This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Main content of bilayer cell membrane is: March 2009 
Select and explain. 
A. Glycerol, B. Cholesterol, C. Cholesterol ester, D. Triacyl glycerol
B: Cholesterol
Answer: B: Cholesterol 
Explanation: Ans. B: CholesterolBiological membranes typically include several types of lipids other than phospholipids. A paicularly impoant example in animal cells is cholesterol, which helps strengthen the bilayer and decrease its permeability.Cholesterol also helps regulate the activity of ceain integral membrane proteins. Integral membrane proteins function when incorporated into a lipid bilayer.Because bilayers define the boundaries of the cell and its compaments, these membrane proteins are involved in many intra- and inter-cellular signaling processes.