Here's why:

* Concentration Gradient: This refers to the difference in concentration of a substance between two areas. Solutes naturally move from areas of high concentration to areas of low concentration.

* Passive Transport: This means that no energy is required for the movement. The concentration gradient itself provides the driving force.

There are several types of passive transport, including:

* Simple Diffusion: Movement directly across a membrane.

* Facilitated Diffusion: Movement aided by membrane proteins.

* Osmosis: Movement of water across a semi-permeable membrane.

In contrast, active transport requires energy to move solutes against their concentration gradient, from low concentration to high concentration.