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A2.2.12 (HL)—Endosymbiosis origin of eukaryotes

Endosymbiotic theory explains mitochondria and chloroplasts as descendants of engulfed bacteria, supported by their genomes, ribosomes, division, membranes, size, and phylogenetic relationships.

Syllabus
First assessment 2025
Objective
A2.2.12
Level
HL

Exam analysis

Chance of appearing11%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper1
Typical marks1–4

Common command terms

  • Discuss
  • Explain
  • Outline
  • Identify
  • Describe

Scoring notes

Common mistake
Listing organelle features without explaining how they support bacterial ancestry.

Recent exam appearances

November 2024Paper1 ["HL"] · TZ04[ 1 ]A2.2.12 (HL)—Endosymbiosis origin of eukaryotes
November 2023Paper2 ["HL"] · TZ27(b)[ 5 ]A2.2.12 (HL)—Endosymbiosis origin of eukaryotes
May 2023Paper1 ["HL"] · TZ24[ 1 ]A2.2.12 (HL)—Endosymbiosis origin of eukaryotes
May 2023Paper1 ["HL"] · TZ14[ 1 ]A2.2.12 (HL)—Endosymbiosis origin of eukaryotes
November 2021Paper1 ["HL"] · TZ02[ 1 ]A2.2.12 (HL)—Endosymbiosis origin of eukaryotes
Practice this objective

Coverage 2014–2024 · Updated 15 Jul 2026

Concept essentials

  • Mitochondria descend from aerobic bacterial endosymbionts.
  • Chloroplasts descend from photosynthetic cyanobacterial endosymbionts.
  • Circular DNA, 70S ribosomes, and binary fission support bacterial ancestry.
  • Double membranes and molecular homology provide additional evidence for engulfment.
ConceptIB Biology HL