How is diffusion best defined?

Diffusion is best defined as the movement of particles from an area of high concentration to an area of low concentration. This process occurs due to the random movement of particles, and it continues until there is an equal distribution of the particles throughout the available space. This natural tendency to spread out is crucial in numerous biological processes, such as gas exchange in the lungs where oxygen diffuses from the alveoli (high concentration) into the blood (low concentration), and carbon dioxide moves in the opposite direction.

The other definitions do not accurately capture the essence of diffusion. Active transport involves the movement of molecules against their concentration gradient, requiring energy input, which contrasts with the passive nature of diffusion. The absorption of nutrients refers to the process by which substances are taken up from the digestive tract into the bloodstream, which may involve various mechanisms including diffusion, but is not synonymous with it. Filtration typically involves the movement of substances across a barrier, commonly in the context of kidneys filtering blood, which also differs fundamentally from diffusion as it is a pressure-driven process.