Aim for FiveBiology

Conceptual analysis

Membranes in the cold

  • Units 1 and 2
  • 4 points
  • About 10 minutes

You can use a calculator on this question, just like on exam day.

A short question about a biological system that gets disrupted. You describe and explain the normal process, then predict what the disruption causes and justify that prediction. On the exam: Question 4 of 6 in Section II; about 10 minutes suggested.

The question

Many bacteria change the fatty acids in their membrane phospholipids when the temperature changes. At low temperatures, they use an enzyme called a desaturase to add double bonds between carbons in the fatty acid tails. A mutant strain of a bacterium has no functional desaturase, so its phospholipids contain only saturated fatty acids. Researchers grow the normal and the mutant strains at 10°C.

Suggested time: 10 minutes

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Part (a)

1 point

Describe how phospholipids are arranged in a plasma membrane.

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Part (b)

1 point

Explain how double bonds in fatty acid tails affect the fluidity of a membrane.

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Part (c)

1 point

Predict the fluidity of the mutant strain's plasma membrane at 10°C compared with the normal strain's plasma membrane at 10°C.

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Part (d)

1 point

Justify your prediction in part (c).

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