Which of the following statements accurately distinguishes between active and passive transport?

A. Passive transport moves molecules randomly through the membrane regardless of concentration; active transport relies solely on diffusion: Passive transport is strictly governed by concentration or electrochemical gradients, not random movement. Active transport is the opposite of diffusion, as it requires a protein pump and energy. This choice incorrectly defines the fundamental physics of both processes.

B. Active transport and passive transport both require energy to move substances through the membrane: Only active transport requires the expenditure of cellular energy, typically in the form of adenosine triphosphate. Passive transport utilizes the kinetic energy already present in the concentration gradient. This distinction is the core difference between the two transport categories.

C. Active transport moves molecules against their concentration gradient using ATP, while passive transport moves molecules down their gradient without using energy: Active transport functions like a pump to move solutes from low to high concentration areas. Passive transport, including osmosis and facilitated diffusion, occurs spontaneously as molecules move toward equilibrium. This is the scientifically accurate distinction between the two.

D. Active transport moves molecules with the concentration gradient and requires energy; passive transport moves molecules against the gradient without energy: Moving with a gradient does not require energy, making the first part of this statement contradictory. Furthermore, moving against a gradient is physically impossible without an external energy source. This choice reverses the logic of thermodynamics.