How does diffusion differ from endocytosis and exocytosis?
Diffusion is passive, uncontrolled by the cell for nonpolar molecules. While endo/exocytosis is selective and energy dependent.
The cellular membrane of the cell is composed of nonpolar phospholipids that form a bilayer. Because of that, it blocks polar molecules from passing through but allows water and nonpolar molecules to pass.
Diffusion is the movement of molecules down a concentration gradient. It occurs passively without any control from the cell itself.
Endo/Exocytosis is the absorption or secretion of polar molecules through the cell membrane. It is an energy consuming process where the cell has complete control over it.
In case of exicytosis, the cell will direct a secretion vesicle that contains polar substances into the cell membrane. These vesicles are composed of the same phospholipids as the cell membrane. Making possible to fuse those vesicles with the membrane and expelling their content into the extracellular space.
In case of endocytosis, the opposite occurs. Part of the cell membrane engulfs the demanded substances and drag it inside as a new vesicle.