The correct answer is: d. membrane thickness
Cholesterol is the second major lipid in the structure of the cell membrane (phospholipids are the major lipid components).
Cholesterol is rigid, so it gives the cell membrane extra support (strength and hardness) by immobilizing some of the lipid molecules around them. This rigidity makes it harder for small molecules to pass through the membrane, so it also has the influence on cell membrane’s permeability (the membrane becomes effective barrier to ions).
Cell membrane’s fluidity is also under the influence of the cholesterol which helps generate some extra space between the lipids allowing them to move freely throughout the membrane.
