While the kingdoms (Monera, Protista, Fungi, Plantae, and Animalia) have varying levels of complexity and organization, they all share the fundamental building block of life: the cell.

Here's why:

* Cell Theory: The cell theory states that all living things are composed of cells, and that all cells come from pre-existing cells. This is a fundamental principle in biology.

* Basic Functions: Cells carry out all the basic functions of life, including metabolism, growth, reproduction, and response to stimuli. Regardless of the kingdom, these functions are carried out within the cell.

* Evolutionary History: All life on Earth is thought to have evolved from a single common ancestor, and the cell is the fundamental unit that has been passed down through the evolutionary process.

While cells can be organized into tissues, organs, and organ systems in more complex organisms, the cell remains the basic unit of life for all kingdoms.