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