Cell membrane structureEdexcel International A Level Biology: Subtopic test

1. 1
   Cell surface membranes are made mainly of phospholipids and proteins. Each phospholipid molecule has a phosphate-containing head that attracts water and two fatty acid tails that repel water.

   (a)

   How are the phospholipid molecules arranged in a cell surface membrane?

   [1 mark]

   • AIn a single layer, with the hydrophobic tails facing the watery surroundings
   • BIn a bilayer, with the hydrophilic heads facing inwards and the tails facing the watery surroundings
   • CIn a bilayer, with the hydrophilic heads facing the watery surroundings and the hydrophobic tails facing inwards
   • DIn two layers, with heads and tails mixed at random

   (b)

   Which of these substances can diffuse directly through the phospholipid bilayer without the help of a membrane protein?

   [1 mark]

   • AOxygen
   • BSodium ions
   • CGlucose
   • DAmino acids

   (c)

   Explain why sodium ions cannot diffuse directly through the phospholipid bilayer.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In an experiment on cell fusion, a mouse cell and a human cell were joined to make one hybrid cell. The membrane proteins of the mouse cell were labelled with a red fluorescent dye and those of the human cell with a green dye. Immediately after fusion the red and green labels were in separate halves of the hybrid cell. After 40 minutes at 37 °C the two labels were completely mixed over the whole surface. When the experiment was repeated at 0 °C the labels stayed in separate halves.

   (a)

   What can be concluded from the mixing of the red and green labels at 37 °C?

   [1 mark]

   • AMembrane proteins are fixed in position
   • BMembrane proteins form continuous layers on both surfaces of the membrane
   • CThe dyes reacted with each other to form a new colour
   • DMembrane proteins are able to move within the membrane

   (b)

   Which statement best explains why the labels did not mix at 0 °C?

   [1 mark]

   • AThe membrane proteins were denatured and dissolved in the cytoplasm
   • BThe phospholipids have less kinetic energy, so the membrane is less fluid and the proteins cannot move
   • CThe dyes cannot be seen at low temperature
   • DThe phospholipid heads became hydrophobic

   (c)

   Explain how the results at 37 °C support the fluid mosaic model of membrane structure.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates the effect of temperature on the permeability of beetroot cell membranes. She cuts identical discs of beetroot, rinses them, and places one disc in 10 cm³ of distilled water in each of five test tubes held in water baths at 20, 30, 40, 50 and 60 °C for 10 minutes. She then removes the discs and measures the absorbance of the liquid in each tube with a colorimeter. Absorbance rose slightly between 20 °C and 40 °C and then rose sharply above 50 °C.

   (a)

   Explain why the absorbance of the liquid increases when the membranes of the beetroot cells become more permeable.

   [3 marks]

   (b)

   Explain the sharp rise in absorbance above 50 °C and state two variables the student must control to make this a valid investigation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Ethanol is an ingredient of some disinfectants. A researcher places identical beetroot discs for 20 minutes at 25 °C in ethanol solutions of concentration 0, 10, 20, 30 and 40% by volume. She measures the absorbance of each solution using a colorimeter and finds the following readings in order: 0.02, 0.05, 0.18, 0.52 and 0.96. Only one disc was used at each concentration.

   (a)

   Describe the pattern shown by the data and explain, with reference to the structure of the membrane, the effect of increasing ethanol concentration on the beetroot cells.

   [6 marks]

   (b)

   Evaluate how far the data support the conclusion that ethanol increases the permeability of beetroot membranes, and suggest how the confidence in the conclusion could be improved.

   [6 marks]

   Total for question 4: 12 marks

End of questions