How does cholesterol maintain optimal fluidity in cell membranes?

[JimmyRay]

hi, my textbook shows how cholesterol sort of patches the cell membrane together, keeps it at optimal fluidity... I was just wondering how cholesterol is able to do this... I am in grade 11 biology I am not being tested on this or whatever I was just wondering if anyone knew...

 

[Monique]

Well, membranes are bilayers of lipids. A lipid has a polar head group and two non-poloar tail groups, the head is pointed outward to the water and the tails inward of the bilayer.

Cholesterol make the membrane more rigid and prevents phase transitions that could lead to crystallization. The reason that cholesterol does this, is because of its structure. Cholesterol has a very small head group and a bulky tail group with a rigid steroid ring structure, which cause it to be bend. This bending immobilizes surrounding phospholipids, thus making the membrane less fluid.

Eukaryotic plasma membranes (such as the human) contain especially large amounts of cholesterol, up to one molecule for every phospolipid molecule.

Cholesterol also aids in the formation of lipid rafts, which are microdomains also containing sphingolipids (with long saturated fatty hydrocarbon tails). These lipid raft domains are thicker than other parts of the lipid bilayer, and are thus able to accommodate other membrane proteins.

 

[JimmyRay]

hi thanks for replying

Well ok I understand that cholesterol makes the membrane less fluid, like as you said due to the bending... well it makes it LESS fluid when there's a lot of energy in the cell membrane (temperature high) ok which is how it keeps the cell membrane together, but how does it also make it FLUID at the same time? when temperature is low...

 

[Monique]

That is because for a phase transition to occur from a liquid state to a rigid crystalline (gel) state, the hydrocarbon tails need to be closely packed and ordered.

1) cholesterol prevents interaction between hydrocarbon tails of lipids
2) cholesterol is bend and thus prevents close packing

So the membrane remains fluid at lower temperatures, than would normally be allowed.

 

[JimmyRay]

Ohh ok, I see, thank you.