Simplify the following text for a Grade 11 native English-speaking person.
Original: All chloroplasts are thought to derive directly or indirectly from a single endosymbiotic event ( in the Archaeplastida ) , except for Paulinella chromatophora , which has recently acquired a photosynthetic cyanobacterial endosymbiont which is not closely related to chloroplasts of other eukaryotes . In that they derive from an endosymbiotic event , chloroplasts are similar to mitochondria , but chloroplasts are found only in plants and protista .
Simplified: Almost all chloroplasts are thought to derive directly or indirectly from a single endosymbiotic event .