CONCEPT
Chloroplast
The photosynthetic organelle in plant cells — a second endosymbiotic merger, in which a eukaryote engulfed a cyanobacterium and integrated it, producing the entire plant kingdom.
The chloroplast is the membrane-bound organelle in plant cells and algae that performs photosynthesis, converting light energy into chemical energy stored in glucose. Like mitochondria, chloroplasts originated through endosymbiosis: a mitochondria-bearing eukaryote engulfed a photosynthetic cyanobacterium approximately 1.5 billion years ago. The bacterium survived, integrated, and became a permanent resident. Chloroplasts retain their own circular DNA, their own ribosomes, and a double membrane reflecting the original engulfment. They replicate independently but are coordinated with the host cell's division. The chloroplast endosymbiosis was even more transformative than the mitochondrial one: it gave eukaryotes access to solar energy, enabling them to colonize terrestrial environments and produce the oxygen-rich atmosphere that complex animal life requires. The chloroplast is Margulis's second canonical case of symbiogenesis — proof that radical merger is not a one-time fluke but a repeatable mechanism driving major evolutionary transitions.
In The You On AI Field Guide
The chloroplast endosymbiosis is more complex than the mitochondrial one because it occurred in multiple independent lineages. The primary endosymbiosis — a eukaryote