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Your NCERT cell chapter says mitochondria have their own circular DNA and 70S ribosomes, and divide by fission. It does not say why a structure inside your cells would look so much like a bacterium. The answer is the endosymbiotic theory, and the scientist who brought it back to life was Lynn Margulis. This post explains her argument, which parts survived and how to use the evidence in an answer.
Who was Lynn Margulis?
Margulis was born on 5 March 1938, according to her National Medal of Science record, and died in 2011, as Nature's obituary records. She received the National Medal of Science in 1999, listed with the University of Massachusetts, for "her outstanding contributions to understanding of the development, structure, and evolution of living things" and for her abilities as a teacher and communicator of science.
What did she propose in 1967?
Michael Gray's essay Lynn Margulis and the endosymbiont hypothesis: 50 years later describes the paper. Writing as Lynn Sagan, she published On the origin of mitosing cells in the Journal of Theoretical Biology in 1967. Gray reports that it had reportedly been rejected by more than a dozen journals first.
She proposed that "three fundamental organelles: the mitochondria, the photosynthetic plastids and the (9 + 2) basal bodies of flagella were once themselves free-living (prokaryotic) cells." In her scenario, an aerobic bacterium was taken into a host that could not use oxygen, and the partnership became permanent. On plastids she was blunt: plant cells "did not evolve oxygen-eliminating photosynthesis"; they "acquired it by symbiosis" from blue-green algae, which we now call cyanobacteria.
The idea was not new. Gray notes that versions of it had appeared in the late 19th and early 20th centuries and then faded. What Margulis added was a complete account of how the eukaryotic cell was assembled, and years of argument for it. Gray describes a spirited debate through the 1970s and 1980s between people who thought organelles came from outside the cell and those who thought they formed from within.
What evidence supports her today?
The strongest evidence comes from the organelles' own genomes. Gray writes that the genes they carry, and how those genes are arranged and expressed, show "with a high degree of certainty" that mitochondria come from a group of bacteria called alpha-proteobacteria and plastids from cyanobacteria. One group of single-celled eukaryotes, the jakobids, has a gene-rich mitochondrial genome that "strongly resembles a shrunken bacterial genome".
The flagellum is a different story. No genome has ever been found in the flagellar apparatus, Gray points out, so there is nothing to compare with bacteria, and the idea is not supported.
Worked reasoning: evidence from your NCERT chapter
NCERT does not name the endosymbiotic theory, but the Class 11 chapter Cell: The Unit of Life lists the features you need. For each, ask what a bacterial origin would predict.
Own DNA. The mitochondrial matrix has "single circular DNA molecule", and chloroplasts have "small, double-stranded circular DNA molecules". Bacteria also have circular DNA. A structure made from the cell's own membranes would have no reason to carry its own genome.
Own ribosomes. Mitochondria and chloroplasts have 70S ribosomes, while the cytoplasm has 80S ribosomes. NCERT gives 70S as the bacterial type.
Division. "The mitochondria divide by fission." New mitochondria come from old ones, as bacteria come from bacteria.
Two membranes. Both organelles are double membrane bound, which fits a cell taken in by another.
A good answer names the feature, states the bacterial parallel and says what it suggests. "Mitochondria have circular DNA like bacteria, suggesting they descend from a bacterium" is better than "mitochondria are bacteria".
Common mistakes:
Adding S values. A 70S ribosome has 50S and 30S subunits, and an 80S ribosome has 60S and 40S. 50 + 30 is not 70, because S, the Svedberg unit, is a sedimentation coefficient, which NCERT describes as indirectly a measure of density and size. It is not a mass you can add.
Saying Margulis invented the idea. She revived and extended an older one.
Treating the whole 1967 paper as accepted. The flagellum part is not.
Stopping at the old picture. Gray notes that how the partnership began, and what kind of cell the host was, are still debated.
Our guide to the cell as the unit of life covers the other organelles in the chapter.
What can a student take from her way of working?
Collect evidence from many fields. Her case drew on cell structure, biochemistry and the rise of oxygen in the atmosphere.
Keep going after rejection. A paper turned down many times is now seen, in Gray's words, as marking the modern renaissance of the endosymbiotic theory.
Accept that part of a big idea can be wrong. The mitochondria and plastid parts stand even though the flagellum part does not.
Make claims that evidence can check. Later genome sequencing tested her ideas directly.
When a structure is hard to picture
Double membranes and ribosome subunits are easier to remember when you can see them in three dimensions. The Fermi Concept Library has every concept in biology with its own lesson and its place in the map, and the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than to read.
Try it now: write two sentences explaining how circular DNA in chloroplasts supports a bacterial origin, without using the word "proves".
