What is the effect of cholesterol in the cell membrane?
a) cholesterol moderates membrane fluidity at low and high temperatures.
b) cholesterol enables a sharp increase in fluidity at the transition temperature.
c) cholesterol reduces permeability to polar molecules over a wide temperature range.
d) cholesterol decreases membrane fluidity at lower temperatures.

a) Cholesterol moderates membrane fluidity at low and high temperatures.

Explanation:

Cholesterol is amphipathic sterol with a polar head group and non-polar steroid nucleus and hydrocarbon chain. Membranes are lipid bilayer with proteins embedded in it. An increase in temperatures makes membranes more fluid by disrupting the tight packing of non-polar tails of membrane phospholipids. At lower temperatures, tight packing of phospholipid tails makes the membranes more solid.

However, to maintain the functions of the membrane, its fluidity should be maintained within the range. Cholesterol is a moderator of membrane fluidity. Its interactions with amphipathic phospholipids make the membrane less fluid at higher temperatures and more fluid at lower temperatures.