What is the smallest unit of biological organization that is alive

The cell! We're all made of them, but you can't see them.

The Cell, it is the smallest structural and functional unit of living things that can exist on it's own. This is why it is often called the building block(s) of life. There are several organisms, such as bacteria or yeast, which are unicellular, which means they only contain one cell, while most things, such as mammalians, are multicellular, meaning they are made up of more than one cell.