Is there an evolutionary reason for the 5 electron transport complexes in plants and animals?
Your latter assumption is the best we have so far, to my understanding.
Here are a few excerpts from &quot;Early Evolution of Photosynthesis&quot; 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: &quot;Do photosynthetic and respiratory electron-transport chains share redox proteins?&quot;

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 (&gt;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.