Which statement best describes diffusion?

• A. Transport against a gradient
• B. Movement of particles from high to low concentration
• C. Bulk water movement
• D. Movement through plant phloem

Answer: (B)

Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration as a result of random thermal motion, and it does not require cellular energy. This means substances spread down their concentration gradient until equilibrium is reached, driven purely by opportunities created by molecular motion rather than active forces. This is why the description “movement of particles from high to low concentration” best captures diffusion.

The other processes involve energy or different mechanisms: moving against a gradient is active transport and needs energy; bulk water movement describes osmosis or bulk flow, not diffusion; movement through plant phloem refers to phloem transport (translocation), a bulk process driven by pressure differences rather than simple diffusion.