The main difference between simple and facilitated diffusion lies in how substances move across cell membranes.
Simple diffusion is the process where molecules move from an area of high concentration to an area of low concentration without the assistance of any proteins. This type of diffusion relies solely on the kinetic energy of the molecules, which allows them to pass through the lipid bilayer of the cell membrane freely.
On the other hand, facilitated diffusion also moves substances from high to low concentration, but it requires the help of specific transmembrane proteins to assist the movement of larger or polar molecules that cannot easily cross the lipid bilayer. This method does not require energy, as it still follows the concentration gradient, allowing molecules to pass through channels or carrier proteins.
In summary, while both types of diffusion involve movement along the concentration gradient, simple diffusion occurs directly through the membrane, whereas facilitated diffusion needs specific proteins to assist in the process. There is no energy expenditure required for either type of diffusion.