Here's why:
* Passive transport relies on the second law of thermodynamics, moving molecules from an area of high concentration to an area of low concentration. This movement is driven by the concentration gradient itself, so the cell doesn't need to expend energy.
There are different types of passive transport:
* Simple diffusion: Movement of molecules across a membrane directly from high to low concentration.
* Facilitated diffusion: Movement of molecules across a membrane with the help of membrane proteins, still driven by the concentration gradient.
* Osmosis: Movement of water molecules across a selectively permeable membrane from an area of high water concentration to an area of low water concentration.
In contrast to passive transport:
* Active transport requires the cell to expend energy (usually in the form of ATP) to move molecules against their concentration gradient (from low to high concentration).
Let me know if you'd like more detail on any of these concepts!
