Remember, a membrane is a semi-permeable membrane. There are some substances that can cross the membrane and others have to take different route. It all comes down to their polarity. If we were to examine a membrane made of phospholipid bilayers, one side of it has hydrophilic heads and the inside has hydrophobic tails. The membrane is therefore polar, allowing polar substances to enter in and out of the cell. Non polar substances, such as water have to take a different route, through special proteins called aquaporins.
