Which term describes movement across a membrane that does not require energy?

• A. Passive
• B. Active
• C. Facilitated Diffusion
• D. Osmosis

Answer: (A)

Movement across a membrane without using cellular energy is known as passive transport. This means substances move down their concentration or electrochemical gradient, from higher to lower concentration, driven purely by the gradient rather than by ATP or other energy sources.

Passive transport covers several specific processes, including diffusion directly through the lipid bilayer and diffusion through membrane proteins (facilitated diffusion), as well as osmosis, which is the diffusion of water. The key idea is no energy input from the cell is required, just movement along the gradient.

Active transport, in contrast, requires energy to move substances against their gradient, so it doesn’t fit. Facilitated diffusion and osmosis are examples of energy-free movement, but the term that describes the general category of all such processes is passive transport, making it the best fit for the question.