An ancient prokaryote developed an endomembrane system
by infolding of its plasma membrane.
Infolding also allowed it to engulf smaller cells by endocytosis.
The smaller cell may escape digestion and develop a symbiotic relationship:
- An aerobic endosymbiont may evolve into a
and allow its anaerobic host to use oxygen as an energy source.
- Another endosymbiotic event may allow a photosynthetic
prokaryotes to become a
(a pigment-bearing organelle such as a