Animation showing the origin of cell organelles according to endosymbiotic theory. Initially, an early cell ingests a free-living aerobic bacterium (red) but does not digest it. The bacterium thrives in the cell, digesting food molecules and using the energy to create ATP. Some of this was released into the host cell, which provided it with energy. Over time, the cells became dependent on each other, and the bacteria evolved into what is now known as a mitochondrion (seen in inset). A similar process occurred with a cyanobacterium (green), which photosynthesised within the cell and eventually evolved into a chloroplast. Evidence for this theory includes the fact that both mitochondria and chloroplasts contain their own DNA. It is thought that mitochondria evolved before chloroplasts, as all eukaryotic cells contain mitochondria, but not all contain chloroplasts.
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