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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

Endosymbiosis Explains Mitochondria and Chloroplasts

HL only

Endosymbiosis explains mitochondria and chloroplasts as descendants of bacteria incorporated into early eukaryotic cells.

A common unicellular eukaryotic ancestor already had a nucleus and sexual reproduction. Mitochondria then arose by endosymbiosis; chloroplasts arose later in some eukaryotic lineages.

Observation in mitochondria/chloroplasts Endosymbiotic inference
70S ribosomes Resemble bacterial translation machinery
Naked circular DNA Resembles bacterial chromosomes
Replication within the cell Retains bacterial-like division capacity
Double membrane and bacterial sequence similarity Consistent with engulfment and bacterial ancestry

Mitochondrial circular DNA and 70S ribosomes are independent evidence lines; together they support bacterial ancestry more strongly than organelle shape alone.

Modern organelles are dependent parts of eukaryotic cells, not free-living bacteria. Chloroplast endosymbiosis occurred only in lineages that possess plastids.

Endosymbiosis origin of eukaryotes

HL only

Assessment in practice

1–3 marks
How it is assessed

This objective is assessed through essay response, commonly using Discuss / Explain / Outline.

Command terms

Discuss / Explain / Outline / Identify / Describe

What earns marks

Build the answer around this relationship: Mitochondria descend from aerobic bacterial endosymbionts.

Watch for

Listing organelle features without explaining how they support bacterial ancestry.

Representative question

Question 1

[Maximum number: 6]

Explain the endosymbiotic theory for the origin of eukaryotes and the evidence for it.

Complex Cell Origins

HL only

The HL extension asks how complex cell organization could arise and become useful. Endosymbiosis explains the origin of mitochondria and chloroplasts using bacterial evidence. Differentiation explains how cells with the same genome become specialized through different gene expression and proteomes. Multicellularity explains why adhesion, communication, and differentiation allowed larger bodies and division of labour.

  • Endosymbiosis: organelle origin supported by bacterial-style evidence.
  • Differentiation: same genome, different gene expression, different proteome.
  • Multicellularity: adhesion, communication, differentiation.
  • Advantages: larger body size and cell specialization.

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.