Diffusion 

It is important to understand that:

• All particles are constantly in motion due to kinetic energy that they possess
• Motion is random, no set pattern
• Particles are constantly bouncing of one another and off other objects
• Particles will distribute themselves equally as a result of diffusion
• Only non polar molecules can diffuse straight through the membrane surface such as oxygen

Simple Diffusion:
passive as energy comes rom an inbuilt motion of particles (natural)  

THE NET MOVEMENT OF PARTICLES/IONS FROM A REGION WHERE THEY ARE MORE HIGHLY CONCENTRATED TO ONE WHERE THEIR CONCENTRATION IS LOWER UNTIL EVENLY DISTRIBUTED

Facilitated Diffusion

The movement of molecules such as charged ions and polar molecules do not diffuse easier due to their hydrophobic nature of the fatty acids tails in the phospholipid membrane, so transmembrane channels and carriers span the membrane (facilitated diffusion).
This occurs when a molecule isn't lipid soluble, polar, charged or is too big

Similar to simple diffusion as:

• Passive process relies on inbuilt kinetic energy of diffusing  molecules
• Occurs down a conc gradient

But differs as:

• Only occurs at specific points on the membrane where protein channels and carrier proteins are present

Carrier proteins

When a molecule is specific (complementary) to the molecule present the bind. This causes the protein to change shape in a way that the molecule is released inside the cell membrane. 

This uses the kinetic energy of the particles itself

It occurs down a concentration gradient where the solute concentration inside the cell membrane must be lower than outside for it to diffuse in. 

Channel protein 

These form water filled hydrophobic channels across the cell membrane. These allow water soluble ions to pass through. Channels will only open to specific molecules and so if the specific ion isn't present it will remain closed. Ions binds to proteins causing it to change shape in a way that closes it on one side and opens up one another.