Answer
(d) changes in shape and position of carrier molecules in the plasma membrane
Work Step by Step
Solutes can be pumped in and out of the cell, against their electrochemical gradients, because they attach to carrier molecules in the plasma membrane. These carrier molecules change their shape and position based on whether they are pumping solutes in or out of the cell, and which solutes they are carrying.
While exocytosis does remove materials from the cell, it is not solute-pumping. Phagocytosis is a form of "cellular eating" and does not involve solute-pumping. Electrical forces in the cell membrane affect the electrochemical gradient, but the active transport type of solute-pumping is not accomplished due to those forces.
