bio

Plastid

The double-membraned organelle where plants photosynthesise — the chloroplast is its most familiar form. It was once a separate organism. Between roughly 1.6 and 1.9 billion years ago a heterotrophic eukaryotic cell engulfed a cyanobacterium and did not digest it; most of that bacterium's genes migrated to the host nucleus and the remainder shrank into an organelle. The evidence is still there: two membranes, a genome of its own, and in the glaucophyte algae even a surviving bacterial peptidoglycan wall between the membranes.