• Lipid Bilayer

The cell membrane is composed of a lipid bilayer that consists of two layers of phospholipids. These phospholipids have hydrophilic heads and hydrophobic tails. The hydrophilic heads face outward and interact with the extracellular and intracellular environments, while the hydrophobic tails face inward and interact with each other to form a nonpolar environment.

• Polar Molecules

Polar molecules have a net dipole moment due to the uneven distribution of electrons. They have a hydrophilic nature and interact with water molecules. Polar molecules include ions, sugars, and amino acids.

• Polar Molecules Cannot Pass Through Nonpolar Lipid Bilayer

The nonpolar environment of the lipid bilayer repels polar molecules, preventing them from passing through. The hydrophobic tails of the phospholipids repel the polar molecules, while the hydrophilic heads attract them. This creates a barrier that prevents the polar molecules from crossing the membrane.

• Polar Molecules Need Help to Cross the Membrane

Polar molecules need the help of transport proteins to cross the membrane. These transport proteins create a channel that allows polar molecules to pass through. The transport proteins are specific to the type of molecule they allow to pass through. For example, glucose transporters only allow glucose molecules to pass through.

In conclusion, the nonpolar environment of the lipid bilayer in the cell membrane prevents polar molecules from passing through. Polar molecules need the help of transport proteins to cross the membrane.