為什麼鈉離子不能穿過磷脂雙分子層

first, 磷脂雙分子層 = phospholipid bilayer. This is a lipid which make the cell membrane in cells.
So, why ions cannot go through this membrane?
Ions are large particles while the phospholipid bilayer is a dense layer. Thus, they cannot pass through the membrane simply by diffusing from the membrane. In fact, not only ions that cannot pass through the bilayer but also some large molecules like starch.
in order to transport the ions through the membrane, ions have to use the channel protein as a passage to diffuse in and out the cell.

hope i can slove your problem.

This is because the ions cannot dissolve in the fat soluble matter . Also due to the ions have charge & is repelled by the phospholipid bilayer .

this is the problem due to polarity.(with repulsion)
if ions "wanted" to go through phospholipid bilayer,they have to go through the channel protein.

2011-10-18 12:50:31 補充：
hydrophobic/
hydrophiilic

磷脂雙分子層帶有疏水尾端(不溶於溶水性的離子中)，所以不能穿過磷脂雙分子層。