One of the roles of plasma membrane proteins is see section 6.4 ( page 131) .

One of the roles of the plasma membrane protein is to FACILITATE THE PASSAGE OF SOME SELECTED MOLECULES WHICH COULD NOT OTHERWISE CROSS THE LIPID BILAYERS.
One of the function of the proteins that are embedded within the phospholipid bilayers of the cell membrane is to act as carriers to some substances which have to enter the cells but which can not enter the cells on their own.