Phospholipid Bilayer Membranes are Selectively Permeable

Question

Phospholipid bilayer membranes are selectively permeable. What does that mean?
A) They allow everything nonpolar to cross.
B) They allow everything but water to cross.
C) They allow only water to cross.
D) They allow some things to cross while restricting others.

Accepted Answer

Selectively permeable means that the membrane allows some things to cross while restricting others. Specifically, phospholipid bilayer membranes are permeable to small, nonpolar molecules like oxygen and carbon dioxide, but they restrict the passage of larger polar molecules like ions and many types of proteins.