Did you know that two of the organelles could have been bacteria in the past before turning into organelles?
The endosymbiotic theory suggests that mitochondria and chloroplasts, key organelles in eukaryotic cells, were once free-living bacteria that formed a mutualistic relationship with a host cell billions of years ago. Over time, these bacteria became an integral part of the cell, evolving into the mitochondria and chloroplasts we know today.
I am guessing that we all know there are 2 types of cells.
Prokaryotes
Eukaryotes
Billions of years ago, there were only prokaryotes. They don't have a nucleus or membrane-bound organelles, and their DNA swims in the cytoplasm. Let's say that there were three types of eukaryotes. One could produce its food with light energy, another could use oxygen to produce ATP on its own, and the last could consume others, so they were bigger than the others.
The Endosymbiotic Theory suggests that one of these larger cells somehow engulfed the other smaller cell that could produce ATP. And this cell survived instead of being digested in the larger cell. And they lived together. This cell is believed to be the ancestor of a heterotrophic eukaryote cell. After that, some of the same large cells that had engulfed one bacteria decided to engulf the one that could photosynthesize, and they also didn't get digested. So this cell is believed to be the ancestor of an autotrophic cell.
Let's look at the reasons why are we assuming that this may happened.
1) Unlike the rest, these two organelles have double-bounded membranes. The two membranes would be the vesicle from the endocytosis and the cell membrane of the engulfed cell.
2) Mitochondrial and chloroplast contain plasmid-like, circular DNA that has genes independent from those found in the eukaryotic nucleus. These are thought to be conserved genes from the original engulfed cell.
3) These organelles have 70s ribosomes.
These are the reasons why Endosymbiotic Theory came up.
How do you feel about some random bacteria turning into an organelle and now living in millions of your cells? :)