Diffusion is defined as the net movement of substances from high concentration to low concentration until equilibrium, and is passive.

Diffusion is defined as the net movement of substances from high concentration to low concentration until equilibrium, and is passive.

• A. Diffusion is the net movement of substances from low concentration to high concentration, requiring energy
• B. The net movement of substances from high concentration to low concentration until equilibrium, and is passive
• C. Diffusion is movement through protein channels only
• D. Diffusion always requires water as solvent

Answer: (B)

Diffusion is driven by the random movement of particles and moves substances down their concentration gradient—from areas of higher concentration to areas of lower concentration. It is a passive process, meaning it does not require cellular energy. The net movement continues until concentrations are equal on both sides (dynamic equilibrium), though individual molecules keep moving. This definition fits because diffusion describes the downhill flow along the gradient and doesn't use energy. The statement also notes that the process is passive, which matches how diffusion occurs in cells or in solutions without pumps. Context can help you see how diffusion works in real life: small nonpolar molecules can diffuse directly through the lipid bilayer, while polar or charged substances may diffuse via facilitated diffusion through channel or carrier proteins—still down the gradient and without energy input. It’s important to distinguish diffusion from osmosis (the diffusion of water) and from active transport (which moves substances against the gradient using energy). Why the other ideas don’t fit: moving from low to high concentration would require energy and is not diffusion. Diffusion is not limited to movement through protein channels—many substances diffuse through the membrane by passing directly through the lipid bilayer. And diffusion does not require water as a solvent; it occurs for gases and other liquids as well, with osmosis being the specific case of water diffusion.

Diffusion is driven by the random movement of particles and moves substances down their concentration gradient—from areas of higher concentration to areas of lower concentration. It is a passive process, meaning it does not require cellular energy. The net movement continues until concentrations are equal on both sides (dynamic equilibrium), though individual molecules keep moving.

This definition fits because diffusion describes the downhill flow along the gradient and doesn't use energy. The statement also notes that the process is passive, which matches how diffusion occurs in cells or in solutions without pumps.

Context can help you see how diffusion works in real life: small nonpolar molecules can diffuse directly through the lipid bilayer, while polar or charged substances may diffuse via facilitated diffusion through channel or carrier proteins—still down the gradient and without energy input. It’s important to distinguish diffusion from osmosis (the diffusion of water) and from active transport (which moves substances against the gradient using energy).

Why the other ideas don’t fit: moving from low to high concentration would require energy and is not diffusion. Diffusion is not limited to movement through protein channels—many substances diffuse through the membrane by passing directly through the lipid bilayer. And diffusion does not require water as a solvent; it occurs for gases and other liquids as well, with osmosis being the specific case of water diffusion.