Practice question

Chloroplasts trace their ancestry to primary endosymbiosis where a heterotrophic protist containing mitochondria captured a photosynthetic cyanobacterium related to nitrogen-fixing filamentous types, approximately 1.6 billion years ago. Cyanobiont retained photosynthetic apparatus including thylakoids containing chlorophyll a and phycobilisomes, Calvin cycle enzymes, and circular genome. Host integrated symbiont through gene transfer and protein targeting via TOC TIC translocons. Molecular phylogenies of 16S rRNA and photosystem proteins cluster plastid genes with cyanobacteria, not proteobacteria, confirming cyanobacterial origin of Archaeplastida plastids leading to red algae, glaucophytes, and green plants.