Just by looking at a picture of each cell type, it is easy to see that eukaryotic cells are much more complex than prokaryotic cells. How did they get that way? The theory of cellular evolution increasing cell complexity is called the Endosymbiosis Theory. The prefix “endo” means inside and the root word “symbiosis” means living together. In this case, one organism is literally living inside of another. Let’s explore a little more about what that means.

If you look at a eukaryotic cell, you may notice that there are a few membrane-bound organelles that are highly specialized. The mitochondria perform respiration and convert chemical energy into kinetic energy. In plant cells, the chloroplasts are highly specialized to perform photosynthesis, converting radiant energy into chemical energy. The Endosymbiosis Theory proposes that these energy factory organelles may have once been prokaryotic organisms that lived on their own. Photosynthetic bacteria and oxygen-breathing bacteria could have been the ancestors to chloroplasts and mitochondria organelles.

These ancient prokaryotes were vulnerable and could not defend themselves against predators. The larger prokaryotic cells did not have a way to convert energy, making them inefficient. Similar to mutualism, the two cell types joined together benefitting from the relationship. The small prokaryotes were now protected and the larger prokaryotes had energy factories living inside of them. Eventually, as the now eukaryotic cells reproduced, the prokaryotes were replicated within them as well. It is thought that these small, ancient prokaryotes eventually became the mitochondria and chloroplasts studied today.