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The electron transport chains of both the light reactions of photosynthesis (in plants) and oxidative phosphorylation (in animals) both contain 5 complexes including ATP synthase, as shown below.

Oxidative phosphorylation Image Source

Light Reaction Image Source

Is this coincidence, or does this possibly suggest that both chloroplast and mitochondrion precursors derived their energy-obtaining mechanisms via an ancient evolutionarily-related metabolic pathway which diverged?

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Your latter assumption is the best we have so far, to my understanding.

Here are a few excerpts from "Early Evolution of Photosynthesis" published in Plant Physiology, October 2010 (emphasis mine):

There is suggestive evidence that photosynthetic organisms were present approximately 3.2 to 3.5 billion years ago...

Overwhelming evidence indicates that eukaryotic photosynthesis originated from endosymbiosis of cyanobacterial-like organisms, which ultimately became chloroplasts (Margulis, 1992). So the evolutionary origin of photosynthesis is to be found in the bacterial domain.

And then there's the abstract from this paper: "Do photosynthetic and respiratory electron-transport chains share redox proteins?"

In purple nonsulfur bacteria and cyanobacteria, there is close interaction between the photosynthetic and respiratory electron-transport chains, which share identical redox proteins.

Purple bacteria and cyanobacteria are some of the oldest known lineages (>3BY, whereas Mitochondria parallel Eukaryotic origins at about 1.4BY - and the endosymbiosis which created Chloroplasts probably came after [plants have both Chloroplasts and Mitochondria, whereas most Eukaryotes just have Mitochondria]), and since the respiratory chain and photosynthetic chain share identical proteins in cyanobacteria and purple bacteria it's not a huge leap to assume that - yup - the 5-protein structure is probably the template used by both Chloroplasts and Mitochondria because of a common source.

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The second paper and associated paragraph is EXACTLY what I was looking for. +1, thanks. –  LanceLafontaine Oct 4 '12 at 17:48
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Well, according to the endosymbiosis theory both mitochondria and chloroplasts derive from ancestral bacteria that invaded/were phagocytosed (fancy word for eaten) by eukaryotic cells.

So, the electron transport chains of both mitochondria and chloroplasts would indeed share a common bacterial ancestor.

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