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When molecules move from an area of high concentration to an area of low concentration, this is termed as simple diffusion. This means that no energy is required to move the molecules through the membrane.

When molecules move from an area of low concentration to an area of high concentration, through a transmembrane this is called facilitated diffusion. Additionally, the molecules are aided to move across the membrane by a protein on the membrane.

  Simple diffusion Facilitated diffusion
Definition The passing through from higher concentration to lower concentration small- hydrophobic, non- polar molecules, without assistance. The process of movement/transport of larger hydrophilic,charged molecules from the inside and outside of the cell thru the aid of transport proteins
How is movement initiated? Thru force Thru membrane proteins
Occurs in Prokaryotes Eukaryotes

The difference therefore is that in simple diffusion, molecules move from high to low concentration gradient, while in facilitated diffusion, molecules move from low to high concentration by an aid of a protein through the simple diffusion, no aid is needed while in facilitated as the name suggests, needs to be aided to move across. This means that simple diffusion is dependent on the difference of saturation or concentration, once the equilibrium is reached then the process cannot continue. Facilitated diffusion is not dependent on the concentration difference thus can allow the difference from one side to the other