Which description defines a concentration gradient and its effect on diffusion?

• A. A gradient of pH across membranes
• B. Movement of water against its concentration gradient
• C. A difference in solute concentration between two regions; substances diffuse down their gradient from high to low concentration
• D. Equal concentration across a membrane

Answer: (C)

A concentration gradient is a difference in solute concentration between two regions that creates a driving force for diffusion. In diffusion, particles move randomly, but there is a net flow from where their concentration is high to where it is low, until the concentrations equalize. This movement is passive and does not require energy.

The described statement captures the essence: there is a difference in solute concentration, and substances diffuse down that gradient from high to low concentration. That’s exactly how diffusion works.

The other ideas don’t fit as well: a pH gradient is a specific type of gradient, but diffusion is defined by solute concentration differences in general; a gradient of pH doesn’t describe diffusion universally. Movement of water against its gradient refers to osmosis and water moving to higher solute concentration (not diffusion of solutes). Equal concentration across a membrane means no gradient, so no net diffusion.