Overview: The cell is the basic unit of life, and our understanding of it has advanced as science, and the tools available to scientists, has advanced. In this episode of Crash Course Biology, we’ll take a look at the difference between prokaryotic and eukaryotic cells, take a guided tour of the eukaryotic cell, and learn why most cells are small. We’ll explore the eukaryotic cell’s surprising beginnings through an endosymbiosis that occurred about 1.5 billion years ago.