2.11 Origins of Cell Compartmentalization

The endosymbiotic theory

The endosymbiotic theory explains where mitochondria and chloroplasts came from: they were once free-living prokaryotes engulfed by a larger host cell. Instead of being digested, they lived on inside it, eventually becoming permanent organelles.

The evidence

Mitochondria and chloroplasts have their own circular DNA, their own ribosomes (similar to bacteria), and double membranes, and they reproduce by binary fission — just like bacteria. Meanwhile, the endomembrane system (ER, nuclear envelope) is thought to have formed from infoldings of the plasma membrane.

Key terms

Endosymbiotic theory

Mitochondria and chloroplasts descended from free-living prokaryotes engulfed by a host cell.

Evidence for endosymbiosis

Own circular DNA, own bacteria-like ribosomes, double membranes, binary fission.

Double membrane

Two membranes around mitochondria/chloroplasts, consistent with being engulfed.

Binary fission

The bacteria-like division by which mitochondria and chloroplasts reproduce.

Endomembrane system origin

Thought to arise from infoldings of the plasma membrane (ER, nuclear envelope).

Try an AP-style question

Which observation provides the strongest evidence that mitochondria descended from free-living bacteria?

1. Mitochondria produce ATP for the cell
2. Mitochondria are surrounded by the endoplasmic reticulum
3. Mitochondria are found in nearly all eukaryotic cells
4. Mitochondria have circular DNA and ribosomes similar to those of bacteria