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!

[fineline]

Great video, I'm not a biology major and even I understood it. Best regards from Pakistan

[marcusabston]

It's a lot that goes into this especially with the thought of mitochondria or even chloroplasts simply just becoming organelles. This means these prokaryotes died, but somehow the cell kept them in production. It also doesn't explain the purpose of becoming multicellular which is very complex within itself. This entire theory focuses on need but evolution itself is reactionary not done out of need. It would also be impressive for more than one cell to do this exact same thing, and lead to the creation of the eukaryotic group itself

[leia]

thanks this made a lot more sense than googling it

[abdussamad]

Mashallah.great professor of biology.abdussamad from Pakistan

[nicolemandziara]

I'm very confused about this theory. Prokaryotes have cell walls in addition to their cell membranes. They take up nutrients through diffusion even secreting enzymes to break down larger molecules outside their membrane. If they have a rigid cell wall, how can it fold in to create the endomembrane system? Also, how does it use the process of phagocytosis to engulf an aerobic prokaryote if it has a rigid cell wall? If this is how it happened, wouldn't mitochondria be a four membrane bound organelle... the cell membrane and cell wall from the engulfed prokaryote plus the cell wall and cell membrane from the prokaryote that did the ingesting? I understand the evidence that is used to support this theory, I just don't get how this could have happened.

[rodriguezfamily]

thank you for the details love your vids

[ronaksingha]

Can you do a video about membrane proteins??? It would be very helpful

[StaminatorBlader]

what i dont understand is how the organelle reappears in the cells offspring...how would it become part of the cells genome? its not like they just passed on the mitochondria the way we do microorganisms that live exclusively inside us. how does the one cell that incorporated the other then reproduce with that second cell already there at birth? i understand cells dont reproduce sexually but im not a biologist so i dont have the right words lol. at what point is it no longer a symbiont jumping ovee from parent to offspring and becomes part of the same organism and how is that even possible? like the microbes in me that my mother passed on to me arent part of me theyre a seperate thing. i would really appreciate an explanation.

[Thee-_-Outlier]

Didn't they prove they can live separately for a short bit, a week I believe? It was a specific symbiote they used I forget the name but it's one that doesn't consume new chloroplasts instead the same chloroplasts produce each new generation inside the symbiote replacing themselves. This is apparently unusual for a chloroplast symbiote. Anyways, studying that specific symbiote it lived separate from its chloroplasts for a week and didn't produce new ones or anything, inferring these are separate organisms but at this point only live apart for short periods, or perhaps evolved even further to not reproduce separately idk, I'm guessing now

[Stephknight]

What is the hypothesized steps for the evolution of Eukaryotic cells through endosymbios?