Here's why:

* Cell walls are tough, rigid structures that surround plant cells. They are primarily made of cellulose, a complex carbohydrate.

* Cell walls provide structural support and protection for the cell, helping to maintain its shape and prevent it from bursting due to internal pressure.

* Cell walls also play a role in regulating the movement of water and other substances into and out of the cell.

While other components like the vacuole and cytoskeleton contribute to cell shape and function, the cell wall is the primary source of rigidity in plant cells.