How is diffusion defined?

Diffusion is defined as the movement of particles from an area of higher concentration to an area of lower concentration. This process occurs spontaneously due to the random motion of particles, seeking to reach a state of equilibrium where the concentration of particles is uniform throughout a given space.

In biological systems, diffusion is crucial for many physiological processes, such as the exchange of gases in the lungs and the movement of nutrients and waste products across cell membranes. This type of movement does not require energy input, as it relies on the inherent kinetic energy of molecules.

The other options describe concepts that do not align with the definition of diffusion. Movement from low to high concentration or against a concentration gradient involves active transport mechanisms and requires energy, which is not characteristic of diffusion. Additionally, diffusion is not limited to gases; it also occurs in liquids and solids, making the idea that it only involves gases a misunderstanding of the broader context in which diffusion occurs.