Final answer:
Molecules passively transported across a membrane move from a region of higher concentration to one of lower concentration until equilibrium is reached. This process, known as diffusion, does not require cellular energy.
Step-by-step explanation:
When molecules are passively transported across a membrane, the direction of their movement is dictated by the molecules' tendency to move toward a region of lower concentration. This process is known as diffusion, which is a type of passive transport. Molecules will continue to move down their concentration gradient until the concentrations on both sides of the membrane are equal. Diffusion releases free energy that is dependent on the relative concentrations of the solutes.
Substances that can pass easily through the cell membrane do so by simple diffusion, while others may require facilitated transport to move through specific protein channels in the membrane. This movement does not require cellular energy, distinguishing it from active transport that does. For instance, glucose utilizes facilitated transport via glucose transporters to enter cells.