As a matter of course, {HEAD ~ cation exchange membranes} permit {TAIL ~ selective permeation} of cations alone 
permit
As a matter of course , { HEAD ~ cation exchange membranes } permit { TAIL ~ selective permeation } of cations alone
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH