The Endosymbiotic Theory Helps to Explain the Origin of Which Structures?
Mitochondria and chloroplasts. The endosymbiotic theory proposes these double-membrane organelles descended from free-living prokaryotes — aerobic bacteria and cyanobacteria — that were engulfed by an ancestral eukaryotic cell and became permanent internal symbionts rather than being digested.
The answer
The endosymbiotic theory explains the origin of mitochondria and chloroplasts. It proposes that these organelles were once free-living prokaryotes. An ancestral host cell engulfed an aerobic (oxygen-using) bacterium that became the mitochondrion, giving the host the ability to produce energy efficiently. Separately, in the lineage leading to plants and algae, a host cell engulfed a photosynthetic cyanobacterium that became the chloroplast. Instead of being digested, these engulfed cells survived, reproduced inside the host, and over evolutionary time became permanent, interdependent organelles.
The theory was popularized and developed by biologist Lynn Margulis in the 1960s (in her paper On the Origin of Mitosing Cells), building on earlier ideas from Konstantin Mereschkowski and Ivan Wallin.
The evidence that supports it
Several independent lines of evidence make this theory compelling, and understanding them is what separates real understanding from memorizing the answer:
- Their own DNA: mitochondria and chloroplasts each contain a small, circular DNA molecule resembling a bacterial genome, not the linear chromosomes of the nucleus.
- Their own ribosomes: these organelles have 70S ribosomes (the prokaryotic type) rather than the 80S ribosomes found in the eukaryotic cytoplasm, and antibiotics that target bacterial ribosomes can affect them too.
- Binary fission: they reproduce independently of the cell by dividing in two, the same way bacteria do, rather than being built from scratch by the cell.
- Double membranes: each has two membranes — the inner one resembles a bacterial plasma membrane, while the outer one is thought to derive from the host's engulfing vesicle.
- Size and similarity: they are roughly the size of bacteria, and their genetic sequences most closely match specific bacterial groups (mitochondria to alpha-proteobacteria, chloroplasts to cyanobacteria).
Why not the other structures?
Organelles like the nucleus, endoplasmic reticulum, and Golgi apparatus are not explained by endosymbiosis. These are part of the endomembrane system and are thought to have arisen from infoldings of the cell's own plasma membrane, not from engulfed bacteria. They have single membranes, no independent DNA, and no ribosomes of their own, so they fail every test the theory relies on. Likewise, structures such as ribosomes in general, the cytoskeleton, or the cell wall are not products of endosymbiosis. Only mitochondria and chloroplasts carry the tell-tale bacterial signatures — their own genome, prokaryotic ribosomes, division by fission, and double membranes — which is exactly why the endosymbiotic theory is invoked specifically to explain them.
The Endosymbiotic Theory Helps to Explain the Origin of Which Structures?
Frequently asked
What evidence supports the endosymbiotic theory?
Mitochondria and chloroplasts have their own circular DNA, prokaryotic-type 70S ribosomes, and double membranes, and they reproduce by binary fission like bacteria. They are also bacteria-sized, and their genes most closely match alpha-proteobacteria (mitochondria) and cyanobacteria (chloroplasts).
Who proposed the endosymbiotic theory?
Biologist Lynn Margulis developed and popularized the modern endosymbiotic theory in the 1960s with her 1967 paper 'On the Origin of Mitosing Cells.' She built on earlier ideas from Konstantin Mereschkowski and Ivan Wallin from the early 20th century.
Why do mitochondria and chloroplasts have their own DNA?
Because they descended from free-living prokaryotes that had their own genomes. When engulfed, they retained a small circular DNA molecule resembling a bacterial chromosome. Over time many genes moved to the nucleus, but each organelle still keeps some of its own DNA and can partly self-replicate.
Do mitochondria and chloroplasts have double membranes?
Yes. Each is surrounded by two membranes. The inner membrane resembles the original bacterium's plasma membrane, while the outer membrane is thought to derive from the host cell's vesicle that engulfed it. This double membrane is a key piece of evidence for endosymbiosis.