Simple Diffusion

SIMPLE DIFFUSION
Molecules diffuse across the cell membrane

Rate of diffusion is determined by:

• Size: smaller molecules diffuse across faster than large ones
• Polarity: the less polar, the faster it diffuses across the membrane
• Charged molecules: do not diffuse across hydrophobic interior

Types of molecules

• Nonpolar molecules: hydrophobic, diffuse rapidly
  i.e. oxygen, carbon dioxide, nitrogen and steroid hormones
• Small, uncharged polar molecules: diffuse across
  i.e. water, glycerol and ethanol
• Large, uncharged polar molecules: do NOT diffuse across
  i.e. amino acids, glucose and nucleosides
• ions (charged molecules): cannot diffuse across
  ie. Hydrogen, sodium, potassium, calcium and chloride ions