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Diffusion is a mechanism used to transport materials across cell membranes, using no energy (ATP).

There are three types of diffusion[1]:

Simple 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 concentration. 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:

Facilitated Diffusion

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[2]:


  1. Biochemistry (6th Edition) Berg et. al. W. H. Freeman and Company New York
  2. LODISH, H et al. (2008) Molecular Cell Biology, 6th ed. p439-440. New York: W.H. Freeman and Company
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