Question

The phospholipid bilayer

• A. Is readily permeable to large, polar molecules
• B. Is entirely hydrophobic
• C. Is entirely hydrophilic
• D. Has different lipids in the two layers

Answer 1

Answer: (D)

Has different lipids in the two layers

The plasma membrane of the cell is made up of phospholipid bilayers with embedded proteins. Membrane lipids are asymmetric in their distribution on the two faces of the bilayer, although the asymmetry, unlike that of membrane proteins is not absolute. In lipid layers, outer head part is the hydrophilic and inner tail part is hydrophobic. In the case of proteins, the asymmetry reflects a specific functional role of different proteins whereas the asymmetry in lipids accounts for differences in properties of the constituent lipid portion of the membrane which consequently describes the structure of bilayer. In the plasma membrane, certain lipids are typically found primarily in the outer face of the bilayer and others in the inner or cytoplasmic face. Sphingomyelin and phosphatidylcholine are present in higher amounts towards the outer face while phosphatidylethanolamine and phosphatidylserine are present in higher amounts on the inner or cytoplasmic side.