Non-polar molecules e.g. oxygen and carbon dioxide, and very small molecules e.g. water, can pass through the membrane unaided. Water soluble substances generally are unable to pass through by simple diffusion due to the hydrophobic nature of the centre of the phospholipid bilayer.
Diffusion: the net movement of a substance from where it is in a higher concentration to where it is in a lower concentration.
Diffusion across membranes is affected by a number of factors:
- the concentration gradient - the greater the concentration gradient across the membrane, the faster the diffusion
- the size of the molecule - small molecules diffuse faster than larger molecules
- the temperature - normally takes place quicker at higher temperatures, as high temps give the diffusing molecules…