Here's why:
* Diffusion: The movement of particles from an area of high concentration to an area of low concentration, driven by the random motion of particles. It requires no external energy.
* Active transport: The movement of particles against their concentration gradient (from low concentration to high concentration). This process requires energy, typically supplied by ATP.
Other processes that could be considered "opposite" of diffusion, depending on the context:
* Osmosis: The movement of water across a semipermeable membrane from a region of high water concentration to a region of low water concentration. While it's technically driven by a concentration gradient, it's often considered distinct from diffusion because it involves water movement.
* Filtration: The movement of fluids and small solutes through a membrane, driven by pressure differences. Unlike diffusion, filtration isn't driven by concentration gradients but by physical pressure.
Ultimately, the best "opposite" of diffusion depends on the specific context you're considering.