Facilitated diffusion (also known as facilitated transport or passive-mediated transport) is the process of spontaneous passive transport(as opposed to active transport) of molecules or ions across a biological membrane via specific transmembrane integral proteins
Passive transport is the movement of solute across a membrane down an electrochemical gradient (from the side of the membrane with a high concentration of solute to the side with a low concentration).
Active transport is the movement of solutes across a membrane against an electrochemical gradient (from the side of the membrane with a low concentration of solute to the side with a high concentration), which requires energy. Simple diffusion is the unassisted diffusion of a lipid-soluble solute (shown as blue dots) across a lipid bilayer.
Transport via an ion channel involves a protein-lined pore (shown in purple) spanning the lipid bilayer through which select ions (shown as green dots) are transported. Facilitated diffusion involves transport of a solute (shown as green triangles) across a membrane by a carrier protein (shown in purple). Active transport involves transport of a solute (shown as green squares) against an electrochemical gradient by a pump protein (shown in purple). This process requires energy.
The process is called endocytosis (into the cell) and excocytosis (out of the cell). Large particles cannot diffuse through the plasma membrane, so the cell must engulf or expel them in this manner. In endocytosis, the cell engulfs particles by surrounding them with the plasma membrane to form what is called a vesicle. The vesicle can then be transported throughout the cell, or digested in the vesicle for nutrients.
If a large particle needs to be transported out of the cell, it is packaged by the endoplasmic reticulum, and then "buds off" the golgi apparatus (an internal membrane system), which creates a vesicle that is transported to the plasma membrane. The vesicle fuses with the plasma membrane and the contents are exported out. (exocytosis
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