The presence of a separate genome, along with similarities between the structures of the chloroplast and photosynthetic cyanobacteria, prompted scientists to propose that chloroplasts originated as the result of an early eukaryotic cell engulfing a prokaryotic cyanobacterium. This proposal has recently received nearly unequivocal support in view of the remarkable similarities in sequences in genes that occur within chloroplasts and cyanobac-teria. The evidence suggests that this event, called endosymbiosis, happened once, or a few times, about one billion years ago and that chloroplasts in all photosynthetic eukaryotic organisms are descendants of this event. Shortly after the cyanobacterium became resident within the host cell, much of the genetic information in the bacterium was transferred to the nucleus of the host. Following this endosymbiosis event, as photosynthetic organisms evolved, their chloroplasts diverged as well.
The divergent evolutionary heritages of chloroplasts in various organisms has led to a collection of unique properties. Most of the variety occurs among the algae. Light-harvesting complexes in green algae (Chloro-phyceae) contain chlorophylls a and b bound to proteins within the membrane that are very similar to higher plants. The red algae (Rhodophyceae) are similar to green algae except that they contain phycobilisomes as major light-harvesting complexes attached to the surface of thylakoid membranes and do not contain chlorophyll b, which is similar to cyanobacteria. Brown algae (Phaeophyceae), yellow-green algae (Chrysophyceae), diatoms (Bacil-lariophyceae), and dinoflagellates (Dinophyceae) contain proteins similar to those in light-harvesting complexes in green algae; they differ in that they contain chlorophyll c instead of chlorophyll b. These latter algal families contain an additional pair of membranes surrounding the chloroplast. These extra membranes are thought to have originated when a second eukaryotic cell engulfed an entire chloroplast-containing eukaryotic alga. SEE ALSO Cells; Chlorophyll; Endosymbiosis; Photosynthesis, Carbon Fixation and; Photosynthesis, Light Reactions and; Plastids.
J. Kenneth Hoober
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