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Rickettsiales are widely regarded as the ancestor of eukaryotic mitochondria through endosymbiosis. Cyanobacteria are widely regarded as the ancestor of eukaryotic chloroplasts through endosymbiosis.

These two organelles bear striking similarities in terms of structure and mechanics. There seems to be plenty of information regarding speculation on the endosymbiosis that led from these free-living organims to the organelles. However I can't seem to find much on speculated relationships between these organisms in terms of common structures.

My Quesions : Are the electron transport systems and chemiosmosis simply ubiquitous phenomena in prokaryotes, or are these similarities evidence of highly conserved common ancestry? Or is there another answer? Is there any information regarding the phylogenetics of Rickettsiales and Cyanobacteria. Do these two organisms belong to a single small clade or were these biochemistries (ETCs, ATP Synthases, etc) ubiquitous at some point and have simply been preserved in these two organisms today?

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photosynthesis and respiration both use electron transport to generate ATP. However, in photosynthesis the proton is produced by photolysis of water whereas in respiration it is generated by oxidation of organic molecules, The ATP synthase is essentially the same. Also, the electron carriers like quinones and cytochromes are very similar. The different proton source in these two reactions must be a resultant of habitat among other factors. Assuming that you already know these details, I am unsure of what exactly you are asking. –  WYSIWYG Nov 4 '13 at 19:02
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Rickettsia and Cyanobacteria are not part of a small clade that share these features. ATP synthases are present across all domains of life, and they are all homologous, i.e. derived from a common ancestor. The means its fairly likely that the last universal common ancestor had some form of ATP synthase.

ETCs and ATP synthases are part of the chemiosmotic theory put forward by Mitchell. Whats beautiful about this mechanism is that you just need a single ATP synthase to make your energy, and it can be used regardless of your metabolism, be it a Cyano or a Rickettsia. All you need to do is somehow couple your metabolism to a proton motive force across your membrane, and then you just let the protons fall back down their electrochemical gradient, powering the formation of ATP.

Organisms from all corners of the tree of life use this strategy, so this is not some special common feature linking the organisms that ultimately became eukaryotic organelles. Its worth noting that there are alternatives to ETCs and proton motive force-based ATPases, you can make ATP by substrate-level phosphorylation like in fermentation metabolisms.

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