Mitochondria and chloroplasts originated from free-living prokaryotic cells that were engulfed by an ancestral eukaryotic cell but were not digested. A permanent, mutually beneficial relationship developed, and the engulfed cells eventually became organelles.
The Endosymbiotic Mechanism
The endosymbiotic theory proposes these stages:
- An ancestral eukaryotic cell engulfed an aerobic prokaryote by endocytosis.
- The prokaryote remained inside the host and carried out aerobic respiration, supplying ATP.
- The host supplied protection and organic compounds to the prokaryote. Over evolutionary time, it became a mitochondrion.
- Later, an ancestral photosynthetic eukaryote containing mitochondria engulfed a photosynthetic prokaryote.
- This prokaryote supplied carbohydrates produced by photosynthesis and eventually became a chloroplast.
Therefore, mitochondria evolved before chloroplasts. All major eukaryotic groups have mitochondria or evidence of mitochondria, whereas chloroplasts occur only in photosynthetic eukaryotic lineages.
Evidence for Endosymbiosis
| Evidence | Link to prokaryotic ancestry |
|---|---|
| Circular, naked DNA | Resembles prokaryotic chromosomes rather than linear nuclear chromosomes associated with histones |
| 70S ribosomes | Match the ribosome size found in prokaryotes rather than the 80S ribosomes of the eukaryotic cytoplasm |
| Division by binary fission | Mitochondria and chloroplasts replicate in a similar way to prokaryotic cells |
| Double membranes | The inner membrane is associated with the engulfed prokaryote, while the outer membrane is consistent with engulfment by the host |
| Similar size to prokaryotes | Supports their origin as formerly independent cells |
A common misconception is that mitochondria and chloroplasts developed because ancestral cells “needed” them. Evolution is not goal-directed: variations that made the partnership advantageous were retained by natural selection.
Exam Technique
This is AHL content in A2.2 Cell structure. For an “explain” question, give the sequence of engulfment, mutual benefit and integration, then support the theory with several specific pieces of evidence. Do not merely state that the organelles “were once bacteria.”