What is diffusion?

Diffusion is defined as the process by which molecules move from an area of higher concentration to an area of lower concentration. This movement continues until there is an equal distribution of molecules throughout the space, achieving equilibrium. In the context of biological systems, diffusion is essential for processes such as gas exchange in lungs and the transport of nutrients and waste products in cells.

The reasoning behind why the movement occurs from high to low concentration is tied to the natural tendency of particles to spread out and occupy all available space, thus moving down the concentration gradient. This process does not require any external energy, as it relies on the kinetic energy of the molecules themselves.

The other options describe movements or processes that either do not align with the typical definition of diffusion or introduce conditions (such as energy input) that are not necessary for diffusion to occur.