Here's why:

* Single-celled: A plant cell is a single, independent unit of life. It contains all the necessary structures (like a nucleus, chloroplasts, and cell wall) to carry out the basic functions of life.

* Multicellularity: Multicellular organisms are made up of many cells working together in a coordinated way. These cells often specialize in different functions, forming tissues and organs.

* Organisms: Organisms are complete, living beings capable of independent existence. A single plant cell is not a complete organism; it is a component of a larger plant.

Think of it this way: A plant cell is like a single brick in a building. The brick is an essential part of the structure, but it cannot stand alone as a complete building.