Endosymbiotic Theory Visualizer
Step through an animated history of a eukaryotic cell engulfing aerobic and photosynthetic bacteria to form mitochondria and chloroplasts, respectively.
THE ORIGIN OF EUKARYOTES
The **Endosymbiotic Theory** proposes that eukaryotic cells evolved through a symbiotic relationship between different types of prokaryotes. Specifically, a large anaerobic host cell engulfed a smaller aerobic bacterium (the ancestor of the **mitochondria**) and later, a photosynthetic bacterium (the ancestor of the **chloroplast**).
EVIDENCE FOR ENDOSYMBIOSIS
Several lines of evidence support this theory, as mitochondria and chloroplasts share remarkable similarities with modern bacteria: 1. **Double Membranes**: Both have an inner membrane (from the original bacterium) and an outer membrane (from the host's vesicle). 2. **Own DNA**: Both possess their own circular DNA, similar to bacterial plasmids. 3. **Independent Division**: Both reproduce via a process resembling binary fission, separate from the cell's mitosis. 4. **Bacterial Ribosomes**: Both have 70S ribosomes, the same size found in prokaryotes.
HOW TO USE THIS VISUALIZATION
1. **Engulfment Phase**: Watch the ancestral host cell surround and take in the aerobic bacterium. 2. **Mutualism Phase**: See how the host provides protection and nutrients while the bacterium provides ATP energy. 3. **Analyze the Organelle**: Zoom in on a modern mitochondrion to see the bacterial 'leftovers' like circular DNA. **Try This**: Look at the DNA of the mitochondrion. Is it linear like the cell's nuclear DNA, or circular? What does this tell you about its evolutionary origin?
AP EXAM CONNECTION
Unit: Unit 2: Cell Structure and Function (Topic 2.11)
Learning Objective: EVO-1.B
COMMON MISCONCEPTIONS
- Thinking the host cell 'ate' the bacteria (it was an engulfment, not digestion).
- Believing the organelles can survive outside the cell today (they have lost too many genes to the nucleus).
- Assuming the nucleus was also formed by endosymbiosis (this is still debated, but not part of the standard theory).
KEY TAKEAWAYS
- Eukaryotes are products of symbiosis.
- Mitochondria evolved from aerobic bacteria.
- Chloroplasts evolved from cyanobacteria.
- Double membranes are a relic of engulfment.
PRACTICE QUESTIONS
Q1 (CONCEPTUAL): Which piece of evidence most strongly supports the bacterial origin of mitochondria?
Show Answer & Explanation
Answer: Mitochondria have their own circular DNA and ribosomes.
Explanation: These traits are characteristic of prokaryotes and are not found in other organelles like the Golgi or ER.
Q2 (CONCEPTUAL): According to endosymbiotic theory, which came first: the evolution of mitochondria or chloroplasts?
Show Answer & Explanation
Answer: Mitochondria.
Explanation: Nearly all eukaryotes have mitochondria, but only some have chloroplasts, suggesting mitochondria were acquired by the common ancestor of all eukaryotes first.
