Diffusion, Osmosis and Active Transport

Substances can enter or leave the plant cell through two known processes these are active and passive transport. Active processes involve the use of energy while passive processes do not involve the use of energy but depends mainly on the permeability of the membrane. Active processes deal with the transfer of molecules and particles whereas passive processes involve diffusion, facilitated diffusion and osmosis.

What is Diffusion?
Diffusion can be defined as the movement of particles from an area where they are at a high concentration to areas where they are at a lower concentration. This will continue until the particles' concentration is uniformed throughout.

What is facilitated Diffusion?
This involves the movement of specific molecules down a concentration gradient (difference in concentration). This is done using a carrier protein, which binds to the molecule allowing it to pass through the membrane. Examples of these include amino acids and glucose.

What is Osmosis?
This is the movement of water molecules across a partially permeable membrane from an area of high water potential to an area of lower water potential. Having a partially permeable membrane means it only allows for some molecules to pass through but not all usually it does not allow for larger solute molecules to pass through.

Active Transport
This is the movement of molecules against a concentration gradient (from lower to higher concentration) with the use of energy. The energy supplied comes from ATP of the mitochondria.

The table below shows examples where the processes mentioned above are operative:

Process	Cases where process is taking place
Diffusion	Gas exchanges in photosynthesis: · More carbon dioxide is outside the leaf than inside so carbon dioxide will diffuse into the leaf while the opposite happens for oxygen.
Facilitated Diffusion	Glucose is too large to pass through pores of the membrane therefore it has to bind to a specific carrier protein in order to pass through.
Osmosis	In the roots of plants water is absorbed from the soil.
Active Transport	Re-absorption of salts in the proximal convoluted tubule.