Practice question

Primary endosymbiosis describes evolutionary event where a prokaryotic cell is internalized by a eukaryote-like host via phagocytosis and retained stably as organelle after gene transfer and metabolic integration. Mitochondrial origin from alphaproteobacterium represents first primary event, followed by plastid origin from cyanobacterium in ancestor of Archaeplastida. Mechanistically engulfment results in two surrounding membranes: inner from symbiont and outer from host vacuole. This definition contrasts secondary events involving eukaryote-eukaryote uptake, emphasizing direct prokaryote to eukaryote transition establishing fundamental eukaryotic organelles with semi-autonomous genomes.