The Ancient Merger: Endosymbiotic Theory Explained

  • 12th Grade
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1. What is the primary focus of the endosymbiotic theory?

Explanation

If eukaryotes possess complex organelles with their own DNA, then those organelles may have originated from external sources. If the endosymbiotic theory proposes that these organelles were once free-living prokaryotes, then it explains the transition from simple cells to complex eukaryotic cells.

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About This Quiz
The Ancient Merger: Endosymbiotic Theory Explained - Quiz

What if I told you that you are actually a walking colony of ancient survivors who decided to team up instead of eating each other? We’re going back billions of years to the most successful hostile takeover in history. With the endosymbiotic theory explained, you’ll realize your energy-producing powerhouses were... see moreonce independent invaders. This is the origin story of complex life, a tale of ancient alliances that changed the trajectory of the planet forever.
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2. The endosymbiotic theory suggests that the ancestral host cell was likely an anaerobic archaean.

Explanation

If early Earth had low oxygen levels, then the first large cells were likely anaerobic. If a host cell engulfed an aerobic bacterium to survive in an oxygen-rich environment, then the endosymbiotic theory correctly identifies the host as an anaerobic organism seeking a metabolic advantage.

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3. Which piece of evidence best supports the endosymbiotic theory regarding organelle membranes?

Explanation

If a host cell engulfs a bacterium via phagocytosis, then the bacterium would be wrapped in the host's plasma membrane. If the resulting organelle has an inner bacterial membrane and an outer host-derived membrane, then the presence of a double membrane is logical evidence for endosymbiosis.

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4. According to the endosymbiotic theory, chloroplasts were acquired before mitochondria in the eukaryotic lineage.

Explanation

If nearly all eukaryotes have mitochondria but only some have chloroplasts, then the acquisition of mitochondria must have happened first in the common ancestor. If a trait is found across a wider group of descendants, then it logically represents an earlier evolutionary event.

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5. The _______ theory explains why mitochondria have their own genome that is distinct from the nuclear DNA of the cell.

Explanation

If an organelle was once an independent organism, then it must have carried its own genetic blueprints. If the endosymbiotic theory states these organelles were once separate bacteria, then the presence of a separate mitochondrial genome is a predicted and necessary outcome of that history.

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6. Modern-day glaucophytes provide evidence for endosymbiosis because their chloroplasts still have a peptidoglycan layer.

Explanation

If peptidoglycan is a unique feature of bacterial cell walls, then finding it in an organelle is a "smoking gun" for bacterial origin. If glaucophyte algae retain this layer between their chloroplast membranes, then it provides direct physical evidence for the endosymbiotic theory.

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7. What is "secondary endosymbiosis"?

Explanation

If primary endosymbiosis involves a eukaryote engulfing a prokaryote, then secondary endosymbiosis is the next level of complexity. If a non-photosynthetic eukaryote engulfs a photosynthetic eukaryote (like a red or green alga), then this describes the "secondary" event common in many protist lineages.

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8. The endosymbiotic theory is supported by the fact that some antibiotics that inhibit bacterial protein synthesis also inhibit mitochondrial protein synthesis.

Explanation

If mitochondria have bacterial-style ribosomes, then they should be susceptible to the same chemicals that target bacteria. If certain antibiotics interfere with mitochondrial function in eukaryotic cells, then this physiological link supports the endosymbiotic theory.

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9. Why was the endosymbiotic theory initially rejected by the scientific community in the early 20th century?

Explanation

If early scientists could not see or sequence organelle DNA, then they lacked the mechanism to prove a bacterial origin. If the scientific consensus at the time focused solely on gradual point mutations, then the idea of "evolution by fusion" was too radical to be accepted without the molecular data we have today.

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10. Which of the following organelles are NOT explained by the endosymbiotic theory?

Explanation

If the theory specifically addresses organelles with their own DNA and double membranes that resemble bacteria, then it applies to mitochondria and chloroplasts. If the Golgi, ER, and lysosomes are part of the internal membrane system likely formed by infoldings of the plasma membrane, then they are not endosymbionts.

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11. Lynn Margulis is the scientist who famously championed and provided modern evidence for the _______ theory in the 1960s.

Explanation

If a theory was neglected for decades until a specific researcher gathered microbiological evidence to support it, then she is the primary figure associated with it. If Lynn Margulis performed this role for the origin of organelles, then she is the champion of the endosymbiotic theory.

