Diffusion is the process in which particles move from a region where they are in higher concentration to a region where they are in low concentration, until uniform concentration is finally achieved. Carbon dioxide (CO₂) moves in and out of the cells by the process of diffusion.

Osmosis is a special type of diffusion. Osmosis is diffusion of water from the region of its higher concentration (pure water or dilute solution) to the region of its lower concentration (strong solution) though a semipermeable membrane. Water moves in and out of the cells by osmosis.

Both diffusion and osmosis are physical or mechanical processes and do not require spending of energy for their performance by the cells.