Question

Permeability is the condition of being capable of having materials flow into and out of a membrane. The permeability of a cell membrane is determined by how easily a molecule can diffuse across the membrane. Usually, only molecules that are fat-soluble can permeate across a cell membrane. Why is this the case?

A.
Water-soluble molecules are too large.
B.
Cell membranes are composed of a lipid bilayer.
C.
Cell membranes are composed of cytosol.
D.
Water-soluble molecules are nonpolar.

Answer 1

Answer:B). Cell membranes are composed of a lipid bilayer.

Explanation: A membrane is a lipid bilayer that surrounds the cell. Lipids have two parts: the fatty acid chain (the tail) and the phosphate group (the head).

The phosphate head groups are polar, face outward and interact with the outside environment of the cell. The fatty acid chain is hydrophobic and nonpolar, face each other in the bilayer thus forming a hydrophobic fluid interior.

The hydrophobic nature of the bilayer interior makes the membrane to permit only nonpolar and hydrophobic molecules to pass through the cell membrane. This is the reason the membrane allows only molecules that dissolve in the lipid bilayer to pass through to enter or leave the cell.