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

Hint: The phospholipid bilayer of the cell membrane is made up of the polar head and nonpolar fatty acid tail. In this lipid bilayer, peripheral and integral membrane proteins are embedded. The inner surface of the proteins is hydrophobic and the outer surface is hydrophilic which decides the permeability of the lipid bilayer.

Complete answer: The phospholipid bilayer consists of two layers of phospholipids – hydrophobic interior and a hydrophilic exterior surface.
Each phospholipid has a hydrophilic or polar head group and hydrophobic fatty acid chains as a tail. This fatty acid tails are represented in the single phospholipid molecule.
So, the entire molecule of phospholipid cannot be hydrophilic or hydrophobic.
The same type of lipids is generally seen in the b0th layers of lipids.
The gases are highly permeable to the lipid bilayer.
Some of the polar molecules also pass the lipid bilayer like water, ethanol, urea but very large polar molecules like glucose, fructose, amino acids, proteins, and ions are not permeable to the phospholipid bilayer.

So, the correct answer is option A. is readily permeable to large polar molecules.

Note: Transport of molecules across the plasma membrane takes place by three methods – active transport, passive transport, and bulk flow.
Passive transport can be again of three types – simple diffusion facilitated diffusion and osmosis.
Bulk flow is of two types – endocytosis and exocytosis.