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Date	May 2019	Marks available	2	Reference code	19M.3.SL.TZ1.1
Level	Standard level	Paper	Paper 3	Time zone	Time zone 1
Command term	Explain	Question number	1	Adapted from	N/A

Question

Membrane structure can be investigated using a technique known as freeze fracture.
Cells are frozen and then split. Fracturing often occurs between the two phospholipid layers of membranes in the cell. An electron micrograph of such a fractured membrane is shown.

[Source: © Science Photo Library]

Using the scale bar, calculate the magnification of the image.

[2]

Explain how electron micrographs such as this helped to falsify the Davson–Danielli model of membrane structure.

[2]

Explain how the amphipathic nature of phospholipids allows them to form bilayers.

[2]

Markscheme

$\frac{{{\text{scale bar length}}}}{{{\text{image size}}}} = \times 660$ ✔

a. the Davson–Danielli model proposed two layers of protein on either side of a lipid bilayer ✔ OWTTE

b. micrographs illustrate proteins in and/or crossing the membrane ✔

a. amphipathic means that they are both hydrophilic and hydrophobic ✔

b. the outside hydrophilic parts are exposed to water ✔

c. hydrophobic parts are away from water in the inside ✔

Water or lack of needs to be mentioned for mpb

Examiners report

Two marks were awarded for an answer within the given range, since many candidates did not show working, and the question did not request it. Many candidates measured the whole diagram for the numerator instead of the scale bar, and others inverted the formula. A formula in words that appeared correct was not credited if the numbers then negated this. Errors in unit conversion were frequent, but credit was given for working.

Descriptions of protein layers either side of the phospholipid bilayer in the Davson-Danielli model were unclear. Embedded or integral proteins were frequently stated for the second marking point.

The terms hydrophilic and hydrophobic were commonly stated, although sometimes assigned to heads and tails incorrectly; however this was not penalised. Orientation towards or away from water was less often explained.

Syllabus sections

Core » Topic 1: Cell biology » 1.3 Membrane structure

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