Evolution of photosynthetic and respiratory prokaryotes and organelles.

Abstract

Common ancestors for mitochondria, chloroplasts, and photosynthetic bacteria (including cyanobacteria) probably existed more than three billion years ago. One ancestral prokaryote may have contained P(chl) Chloroplast a in photochemical reaction centers that drove cyclic electron flow and phosphorylation through membrane-bound components including cytochromes and quinones. Substitution of Chl a for Pchl a and the development of linear electron-transport chains permitted the reduction of NAD/sup +/ and/or NADP/sup +/ for carbon-dioxide fixation. Evolution of photosystem II from photosystem I enabled one prokaryote to evolve oxygen as a byproduct of carbon-dioxide fixation. This organism was the common ancestor of cyanobacteria, Prochloron, and various chloroplasts. A photosynthetic bacterium containing Bchl a appears to have branched off from the Chl a line. This bacterium was the common ancestor of extant respiring bacteria, mitochondria, and purple and green photosynthetic bacteria.