Question

Which kind of molecule gives a cell membrane its fluidity?

Answer 1

Phospholipid.

Answer 2

Cholesterol (Eukaryotes)
Hopanoid (Prokaryotes)

Answer 3

Phospholipid and Lipid Bilayer.

Answer 4

why dont you explain

Answer 5

Basically, the lipid bilayer is composed of 2 layers of phospholipids. A phospholipid, in most texts, is represented by 2 long aliphatic tails attached to a phosphate head. The Phosphate "head" is hydrophilic (likes water) and points towards the outside (for the other layer) and points inwards (for the inner layer). The tails are in the inbetween space between the molecules.

This configuration is quite remarkable if you think about it. A cell memebrane is not a sheet, but rather by individual molecules, which are not joined together chemically (unlike polymers). As you might imagine, this configuration can be rather unstable, but it is very fluid.

To give the cell membrane more integrity, the lipid bilayer has some other molecules. For animals, there are cholesterol molecules which increase the rigidity of the cell membrane. Though there other possible functions for cholesterol as well.

For a better visualization of the lipid bilayer, this site has a cartoon of what a lipid bilayer looks like; as how one represents a phospholid.
http://www.aquaculture.ugent.be/Education/coursematerial/online%20courses/ATA/analysis/lip_phos.htm

For other reading of cholesterol,
http://www.cholesterol-and-health.com/Cholesterol-Cell-Membrane.html

Answer 6

Thanks everybody :) Biology is not my strong suit... :P