Explain how the endosymbiotic theory accounts for the presence 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)

An ancestral eukaryotic cell engulfed an aerobic prokaryote by endocytosis, forming a symbiotic relationship. The engulfed prokaryote was not digested but survived inside the host, providing the host with efficient aerobic respiration and increased ATP production. Mitochondria retain a circular DNA that is similar to prokaryotic DNA and they replicate by binary fission, just like bacteria. Mitochondria are surrounded by a double membrane; the inner membrane is derived from the prokaryote’s plasma membrane, while the outer membrane comes from the host’s endocytic vesicle.

Examiner tips

Use the word ‘engulfed’ and ‘symbiotic relationship’ to hit the first two points. Mention ‘aerobic respiration’ to show the advantage. Cite DNA and binary fission for evidence, and double‑membrane origin for the last point. Keep each point short and to the point. Use UK spelling (e.g., ‘engulfed’).

Common mistakes

Confusing endocytosis with phagocytosis; both are acceptable but phagocytosis is more common. Mixing up the source of the inner and outer membranes. Not mentioning the circular DNA or binary fission as evidence.

Mark scheme (4 marks)

An ancestral prokaryote (host cell) engulfed a smaller aerobic prokaryote by endocytosis / phagocytosis, forming a symbiotic relationship
The engulfed prokaryote was not digested but persisted inside the host, providing an advantage such as aerobic respiration / ATP production to the host
Evidence: mitochondria have their own circular DNA (similar to prokaryotes) and replicate by binary fission
Evidence: mitochondria are surrounded by a double membrane, the inner membrane originating from the prokaryote's plasma membrane and the outer from the host's vesicle / endocytic membrane

Key terms in this question

endosymbiotic theory · mitochondria