All cells are surrounded by a semipermeable membrane, which regulates the passage of materials in and out of the cell. the principle structure of this membrane is
a. two layers of protein with a single layer of lipids in between.
b. a thick wall of cellulose with a thin lipid coating on the inside.
c. two layers of lipids with proteins associated with the lipids.
d. two layers of lipids with a polysaccharide layer in between.