The big idea: How fluid a membrane is depends on its fatty acid tails. Straight tails pack tightly; kinked tails cannot.
The points to remember
- Saturated fatty acids have no C=C double bonds: straight tails that pack tightly.
- Tight packing gives a higher melting point: the membrane is stiffer and stays strong at higher temperatures.
- Unsaturated fatty acids have a kink at each C=C double bond, so their tails cannot pack tightly.
- Loose packing gives a lower melting point: the membrane stays fluid and flexible at low temperatures.
- More unsaturated = more fluid; more saturated = less fluid.
Remember it as: Kinks keep it fluid in the cold; straight tails keep it firm in the heat.
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The big idea: A membrane must stay fluid enough to work, but not so fluid that it leaks. Organisms adjust their fatty acids to the temperature they live at.
Bacteria grown cold
- E. coli grown at low temperature makes more unsaturated fatty acids
- Its membranes stay fluid in the cold
Reindeer legs
- Cells near the hooves, which get very cold, have more unsaturated fatty acids
- Than cells near the warm body
Hot springs
- Bacteria that live in hot springs have more saturated fatty acids
- Their membranes stay strong when hot
Say which way and why: Name the type of fatty acid AND the temperature: unsaturated for cold, saturated for warm. Add that cholesterol also adjusts fluidity in animal cells.
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How it is asked: Paper 2 asks for the effect of fatty acids on fluidity [1], or how membranes are adapted to different temperatures [3]. Paper 1A gives a table of possible changes for a cold habitat.
IB-style questionOutline[3 marks]
Fish living in the Antarctic Ocean, at about −1.8 °C, have cell membranes that still work. Outline how membranes can be adapted to different temperatures.
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See the answer broken into clear points, with guidance on how each point earns marks.