There are three types of diffusion that occur in the human body: simple diffusion, channel diffusion and facilitated diffusion. Each type permits molecules to go into and out of cells, allowing biochemical processes to take place.
Simple diffusion is a passive process that transports small, non-polar molecules through a cell wall. During simple diffusion, a hydrophobic molecule moves through the hydrophobic portion of the cell wall and is engulfed into the cell. Hydrophilic molecules cannot undergo simple diffusion. One example of simple diffusion is osmosis, which is the passive movement of water into and out of a cell. Channel diffusion is also a passive process. It involves the movement of ions and charged particles through a specific channel protein or pore in a cell wall. There is no limit to the number of particles that can travel through each channel.
Facilitated diffusion involves the movement of molecules with the help of a carrier protein. Protein carriers attach to the molecule that needs transporting and carries it across the cell wall. The molecule is moved into the cell along with the carrier protein. The rate of diffusion in facilitated diffusion is, therefore, dependent on the number of carrier proteins present in and around the cell.