A number of distantly related groups of protists, as well as all plants, contain chloroplasts. The chloroplasts come in a number of different types; they may be surrounded by two, three, or four membranes, and they may contain different types of photosynthetic pigments.

Despite these differences, all chloroplasts trace back to the engulfment of an ancestral cyanobacterium by a larger eukaryotic cell—an event called primary endosymbiosis. In endosymbiosis, one organism lives inside another organism.

In the accompanying animation, we focus on this event of primary endosymbiosis and the subsequent evolution of chloroplasts in plants and several types of protists.


In the accompanying animation, we examined the evolution of chloroplasts in a few groups of organisms.

Red algae, green algae, and land plants all have in common a similar type of chloroplast with two surrounding membranes. The two membranes derive from the inner and outer membranes of an ancestral cyanobacterium that was engulfed by an ancestral eukaryote. This primary endosymbiotic event—the formation of a photosynthetic eukaryote—is the origin of all chloroplasts.

The euglenids acquired their chloroplasts from a secondary endosymbiotic event. During euglenid evolution, an ancestral euglenid engulfed a photosynthetic eukaryote, kept the chloroplast, but lost the other elements of the ancient cell. From this history of events, the euglenid chloroplast has a third membrane.

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Textbook Reference: Concept 20.2 Major Lineages of Eukaryotes Diversified in the Precambrian