According to the fluid mosaic model of cell membranes, which of the following is a true statement about membrane phospholipids? a) they can move laterally along the plane of the membrane. b) they frequently flip-flop from one side of the membrane to the other. c) they occur in an uninterrupted bilayer, with membrane proteins restricted to the surface of the membrane. d) they are free to depart from the membrane and dissolve in the surrounding solution. e) they have hydrophilic tails in the interior of the membrane.