I almost flipped when I encountered this sentence in a biology textbook – “The diffusion of sodium is done against a concentration gradient.” Huh? How can that happen? That is simply illogical.
Diffusion is passive transport. It is always along the concentration gradient, never against it. Diffusion is directly related to the difference in concentrations between two areas. This difference in concentration is called the concentration gradient. Molecules will always move along this gradient, that is, from where the molecules have greater concentration to where they are less concentrated. This movement is called diffusion. So if sodium moves by diffusion, then it’s moving down this concentration gradient, not against it. Molecules can only move against their concentration gradient by active transport.