Explain how the endosymbiotic theory accounts for the presence of chloroplasts 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).

Chloroplasts are membrane-bound organelles found in the cells of plants and algae. They contain their own DNA and ribosomes, and are capable of self-replication.

Model answer (4 marks)

An ancestral eukaryotic cell engulfed a photosynthetic prokaryote by endocytosis, but the engulfed cell was not digested. Instead a mutualistic endosymbiotic relationship was established: the prokaryote supplied sugars and other organic molecules to the host, while the host supplied a protected, stable environment and nutrients.

Evidence for this origin is:
1. Chloroplasts are surrounded by two membranes – the inner membrane of the chloroplast and an outer membrane derived from the phagocytic vesicle – matching the double‑membrane structure expected from engulfment.
2. Chloroplasts contain their own circular DNA and 70S ribosomes, identical to those of free‑living prokaryotes, indicating a separate genetic system inherited from the engulfed organism.

Examiner tips

  • Use the word "engulfed" and "endocytosis" to show understanding of the process. Mention the mutual benefits (sugars for host, protection for symbiont). Include both lines of evidence – double membranes and prokaryotic DNA/ribosomes – to cover all points.
  • common_mistakes
  • :
  • Failing to state that the engulfed cell was not digested. Omitting either the double‑membrane evidence or the DNA/ribosome evidence. Using vague terms like "cell" instead of "prokaryote" or "organism".

Mark scheme (4 marks)

  1. An ancestral prokaryote (host cell) engulfed a photosynthetic prokaryote by endocytosis / phagocytosis, rather than digesting it.
  2. A mutually beneficial (endosymbiotic) relationship developed, with the photosynthetic prokaryote providing organic molecules / sugars to the host and the host providing protection / a stable environment.
  3. Evidence for this origin includes the presence of double membranes surrounding chloroplasts, consistent with engulfment by endocytosis forming an outer membrane.
  4. Additional evidence includes chloroplasts possessing their own circular DNA and 70S ribosomes, similar to free-living prokaryotes.

Key terms in this question

endosymbiotic theory · chloroplast

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