This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: membrane structure and function, Subject name: Biochemistry
Bilayer cell membrane contains 
Select and explain. 
A. Cholesterol, B. Triacyl glycerol, C. Cholesterol ester, D. Glycerol
A: Cholesterol
Answer: A: Cholesterol 
Explanation: The phospholipids are arranged in bilayers with the polar head groups oriented towards the extracellular side and the cytoplasmic side with a hydrophobic core (Fig. 2.4A). The distribution of the phospholipids is such that choline-containing phospholipids are mainly in the external layer and ethanolamine and serine containing phospholipids in the inner layer. The cholesterol content of the membrane alters the fluidity of the membrane. When cholesterol concentration increases, the membrane becomes less fluid on the outer surface, but more fluid in the hydrophobic core. The effect of cholesterol on membrane fluidity is different at different temperatures. At temperature below the Tm cholesterol increases fluidity and thereby permeability of the membrane. At temperatures above the Tm, cholesterol decreases fluidity.Ref: DM Vasudevan, page no: 11