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I was previously taught that cholesterol affects the fluidity of a plasma membrane. At high temperatures, cholesterol decreases fluidity and at low temperatures cholesterol increases fluidity. The Khan academy and Wikipedia pages below say the same thing.

https://www.khanacademy.org/video/cell-membrane-fluidity https://www.wikiwand.com/en/Membrane_fluidity

Yet, in a current college course, the textbook ("Molecular Biology of the Cell 6th" by Bruce Alberts et al.) says that cholesterol reduces the mobility of the first few CH2 groups of a phospholipid's two fatty acid chains. In this way, the cholesterol makes the lipid bilayer more rigid and decreases the lipid bilayer's permeability to small, water-soluble molecules. However, it says that cholestrol does not actually make the membrane less fluid.

Is the textbook from my current course just a more nuanced explanation, or have I misunderstood something else?

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  • $\begingroup$ Can you please copy/paste the exact quote? The 4th edition of Alberts clearly says that cholesterol makes the membrane less fluid: "cholesterol tends to make lipid bilayers less fluid at the high concentrations found in most eucaryotic plasma membranes". $\endgroup$ – user24284 Sep 12 '17 at 9:50
  • $\begingroup$ Actually, cholesterol has different effects on the fluidity at different temperatures. When it is cold, cholesterol increases fluidity, preventing the membrane from freezing. When it is warmer, cholesterol decreases fluidity. $\endgroup$ – Tan Yong Boon Sep 13 '17 at 0:43
  • $\begingroup$ Lehninger say the same thing. Sterols decrease fluidity $\endgroup$ – Bakshi Jan 8 '18 at 14:35
  • $\begingroup$ At normal temperatures, cholesterol makes the lipid bi-layer stronger but less fluid because of the way that it forms hydrogen bonds with neighbouring phospholipid and glycolipid heads and fills the space between the bent fatty acid tails. Unfortunately I am still confused about how increasing and decreasing temperature impacts the fluidity. $\endgroup$ – Finnigan Sep 6 '18 at 18:00
  • $\begingroup$ I've posted a brief answer, but I don't have a link to the Bruce Alberts text to address the specific claim of the CH2 groups. $\endgroup$ – James Jan 2 at 10:49
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Cholesterol certainly affects plasma membrane fluidity. In 1978 Cooper found that increasing the ratio of cholesterol to phospholipids decreased the membrane fluidity. This also reduces membrane permeability and reduces the survival of the cells (red blood cells in that case). I imagine this is where the textbooks draw their conclusions.

More recently Rog et al., 2008 demonstrated the importance of the hydroxyl group in the cholesterol by replacing it with a ketone. This affected fluidity and flip flop rate.

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