Simple Diffusion

• Simple diffusion is the net movement of molecules/ions from a region where they are more highly concentrated to a region where their concentration is lower, until equilibrium is reached.
• Simple diffusion is affected by the size of the molecule, the polarity of the molecule (needs to be nonpolar), the surface area available, and the diffusion distance (the thickness of the surface).
• Simple diffusion is a passive process where no ATP is required. A molecule will diffuse down its concentration gradient through a partially-permeable membrane (e.g. small, nonpolar molecules such as oxygen and carbon dioxide).
• Lipid-soluble solutes, e.g. steroid hormones (testosterone, oestrogen, progesterone etc) can diffuse across the phospholipid bilayer because they are soluble in lipids (no protein needed for facilitated diffusion).
• Simple diffusion involves limitations imposed by the nature of the phospholipid bilayer.