14 6 The lipid content of the cell membranes of prokaryotic organisms changes in response to changes in the environmental temperature. (a) Phospholipids form a bilayer in the cell membranes of bacteria. Describe the structure of a phospholipid. 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(b) The cell membranes of most organisms belonging to the domain Archaea are lipid monolayers. The lipid that forms this monolayer is a bipolar lipid. Explain why a bipolar lipid is a suitable molecule to form the cell membrane. 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Turn over 15 (c) The percentage of branched‑chain fatty acids in membranes of bacteria that have different optimum growth temperatures was investigated. The graph shows the results of this investigation. 0 20 40 60 80 100 80 60 40 20 0 Percentage of branched‑chain fatty acids (%) Optimum growth temperature of bacteria / °C (i) Calculate the mean percentage of branched‑chain fatty acids in bacteria whose optimum growth temperature is <20 °C. (1) Answer .............................................................. % (ii) Analyse the data to describe two conclusions that can be drawn from this investigation. 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Explain why it is important that the lipid composition of the membrane of prokaryotic organisms changes if the temperature changes. 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(Total for Question 6 = 10 marks) Turn over 17 BLANK PAGE