Cell membrane structureEdexcel A-Level Biology A: Subtopic test

1. 1
   In 1935 Davson and Danielli proposed that a cell membrane is a layer of phospholipid with a continuous layer of protein covering both of its surfaces. In 1972 Singer and Nicolson proposed the fluid mosaic model after freeze-fracture electron microscopy showed protein molecules lying within the phospholipid bilayer.

   (a)

   Which part of each phospholipid molecule faces the watery cytoplasm and tissue fluid on either side of the membrane?

   [1 mark]

   • AThe hydrophilic phosphate head
   • BThe hydrophobic fatty acid tails
   • CThe whole molecule, lying flat on the membrane surface
   • DEither end, because the orientation is random

   (b)

   Which observation from freeze-fracture micrographs best supports the fluid mosaic model over the Davson and Danielli model?

   [1 mark]

   • AMembranes contain phospholipids
   • BMembranes are about 7 nm thick
   • CMembranes contain two layers of phospholipid
   • DProtein molecules are scattered within the phospholipid bilayer, not only on its surfaces

   (c)

   Explain how scientific understanding of the structure of cell membranes changed between 1935 and 1972.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The cell surface membrane of an animal cell contains phospholipids, cholesterol, channel proteins, carrier proteins and glycoproteins. Cholesterol molecules fit between the fatty acid tails of the phospholipids.

   (a)

   What is the main function of cholesterol in an animal cell membrane?

   [1 mark]

   • AIt forms channels for the diffusion of ions
   • BIt regulates the fluidity of the membrane by restricting the movement of phospholipids
   • CIt acts as a receptor for hormones
   • DIt provides energy for active transport

   (b)

   Which component of the membrane is most likely to act as a receptor for a hormone at the cell surface?

   [1 mark]

   • AA fatty acid tail
   • BA cholesterol molecule
   • CA glycoprotein
   • DA phosphate head

   (c)

   Explain why oxygen can diffuse directly through the phospholipid bilayer, but sodium ions cannot.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates the effect of ethanol on cell membranes using beetroot, which contains a red-purple pigment (betalain) inside its cells. She cuts equal discs of beetroot, rinses them and places each in 5 cm³ of ethanol solution for 30 minutes. She then measures the absorbance of each solution in a colorimeter with a blue-green filter. Her results for ethanol concentrations of 0, 10, 20, 30 and 40% were absorbances of 0.05, 0.08, 0.20, 0.52 and 0.95.

   (a)

   Describe the trend in the results and explain it.

   [3 marks]

   (b)

   Describe how the student should control the investigation so that her results are valid and reliable.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In 1970 Frye and Edidin fused a mouse cell with a human cell. They labelled the membrane proteins of the mouse cell with a red fluorescent marker and those of the human cell with a green fluorescent marker. Immediately after fusion the red and green markers were in separate halves of the hybrid cell. After 40 minutes at 37 °C the colours were completely mixed, but when the experiment was repeated at 1 °C very little mixing occurred. A cell surface membrane must also let oxygen in quickly, keep ions inside the cell, hold hormone receptors in place and remain flexible.

   (a)

   Explain how the results of this experiment support the fluid mosaic model, including the effect of temperature.

   [6 marks]

   (b)

   Describe the structure of a cell surface membrane and explain how this structure gives the membrane the properties described.

   [6 marks]

   Total for question 4: 12 marks

End of questions