How is diffusion defined?

• A. Spontaneous movement of a substance down its concentration gradient
• B. Active transport requiring energy
• C. Movement of solutes through open pores without energy
• D. Movement of molecules against their gradient using carriers

Answer: (A)

Diffusion is the passive, spontaneous spreading of particles from regions of higher concentration to regions of lower concentration, driven by random thermal motion and not requiring cellular energy. This is why it’s described as movement down a concentration gradient without energy input. The other ideas involve using energy (active transport) or describe movement that isn’t inherently about gradient-driven spread, so they don’t define diffusion. Diffusion can occur directly through the membrane for small nonpolar molecules or through pores or channels for others, as long as the driving factor is the concentration gradient and no energy is expended.