Membranes allow some materials to pass while constraining the movement of other molecules.

Semi-permeable

Explanation:

This means it is selective of the molecules it allows to pass through it. Most cells have a semipermeable membrane made of phospholipids. The membrane has pores small enough to allow water molecules to pass through (and anything smaller) but restricts larger molecules such as ions. In addition, polar molecules will find it difficult passing through the membrane on their own due to the hydrophobic inner layer of the phospholipid bilayer structure of the membrane. These molecules require the assistance of transmembrane protein channels.