Which form of transport is concentration gradient dependent?

• A. Passive diffusion
• B. Facilitated diffusion
• C. Active transport
• D. Endocytosis

Answer: (A)

Movement down the concentration gradient without using cellular energy is driven by the difference in concentration on the two sides of the membrane. This describes passive diffusion, where small, nonpolar or lipid-soluble molecules can cross the lipid bilayer directly. The driving force is the gradient itself: the larger the difference in concentration, the faster the net flux, until equilibrium is reached. No transport proteins or energy input are required for this process, which is why it’s the classic example of gradient-driven transport.

Facilitated diffusion also moves substances down their gradient, but it uses specific membrane proteins to help them cross. Active transport moves substances against their gradient and requires energy, so it isn’t solely gradient-driven. Endocytosis is a vesicle-based process that doesn’t depend on the concentration gradient for its driving force.