If you could peer deep into one of the many cells in your body, you’d see little blobs, squiggles, and coils. These are the cell’s organelles, structures that perform specialized functions in cells the same way that the lungs, heart, and other organs do in a body.
Mitochondria are the energy factories found in each cell of fungi, protozoa, insects, and animals. Once nutrients are absorbed or digested, they move in the form of minuscule molecules into the mitochondria, which convert the molecules into chemical energy to power the cell.
Chloroplasts undertake a similar function in the cells of plants, algae, and some protozoa. They capture sunlight and, through a series of chemical reactions called photosynthesis, use the light to make energy.
These organelles are absolutely essential to the existence of all higher life forms on Earth. If all of the mitochondria in our bodies were to suddenly shut down, we would die. The same is true for plants were they to lose their chloroplasts.
As microscopes improved over the years, scientists began noticing striking similarities in the appearances of mitochondria, chloroplasts, and bacterial cells. They discovered that these two organelles contain their own DNA, or gene set, organized very much like the DNA in bacterial cells.
Mitochondria and chloroplasts also reproduce independently from the cells in which they reside, in a manner very like bacterial fission.
Many microbiologists think it is likely that mitochondria and chloroplasts were once free-living prokaryotes (cells that lack a nucleus and organelles) that somehow took up residence in larger cells.
An Invading bacterium may have infected a cell, then become a permanent resident as it adapted to become less virulent, or the target cell became less susceptible. Or both.
Chloroplasts likely first entered a host cell as food before establishing a successful merger with the cell so they were not digested.