Explain the evidence that supports the endosymbiotic origin of mitochondria in eukaryotic cells.

IB DP Biology Standard Level (2023 syllabus) — A2.1 Origins of cells · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Mitochondria contain their own circular DNA, like that of bacteria, indicating a bacterial ancestor.
Mitochondria possess 70S ribosomes, the same size as prokaryotic ribosomes rather than the 80S ribosomes of the cytoplasm.
They are surrounded by a double membrane; the inner membrane is consistent with a retained bacterial membrane inside the host membrane.
Mitochondria divide by binary fission independently of the cell cycle, as prokaryotes do, rather than by mitosis.

Examiner tips

Use the exact terms ‘circular DNA’, ‘70S ribosomes’, ‘double membrane’, ‘binary fission’
Show each point clearly, matching the mark scheme order
Keep sentences concise and factual

Common mistakes

Confusing 70S ribosomes with 80S, or omitting the double‑membrane detail
Failing to mention binary fission or the independent reproduction of mitochondria

Mark scheme (4 marks)

Mitochondria have their own circular DNA, similar to that found in prokaryotes, suggesting a bacterial ancestor.
Mitochondria have their own 70S ribosomes, which are the same size as prokaryotic ribosomes rather than the 80S ribosomes found in the eukaryotic cytoplasm.
Mitochondria are surrounded by a double membrane, with the inner membrane consistent with an engulfed bacterium that retained its own membrane inside the host cell's membrane.
Mitochondria reproduce by binary fission independently of the cell cycle, as prokaryotes do, rather than by mitosis.

Key terms in this question

mitochondria