36 



theory of the origin of these cells (the serial endosymbiotic theory) 

 postulates that they arose by symbiotic association between some 

 unknown pre-eukaryotic cells (urkaryote) and certain types of 

 prokaryotes, then free-living (fig. III-5). These symbioses gave rise to 

 structures recognizable today as intracellular organelles, introduced 

 in one step, not incrementally evolved. The organelles of eukaryotes, 

 thought to have originated from living bacteria, are mitochondria, 

 which react with chemical substrates and atmospheric 2 to produce 

 chemical energy within all types of eukaryotic cells, and the plastids, 

 subcellular sites of photosynthesis which convert radiant energy from 

 light to chemical energy in plants and algae. All these quite com- 

 plex organelles have their own nucleic acid genetic material, and they 

 produce their own distinct proteins. The sequence of some protein 

 molecules in organelles has been compared to that of similar proteins 

 in extant prokaryotes. The amino-acid sequences of certain proteins 

 in the plastids which have been examined indicate a close relationship 

 to certain prokaryotes, particularly to the photosynthetic oxygen- 

 producing cyanobacteria and prochlorons. The relationship of the 



URKARYOTE 



PRESENT DAY 

 PLANT CELL 



Figure III-5.- A schematic illustration of the endosymbiotic theory of the origin 

 of plant cells. Bacterial entities from the cyanobacteria (blue-green algae) group 

 and the purple bacteria group were engulfed by an urkaryote and formed today 's 

 chloroplasts and mitochondria, respectively. Animal cell mitochondria probably 

 have a similar origin, although their ancestry has not been clearly established. 



