What is diffusion?

Diffusion is defined as the process where molecules move from an area of higher concentration to an area of lower concentration. This movement occurs due to the random motion of particles and continues until there is an equal distribution of molecules throughout the space, achieving equilibrium. The significance of diffusion lies in its role in various biological and physical processes, such as the exchange of gases in the lungs and the movement of nutrients and waste in cells.

The other options represent different processes or concepts. The movement against a gradient would imply active transport, which requires energy, while diffusion is a passive process. The act of mixing substances can occur through diffusion but does not fully capture the essence of the process itself. Finally, while flow within a defined space can describe various types of transport, it does not specifically address the concentration gradient that is fundamental to diffusion.