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12. Which of the following is a result of the efficiency gained through the endosymbiotic theory?

Explanation

If endosymbiosis provided a massive increase in available energy (ATP), then cells could support much larger genomes and more complex structures. If these complex cells could then specialize and communicate, then endosymbiosis is the logical prerequisite for the evolution of multicellular organisms.

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13. The internal folds of the mitochondria, called _______, increase surface area for ATP production, similar to membrane structures in aerobic bacteria.

Explanation

If the endosymbiotic theory is true, then the internal structures of organelles should mirror bacterial adaptations. If aerobic bacteria use membrane folds to maximize energy output, then the presence of cristae in mitochondria is a logical continuation of that bacterial architecture.

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14. Mitochondria and chloroplasts reproduce through a process called _______, which is the same way modern bacteria divide.

Explanation

If an organelle is derived from an ancient bacterium, then it should retain bacterial methods of reproduction. If binary fission is the primary method of prokaryotic division and these organelles use it independently of the cell's nucleus, then it supports the endosymbiotic theory.

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15. Which of the following are prokaryotic characteristics found in mitochondria and chloroplasts?

Explanation

If these organelles originated as bacteria, then they should possess bacterial traits. If bacteria have circular DNA and 70S ribosomes (smaller than eukaryotic 80S), then those traits in organelles support the endosymbiotic theory. While they make some proteins, they rely on the nucleus for others.

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16. Mitochondria are thought to have evolved from which specific group of bacteria?

Explanation

If we compare mitochondrial DNA sequences to modern bacterial genomes, then we can find the closest living relatives. If the DNA of mitochondria matches most closely with aerobic alphaproteobacteria, then they are the most logical ancestors according to the endosymbiotic theory.

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17. What provided the "complexity" in the origin of eukaryotic cells as described by endosymbiosis?

Explanation

If a cell gains an organelle that specializes in ATP production, then it has more energy for complex tasks. If this leads to a larger genome and specialized compartments, then the endosymbiotic theory accounts for the rise in biological complexity.

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18. Chloroplasts are believed to have evolved from ancient _______, which were the first organisms to perform oxygenic photosynthesis.

Explanation

If chloroplasts allow plants to turn sunlight into food, then their ancestor must have been a photosynthetic prokaryote. If cyanobacteria are the only prokaryotes capable of this specific type of photosynthesis, then the endosymbiotic theory identifies them as the ancestors of chloroplasts.

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19. The movement of genes from the organelle's genome to the host cell's nucleus is known as endosymbiotic _______ transfer.

Explanation

If organelles have been inside host cells for billions of years, then over time their genomes might shrink as the nucleus takes over some functions. If genes move from the organelle to the nucleus, then this process is logically called endosymbiotic gene transfer.

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20. Mitochondria and chloroplasts can be synthesized from scratch by the cell if they are removed.

Explanation

If these organelles were once independent organisms, then the cell's nucleus shouldn't have the complete instructions to "build" them from nothing. If a cell loses all its mitochondria or chloroplasts and cannot replace them, then this lack of de novo synthesis supports the endosymbiotic theory.

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What is the primary focus of the endosymbiotic theory?
The endosymbiotic theory suggests that the ancestral host cell was...
Which piece of evidence best supports the endosymbiotic theory...
According to the endosymbiotic theory, chloroplasts were acquired...
The _______ theory explains why mitochondria have their own genome...
Modern-day glaucophytes provide evidence for endosymbiosis because...
What is "secondary endosymbiosis"?
The endosymbiotic theory is supported by the fact that some...
Why was the endosymbiotic theory initially rejected by the scientific...
Which of the following organelles are NOT explained by the...
Lynn Margulis is the scientist who famously championed and provided...
Which of the following is a result of the efficiency gained through...
The internal folds of the mitochondria, called _______, increase...
Mitochondria and chloroplasts reproduce through a process called...
Which of the following are prokaryotic characteristics found in...
Mitochondria are thought to have evolved from which specific group of...
What provided the "complexity" in the origin of eukaryotic cells as...
Chloroplasts are believed to have evolved from ancient _______, which...
The movement of genes from the organelle's genome to the host cell's...
Mitochondria and chloroplasts can be synthesized from scratch by the...
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