Overview
This lecture explains the endosymbiotic theory, describing how complex eukaryotic cells may have evolved from simpler prokaryotic cells living together in symbiosis.
Scientific Theories
- A scientific theory is an explanation supported by extensive evidence and repeated testing.
- Theories are different from casual guesses and are backed by many facts but can be updated or disproven.
Prokaryotes vs. Eukaryotes
- Prokaryotes have no nucleus and no membrane-bound organelles ("pro" rhymes with "no").
- Eukaryotes have a nucleus and membrane-bound organelles ("eu" rhymes with "do").
- Both cell types have a cell membrane, cytoplasm, ribosomes, and genetic material, but prokaryotes are simpler.
Endosymbiotic Theory
- The endosymbiotic theory explains how eukaryotic cells evolved from prokaryotic cells that lived in symbiosis.
- Over 2 billion years ago, some large prokaryotes engulfed smaller bacteria without digesting them.
- Engulfed bacteria lived inside the host cell and became symbionts, leading to ancestor heterotroph eukaryotes.
- Some eukaryotes later engulfed photosynthetic bacteria, creating ancestor autotroph eukaryotes.
- Bacteria that used oxygen evolved into mitochondria; photosynthetic bacteria evolved into chloroplasts.
- Mitochondria likely developed before chloroplasts, as nearly all eukaryotes have mitochondria, but only some have chloroplasts.
Evidence Supporting Endosymbiotic Theory
- Mitochondria and chloroplasts have their own DNA, separate from nuclear DNA.
- The DNA of these organelles is arranged similarly to bacterial DNA.
- Mitochondria and chloroplasts are similar in size to bacteria.
- These organelles can divide independently in a way similar to bacterial cell division.
Modern Examples of Endosymbiosis
- Endosymbiosis still occurs today, such as termites having prokaryotes in their gut to help digest wood.
Key Terms & Definitions
- Scientific Theory — an extensively tested explanation for scientific events, supported by evidence.
- Prokaryote — a cell without a nucleus or membrane-bound organelles.
- Eukaryote — a cell with a nucleus and membrane-bound organelles.
- Symbiosis — a relationship where different organisms live closely together, often for mutual benefit.
- Endosymbiosis — one organism living inside another in a symbiotic relationship.
- Mitochondria — organelles responsible for energy production in eukaryotic cells, thought to have evolved from prokaryotes.
- Chloroplasts — organelles in plants/algae for photosynthesis, thought to have evolved from photosynthetic bacteria.
Action Items / Next Steps
- Explore secondary endosymbiosis and other eukaryotic cell structures.
- Review the differences between prokaryotic and eukaryotic cells.