12 at 50 °C, 0.55 at 60 °C and 0.92 at 70 °C. Explain these results in terms of the structure of the cell membranes.

   [6 marks]

   Total for question 4: 12 marks

End of questions