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Date May 2022 Marks available 3 Reference code 22M.2.SL.TZ2.4
Level Standard level Paper Paper 2 Time zone Time zone 2
Command term Explain Question number 4 Adapted from N/A

Question

The Davson-Danielli model of membrane structure was proposed in the 1930s. When electron micrographs of membranes were first produced, they were used as evidence for this model. The micrograph shows two adjacent membranes (indicated with arrows).

[Source: DENNIS KUNKEL MICROSCOPY/SCIENCE PHOTO LIBRARY.]

 

Explain how the appearance of membranes in electron micrographs was used as evidence to support the Davson-Danielli model.

Markscheme

  1. the black lines represent proteins;
  2. forms a ‘sandwich’/2 layers;
  3. there is a clear layer in the centre;
  4. (the clear layer) is composed of phospholipids;
  5. reference to both membranes being similar;

Examiners report

Some students stated that they could ‘clearly see’ the hydrophilic heads and the hydrophobic tails. They were describing the wrong model. Once again, this image came under some criticism for being far too small. Only the better prepared candidates used the evidence from the micrograph, and it was obvious that many students had not encountered this on their course.

Syllabus sections

Core » Topic 1: Cell biology » 1.3 Membrane structure
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