There are several types of passive transport:
* Simple diffusion: Movement of a substance from an area of high concentration to an area of low concentration, following the concentration gradient. This doesn't require any membrane protein assistance.
* Facilitated diffusion: Movement of a substance across a membrane with the help of a membrane protein. This is still passive because it doesn't require the cell to expend energy, but it does require the presence of a specific transporter protein.
* Osmosis: The movement of water across a semipermeable membrane from an area of high water concentration to an area of low water concentration. This is driven by the difference in water potential across the membrane.
All of these processes are driven by the second law of thermodynamics, which states that systems tend towards increasing entropy (disorder). In the case of passive transport, the movement of molecules from high to low concentration increases the overall entropy of the system.
