Here's why:
* Passive diffusion relies on the concentration gradient of the substance. This means the substance moves from an area of high concentration to an area of low concentration. Think of it like a drop of food coloring spreading out in a glass of water.
* No energy input is required for this movement. The natural tendency of molecules to spread out and achieve equilibrium drives the process.
Types of passive diffusion:
* Simple diffusion: Movement across a membrane directly through the lipid bilayer, without the help of any transport proteins.
* Facilitated diffusion: Movement across a membrane with the assistance of transport proteins, but still driven by the concentration gradient.
In contrast, active diffusion requires energy. This is because it involves moving substances against their concentration gradient, from an area of low concentration to an area of high concentration. This requires energy input, usually in the form of ATP.
