When a molecule moves across a membrane without using energy, it's called passive transport.

Here's why:

* Passive means it doesn't require the cell to expend energy.

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

There are several types of passive transport, including:

* Simple diffusion: Movement of a substance from an area of high concentration to an area of low concentration.

* Facilitated diffusion: Movement of a substance across a membrane with the help of a transport protein, but still driven by the concentration gradient.

* Osmosis: 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 more detail about any of these types of passive transport!