Question

Which part of the cell membrane is nonpolar? How does this property contribute to the function of the cell membrane?

Answer 1

Hint: The cell membrane is the outermost layer of an animal cell. It gives protection to the cell. As it is semi-permeable, it only allows certain molecules to pass through. Chemically, the cell membrane is made of lipids, proteins, and carbohydrates associated with lipids or proteins.

Complete answer:
The phospholipids in the cell membrane form a bilayer, which means it is made of two layers of phospholipids. The structure of a phospholipid is made of polar head groups and non-polar hydrophobic tails. These hydrophobic tails do not allow polar molecules to pass through. Thus, for molecules to pass through channels are formed by proteins. These protein channels have hydrophobic amino acids towards the non-polar tails and polar amino acids towards the inside. Such an arrangement makes it possible to have ion channels or water porins in the cell membrane. Thus, giving the cell membrane its property of being semi-permeable.

Hence, the correct answer is option (B).

Additional information:
The other important components of a cell membrane are cholesterol, proteins, and carbohydrates. Cholesterol plays an important role in membrane fluidity and also helps reduce membrane permeability. Proteins on the membrane can either be integrated into the membrane to form channels or may be present on the surface along with carbohydrates as receptors or immunoglobulins. Membrane carbohydrates facilitate cell recognition and adhesion.

Note: S.J. Singer and Garth L. Nicolson in 1972 proposed the fluid mosaic model of the lipid bilayer. When these phospholipids are exposed to an aqueous solution, they self-assemble into a two-layered sheet with the hydrophobic tails pointing towards the center of the sheet.