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Index >> Bacterial Photosynthesis >> Chloroplasts

Chloroplasts

	Chloroplasts - On the basis of resemblances between chloroplasts and prokaryotic cells a hypothesis has been proposed that the ancestors of chloroplasts might have been photosynthetic prokaryotes which subsequently established an endosymbiotic relationship with primitive eukaryote cells. The resemblances between chloroplasts and prokaryotic, cells include structural similarities, the presence of DNA in both, and the presence of similar ribosomes and other components of the proteins synthesis machinery. On becoming organelles of eukaryote cells the chloroplasts lost their genetic independence and came under the partial' influence, of host cell genes
The discovery of the phototrophic eukaryote Cyanophora paradoxa (a flagellate) provides evidence for the endosymbiotic origin of chloroplasts. This organism contains two to four 'cyanelles' which have been interpreted as endosymbiotic Cyanobacteria. The cyanelles contain pigments typical of the Cyanobacteria. They also contain DNA which is 5-10% less than the smallest genome size of the Cyanobacteria. The envelopes of the cyanelles contain muramic acid and diaminopimelic acid, which are characteristic of the peptidoglycan layer of the cell wall of prokaryotes. The cyanelles is therefore a chloroplast which has retained the peptidoglycan wall layer of its prokaryote ancestors.
The similarity between the light harvesting pigment systems of the Rhodopyta (red algae) and the Cyanobacteria suggests that the rhodophytan chloroplast may have evolved from the Cyanobacteria. The chloroplasts of the other algal groups, however, show considerable differences in their light harvesting pigments from those of the Cyanobacteria.
This leads to two possibilities of chloroplast evolution on the basis of the endosymbiotic origin of chloroplasts., One possibility is that the other types of chloroplasts evolved from rhodophytan-type chloroplasts during the early stages in the evolution of photosynthetic eukaryotes. The other possibility is that chloroplasts had a polyphyletic origin, and evolved independently from prokaryotes carrying out oxygenic photosynthesis. The discovery of a connecting link of evolutionary importance favours the second possibility. This connecting link is a unicellular marine organism which develops as an ectosymbiont of the colonial ascidian Didemnum.
	The organism is structurally a prokaryote with a cell envelope like that of a gram negative cell wall. Its light harvesting pigment system is of the chlorophytan type. Both chlorophylls a and b are present in a ratio of about 4.5:1. Phycobiliproteins, which are characteristics of the Cyanobacteria, are absent. The organism was first assigned to the cyanobacterial genus Synechocystis (Lewin, 1975), but was subsequently placed in the Prochloraphyta (Lewin, 1976). The discovery of this prokaryote with a chlorophytan light harvesting pigment system strongly supports the idea of polyphyletic origin of chloroplasts. 'The different types of algal 'pigment systems are apparently' consequences of their independent origin from different photosynthetic prokaryotes. Mitochondria. John and Whatley have suggested that Paracoccus shows most similarities with the probable ancestor of mitochondria

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