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Ah, this one again. Ok... where are you stuck? Do you know what the word "endosymbiosis" means?
Endo is when one organism lives inside the other and the relationship is '''symbiotic'', because the two organisms live together in a close relationship ?
Yep, so what do you think this is saying about chloroplasts?
I'm sorry, This is were I get stuck, I'm my notes all I have with Chloroplasts is "Chloroplats is an oragnelles found inide the eukarytic cells, it had serveral features in common with whole prokorytic cells" Then it goes into details about how it has it's own DNA, their own Ribosomes, and responsible for their own reproductions... It would it be through reproduction as more than one organism come together?
Do you know what prokaryotes are?
And more importantly, can you think of an example of a prokaryote that can photosynthesise?
Prokaryotes are smaller and simpler than the eukaryotic, they're single-celled organisms and they can be either bacteria or Cyanbacteria which make and use their own organic food from sunlight energy through the process of photosynthesis!
Ok, so we've got cyanobacteria as an example of a prokaryote that can photosynthesise. And, to expand on what you wrote in your notes, chloroplasts look a LOT like cyanobacteria. Do you think this is a coincidence?
Probably not a coincidence
Right. Now look up at what you said about endosymbiosis. What do you think might have happened here?
That chloroplast originated from cyanbacteria giving them similar structure perhaps?
Yep, that's what the theory says :) The idea is that free-living cyanobacteria were somehow taken into the cell, where they lived symbiotically and eventually turned into the chloroplasts we see today. You might also be interested to know that this has happened (at least) twice! In plant cells, you get regular chloroplasts with two membranes (like bacteria have), but in a few types of plant-like micro-organisms like Euglena, the chloroplasts actually have THREE membranes. In primary endosymbiosis, some eukaryotic cell engulfed a cyanobacterium and kept it as a symbiont, and then its descendants became plants. Pretty straight forward. But... at some point, a different eukaryotic cell engulfed one of those simple plants and kept IT as an endosymbiont! That's secondary endosymbiosis. And it means that if you look at a Euglena's chloroplast, you see the two membranes of the original cyanobacterium PLUS the membrane of the simple plant cell that its ancestor engulfed.
Thank you! :) I understand much better now
Wait, slight correction: I just looked at the Wikipedia article and I seem to have got a bit confused about the membranes (though I was close). After secondary endosymbiosis, one of the membranes is indeed from the simple plant cell (alga), but another is from the "phagosomal membrane" (so, a membrane used in phagocytosis, I guess). And I guess it's only the innermost one that's left over from the cyanobacterium. But the important thing is that the number of membranes that you see around a chloroplast can be used as evidence for primary or secondary endosymbiosis.