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ATP synthase is used by cells to synthesise ATP from ADP using energy from a proton gradient. Depending on which sites I look at, this is either a universal feature of living cells, or it's "almost universal", used by the vast majority of organisms.

As Superbeast points out in a comment, ATP can also be synthesised on the substrate level via the Krebs cycle. However, I would like to know if there are any organisms that only synthesise ATP this way. In other words, are there any known organisms that entirely lack the ability to produce ATP synthase, and produce ATP entirely through the Krebs cycle, or through some other mechanism not related to ATP synthase?

If there are, I would like some information about them. Are there many such species or just a few? Are they all prokaryotes or all eucaryotes, or some of each? What kind of environments do they live in?

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Right, Krebs cycle, that was kind of obvious I guess. Thanks! But are there organisms that only produce ATP that way? –  Nathaniel May 30 at 7:55

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up vote 3 down vote accepted

As Superbest mentioned, substrate level phosphorylation (SLP) is alternative pathway by which ADP is phosphorylated to ATP. In SLP, phosphorylation is coupled with a highly exergonic chemical reaction. Fermentative organisms (both fully and temporally fermentative) organisms depend on SLP for ATP.

Note: fermentative $\neq$ anaerobic

Fermentation can either be a temporary situation, depending on the growth substrate (for e.g. yeast) or the only option as in the case of amitochondriate organisms like Entamoeba. You can refer this book if it is available to you. See this article too.

You can also see the wikipedia page on fermentation; it mentions the SLP steps.

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If you can add a convincing citation (for the lack of ATP synthase in fermentative organisms) or a link to more information, I will accept this answer. –  Nathaniel May 30 at 14:00
    
i added some info. see if it is fine –  WYSIWYG May 30 at 14:36
    
The linked article appears to be referring to organelles within eucaryotic cells, rather than to the cell as a whole. I'm specifically interested in whether there are any cells that entirely lack ATP synthase, not just in the organelles, but in the outer membrane as well. –  Nathaniel Jun 2 at 2:38
    
Eukaryotic cells don't have ATP synthase on their membrane.. It is always expressed in an organelle. There are reasons to have an organelle. I have given some explanation for this on this post. It is energetically unfavorable for a eukaryotic cell to behave like a huge mitochondria. –  WYSIWYG Jun 2 at 4:15

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