30. 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.

C. two layers of lipids with proteins associated with the lipids. 

Cell membranes contain membrane proteins that  allows all the interaction that occurs within the cell. It makes the cell membrane selectively permeable because of its characteristics as being fluid or its fluidity which means to say that they can move freely within the lipid bilayer. The membrane proteins can also be stored in the lipid bilayer together with the enzymes. It cannot be denied that membrane proteins have a lot of functions based on its type. They can be integral proteins, peripheral proteins and lipid bound proteins. 

Gravitational force i believe