Question

why substances that have hydrophilic moiety find difficult to pass through the membrane??

Answer 1

becoz cell membrane us made up of lipids (fats) hydrophilic substance do not passes from cell membrane

Answer 2

The membrane is a lipid bi-layer structure which means it has two layers made up of lipid molecules which are hydrophobic in nature. Hydrophobic means they are not soluble in water.  

The hydrophobic nature of lipid membrane does not allow any hydrophilic molecules to pass through it.  

But they are specialized structures called pumps, channels which transports material through lipid membrane.