you are studying the entry of a small molecule into cultured animal cells. you determine the rate of movement across the membrane under a variety of conditions and make the following observations: the molecules can move in either direction across the membrane. the molecules always move down their concentration gradient. when atp synthesis is the cell is inhibited, transport of the molecule is not affected. when protein synthesis is inhibited, transport of the molecule is reduced. the mechanism used to get this molecule across the membrane is most likely: