Endosymbiotic theory is supported by mitochondria and chloroplasts having several prokaryote-like features, including circular DNA, 70S ribosomes and independent division. These features suggest that both organelles evolved from free-living prokaryotes engulfed by an ancestral eukaryotic cell.
The Reasoning
This is Additional Higher Level (AHL) content in IB Biology subtopic A2.2, Cell structure. The theory proposes that an ancestral eukaryotic cell engulfed an aerobic prokaryote without digesting it. The two cells formed a mutually beneficial relationship: the internal cell supplied ATP, while the host provided protection and nutrients. Over time, the internal cell evolved into a mitochondrion.
In some eukaryotic lineages, a photosynthetic prokaryote was engulfed later and evolved into a chloroplast. This explains why mitochondria occur across eukaryotes, whereas chloroplasts occur only in photosynthetic lineages.
| Evidence | How it supports endosymbiosis |
|---|---|
| Circular, naked DNA | Resembles the DNA of prokaryotes rather than the linear chromosomes in a eukaryotic nucleus. |
| 70S ribosomes | These resemble prokaryotic ribosomes; the eukaryotic cytoplasm contains 80S ribosomes. |
| Division resembling binary fission | Mitochondria and chloroplasts arise by division of pre-existing organelles rather than being constructed from scratch by the cell. |
| Double membranes | The inner membrane is consistent with the original prokaryotic membrane, while the outer membrane is consistent with engulfment by the host. |
| Similar size to prokaryotes | Their small dimensions are consistent with an origin as bacterial cells. |
A common misconception is that mitochondria and chloroplasts are still independent organisms. They are now organelles: many of their ancestral genes have been lost or transferred to the nucleus, so they depend on the host cell.
Exam Technique
For an explain question, do not merely list features. Link each observation to its inference: for example, “mitochondria contain 70S ribosomes, which resemble prokaryotic ribosomes, supporting their origin from engulfed prokaryotes.” Clearly label this as AHL content.