The movement of molecules from high to low concentration without energy is known as

• A. Active transport
• B. Simple diffusion (passive diffusion)
• C. Osmosis
• D. Facilitated diffusion

Answer: (B)

Diffusion is movement down a concentration gradient without energy input. When this movement happens directly across the membrane without help from proteins, it’s simple diffusion—driven purely by random molecular motion as substances move from higher to lower concentration. Osmosis is a specific case: the diffusion of water across a semipermeable membrane. Facilitated diffusion also moves substances down their gradient but uses membrane proteins (channels or carriers); it’s still energy-free, but its mechanism isn’t unassisted diffusion. Active transport, by contrast, uses energy to push substances against their gradient. So the described process is simple diffusion.