Diffusion is the net movement of molecules from a region of high concentration to a region of low concentration

• This directional movement along a gradient is passive and will continue until molecules become evenly dispersed (equilibrium) 
  

The rate of diffusion can be influenced by a number of factors, including:

• Temperature (affects kinetic energy of particles in solution)
  

• Molecular size (larger particles are subjected to greater resistance within a fluid medium)
  

• Steepness of gradient (rate of diffusion will be greater with a higher concentration gradient)
  

Simple Diffusion

B1.1.13
Ability of non-polar steroids to pass through the phospholipid bilayer

Simple diffusion occurs when small and lipophilic molecules pass between phospholipids to freely cross the bilayer

• Small, non-polar gases (such as oxygen and carbon dioxide) will cross the plasma membrane via simple diffusion

• Additionally, lipophilic molecules (such as non-polar steroids like oestradiol and testosterone) can freely cross the bilayer

Large and charged molecules (such as ions and polar molecules) cannot cross the membrane via simple diffusion

Non-Polar Steroids

Oestradiol

Testosterone