You probably have heard of mitochondria (the powerhouse of the cell) and of chloroplasts (gives the leaves green pigment, used in photosynthesis). But did you also know that according to the endosymbiosis hypothesis, the mitochondria & chloroplast were once free-living cells. They were “swallowed” into other larger cells and became a subcomponent. Evidence for this includes the fact that both these structures have their own DNA and can self-replicate. 

Mitochondria is the prisoner of the cell