The smallest functional and structural unit of all living organisms is the cell.

Here's why:

* Functional: Cells carry out all the essential life processes, such as metabolism, growth, reproduction, and response to stimuli.

* Structural: Cells are the building blocks of all living things. From single-celled organisms like bacteria to complex multicellular organisms like humans, every living thing is made up of cells.

Let me know if you'd like to learn more about cells!