Endosymbiosis. A love story.

Once there was a lonely proto-bacteria who could only respire anaerobically.

And a much smaller proto-bacteria who could respire aerobically and she was the most popular girl in school.

The larger, lonely Prokaryote was intensely jealous of the smaller one so he attempted to ingest her.

While she was in his cytoplasm she said "why are you attempting to digest me, if you are lonely we could just be friends. There is more than enough uneaten food in here to last me forever."

So he agreed and over time she began to produce so much ATP that it leaked out into him and allowed him to respire aerobically too.

And to this day the second bacterias descendants live in your cells.