Cell membrane structureEdexcel A-Level Biology A: Mind map

Exam questions on Cell membrane structure

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.
   Explain how scientific understanding of the structure of cell membranes changed between 1935 and 1972.2 marks
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.
   Explain why oxygen can diffuse directly through the phospholipid bilayer, but sodium ions cannot.2 marks
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.
   Describe the trend in the results and explain it.3 marks