How does cholesterol affect the fluidity of a plasma membrane?
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_fluidityYet, 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?
Your textbook is correct and this is the more nuanced explanation: cholesterol affects the rigidity/stability of the membrane bilayer. This rigidity itself will affect the ability of other molecules (mainly proteins) to move across the bilayer (membrane fluidity). That's what is usually meant by "cholesterol affects fluidity". Membrane itself is fluid in the sense that it will allow for movement of molecules withing it under certain conditions, and this "fluidity" is based on its lipid constitution (more or less cholesterol). Imagine moving through a large crowd of tightly packed people vs the same number of people with some space between them. I also suggest you should look up a structural model of how membrane looks like with and without cholesterol. Hope this helped.
