Which of the following statements regarding the structure and function of cell membranes is correct?

a. The phospholipid bilayer is arranged so that the hydrophilic heads of the phospholipid molecule face the extra- and intracellular fluids.

b. Some proteins integrated into the membrane serve as channels for the passage of nonpolar molecules through the membrane.

c. Peripheral membrane proteins function as channels associated with transport of ions through the membrane.

d. Cholesterol molecules function to transport ions from one side of the plasma membrane to the other.

e. The plasma membrane is a layer of phospholipid molecules with their hyrophilic head groups in contact with the extracellular solution and the hydrophobic tail groups in contact with the intracellular solution.

In general, polar molecules diffuse more rapidly through the lipid bilayer part of cell membranes than do nonpolar molecules. This statement is

a. true.

b. false, because polar molecules diffuse through less rapidly because the bilayer is polar throughout its width.

c. false, because polar molecules diffuse through less rapidly because the bilayer is nonpolar throughout its width.

d. false, because nonpolar molecules diffuse through more rapidly because much of the bilayer is nonpolar.

e. false, because the rate of diffusion of nonpolar and polar molecules through the bilayer is essentially the same as long as the molecules are the same size.

The permeability of the plasma membrane to mineral ions

a. is not influenced by channels formed by proteins.

b. is the same in all cell types.

c. is affected by differences in electrical charge on the two sides of the membrane.

d. is zero in all living cells.

e. is only possible because of the hydrophobic interior of the lipid bilayer.