Molecules are defined as the smallest particles of a chemical compound or an element, and are made up of atoms. Cells are made up of semi permiable membranes that only allow selected materials to move in or out of the cell, therefore due to the big size of molecules compared to atoms the membrane doesn't allow their passage in the cell. Therefore the cell doesn't allow most molecules to diffuse passively to the cell. Secondly, molecules that can enter the cell requires specific carriers carriers that will enable entry of small molecules in the cell.