The movement of materials through a cell membrane without using the cell's energy is called passive transport.

Here's why:

* Passive refers to the fact that the cell doesn't expend any energy to move the materials.

* Transport refers to the movement of substances across a membrane.

Passive transport relies on the second law of thermodynamics, which states that systems tend to move towards a state of higher entropy (disorder). This means that substances naturally move from an area of high concentration to an area of low concentration, down their concentration gradient.

There are several types of passive transport:

* Simple diffusion: The movement of a substance across a membrane from an area of high concentration to an area of low concentration.

* Facilitated diffusion: The movement of a substance across a membrane with the help of a transport protein.

* Osmosis: The movement of water across a semipermeable membrane from an area of high water concentration to an area of low water concentration.

Let me know if you'd like to learn more about any of these specific types of passive transport!