History of Endosymbiosis

As we have already seen, many protists possess chloroplasts. Groups with chloroplasts appear in several distantly related protist clades. Some of these groups differ in the photosyn-thetic pigments their chloroplasts contain. And we've seen that not all chloroplasts have a pair of surrounding mem-branes—in some protists, they are surrounded by three membranes. We now understand these observations in terms of a remarkable series of endosymbioses.

All chloroplasts trace their ancestry back to the engulf-ment of a cyanobacterium by a larger eukaryotic cell (Figure 28.29). This event is known as primary endosymbiosis. The cyanobacterium, a Gram-negative bacterium, had both an inner and an outer membrane. The eukaryote's plasma membrane wrapped around the cyanobacterium as it took it up. The outer membrane and cell wall of the cyanobacterium were eventually lost. Thus, the original chloroplasts had two surrounding membranes—one from the cyanobacterium and one from the eukaryotic host cell.

28.29 A Chloroplast Family Tree One or two primary endosymbioses followed by several secondary and tertiary endosymbioses gave rise to all of today's chloroplasts.

Photosynthetic stramenopiles

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