Answer:
The phospholipids are the first control for allowing the entry of molecules across the semi-permeable membrane. The phospholipids are made up of phosphate heads which are hydrophilic i.e water-loving. The tails which are made up of lipids are hydrophobic i.e water-hating. The lipids are arranged on the inner side whereas phospholipids are present at the outer ends of the structure. They allow selective entry of only small molecules and the larger molecules are blocked from entry.