Which term best describes the net movement of molecules from high to low concentration due to random motion?

• A. Diffusion
• B. Osmosis
• C. Ingestion
• D. Reproduction

Answer: (A)

Diffusion is the net movement of molecules from a region of high concentration to a region of low concentration as a result of their random thermal motion. Particles are constantly moving in all directions, so when there are more particles on one side, more molecules tend to wander into the less crowded area than return, causing the overall spread out of substance until concentrations balance. This process requires no energy input; it’s passive and driven purely by randomness. You can see it in action when a drop of dye disperses in water or when a scent spreads through a room. Osmosis, by contrast, is a specific type of diffusion for water across a semipermeable membrane, moving toward higher solute concentration. Ingestion and reproduction are unrelated to the movement of molecules between areas of different concentrations.