Suppose you are studying the transport of a certain polar molecule across the plasma membrane of cells in culture. over a period of time, you measure the concentration of a polar molecule inside and outside of the cells. you find that the concentration of the molecule is lower in the cell but is gradually increasing. you also measure the atp concentration inside the cell and find that it is not changing. which of the following is probably responsible for the transport of this polar substance into the cell? a) active transport
b) endocytosis
c) facilitated diffusion
d) secondary active transport
e) simple diffusion