A study was conducted to understand the factors controlling the rate at which molecules or ions travel across cell membranes. An artificial
membrane was created that was composed of a phospholipid bilayer only.
The speed at which various substances crossed this membrane was
measured. Some substances can pass through an actual cell membrane
much faster than they passed through the artificial membrane in this study
Which of the following statements best explains this finding?"

a) Actual cell membranes have a much thicker phospholipid bilayer than the artificial
membrane does

b) Actual cell membranes have a variety of proteins embedded in the membrane that are
absent in the artificial membrane

c) Hydrophobic substances spend more time between the two layers of phospholipid in
the artificial membrane than they do between the layers in an actual membrane

d) Hydrophilic substand spend more time attached to the polar region of the
phospholipids in the artificial membrane than they do attached to the polar region of
the phospholipid in an actual membrane