ENDOSYMBIOSIS

Today, mitochondria and chloroplasts would not be able to survive outside a cell, but according to the endosymbiotic theory, they were once independently living cells! The first cells on earth are thought to have appeared around 3.8 billion years ago, 750 million years after the earth’s formation. These cells were prokaryotes – cells lacking organelles or other internal membrane-bound structures. It isn’t until 2.7 billion years ago that eukaryotes - cells with a nucleus enclosed in membranes - appear in the fossil record.
Eukaryotes evolved from prokaryotes. Here is how that is thought to have happened. A prokaryote grew in size, and as it did, it’s surface area to volume ratio decreased. So, to increase the ratio, the cell developed infoldings in its membrane. Eventually, these infoldings pinched off from the cell membrane to form an early endomembrane system surrounding the nucleoid. This was the first membrane-bound nucleus, and hence this was the first eukaryotic cell.
This eukaryotic cell endophagocytosed an aerobic – or oxygen-using – prokaryotic cell, which may have been prey or a parasite. Genomic sequencing of cells today indicates that this prokaryote was from a group of bacteria called the alphaproteobacteria. In any case, this cell avoids digestion and becomes an endosymbiont – in other words, a cell living within another cell.
Funny enough, this endosymbiont became useful to the eukaryote. The aerobic prokaryote was able to use oxygen to make energy – a process called respiration – which was nice because the earth’s oxygen concentrations were increasing at this time due to the activity of cyanobacteria. The aerobic prokaryote benefited from its host because the cytoplasm was full of half-digested food molecules. Digesting these molecules with oxygen, the prokaryote produced so much energy, that some of the ATP leaked into the cell’s cytoplasm – HURRAY!! So as other eukaryotes went extinct with the rising oxygen levels, the eukaryote and the endosymbiont become best pals – with the endosymbiont becoming a mitochondrion. At this point, the endosymbiont became an obligate endosymbiont, meaning it cannot survive on its own outside the cell.
Some time later, the same process occurred with a cyanobacterium, which becomes the chloroplast. This eukaryote was the ancestor of plants and algae. We know that chloroplasts evolved later because plant cells have both mitochondria and chloroplasts, while animal cells have only mitochondria.
But what’s the proof that endosymbiosis happened? First, mitochondria and chloroplasts replicate on their own through something similar to binary fission – and cells cannot create new ones otherwise.p The genome of these organelles is also remarkably similar to those of prokaryotes – mitochondria, chloroplasts, and bacteria can all have a single circular DNA molecule. In addition, porins and cardiolipin are only found in mitochondria, chloroplasts and bacteria.
You may have one more question… How are mitochondria passed from generation to generation? Well, you get your mitochondria from your mom. So next time someone says your genetic info is 50/50, you can tell them that!

Endosymbiosis progression images redrawn from: https://en.wikipedia.org/wiki/Symbiog...
Eukaryotic cell redrawn from: https://commons.wikimedia.org/wiki/Fi...
Mitochondrion redrawn from: https://commons.wikimedia.org/wiki/Fi...