Explain how the endosymbiotic theory accounts for the origin of mitochondria in eukaryotic cells.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
An ancestral eukaryotic cell engulfed an aerobic prokaryote by endocytosis, creating a symbiotic relationship rather than digesting it. The engulfed cell became a mitochondrion, which is surrounded by a double membrane – the outer membrane derived from the host’s plasma membrane and the inner membrane from the prokaryote’s plasma membrane, matching the endocytic origin. Mitochondria retain their own circular DNA and 70S ribosomes, both characteristic of prokaryotes, providing evidence that they were once free‑living bacteria. The symbiosis was mutually beneficial: the host supplied protection and nutrients, while the endosymbiont performed aerobic respiration, producing ATP for the host. Over time this partnership became integrated, giving rise to the mitochondria of modern eukaryotes.
Examiner tips
- Use the word ‘engulfed’ to show endocytosis; mention the double membrane; cite circular DNA and 70S ribosomes; explain mutual benefit and integration.
- Structure answer in clear points matching the scheme; keep terminology precise (e.g., ‘prokaryote’, ‘plasma membrane’, ‘circular DNA’).
Common mistakes
- Confusing the host cell with the endosymbiont; not mentioning the double membrane; omitting the circular DNA or 70S ribosomes; failing to explain the mutual benefit or integration.
Mark scheme (4 marks)
- An ancestral prokaryote (host cell) engulfed a smaller aerobic prokaryote by endocytosis / phagocytosis, forming a symbiotic relationship rather than digesting it.
- The inner membrane of mitochondria is homologous to the plasma membrane of the engulfed prokaryote / mitochondria are surrounded by a double membrane consistent with endocytotic uptake.
- Mitochondria contain their own circular DNA (and 70S ribosomes) similar to prokaryotes, providing evidence of a formerly free-living bacterial ancestor.
- The relationship was mutually beneficial: the host provided protection / organic nutrients, while the endosymbiont's aerobic respiration supplied ATP to the host, leading to integration over evolutionary time.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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