There are three types of diffusion :
The movement of ions and molecules from an area of high concentration to a low concentration across a cell membrane (down their concentration gradient). This agrees with the Second Law of Thermodynamics which says that molecules will spontaneously move towards an area of lower concentration, from an area of higher concetration. Molecules that can travel across cell membranes are called lipophilic molecules as they dissolve in the lipid bilayer.
Molecules which are transported by simple diffusion include:
- Carbon dioxide
- Steroid Hormones - a good physiological example of simple diffusion. Steroid hormones (similar to cholesterol) will move freely across membranes - provided it is down their concentration gradient.
Facilitated diffusion is the diffusion of ions, water or hydrophilic molecules across a membrane with the help of carrier proteins. Within carrier proteins there are three types: transporters (uniporters), counter transporters (antiporters) and co-transporters (symporters). Some counter transporters and co-transporters use ion gradients across the membrane to drive the movement of glucose, amino acids, protons and calcium ions by active transport. This process is known as secondary active transport and is not diffusion.
Molecules which are transported via facilitated diffusion include :