AHL: Cell diversity and endosymbiosis
IB Biology HL· Theme A: Unity and Diversity > Topic 6 AHL· 15 min read
1. Cell Structural Diversity: Prokaryotes vs Eukaryotes★★☆☆☆HL only⏱ 5 min
Prokaryotic Cell
A unicellular cell with no membrane-bound nucleus or organelles, with DNA arranged in a single circular chromosome
Example:
Bacteria and archaea are all prokaryotes
Eukaryotic Cell
A cell with a membrane-bound nucleus holding linear chromosomes, and specialized membrane-bound organelles
Example:
Plant, animal, fungal and protist cells are eukaryotic
Prokaryotes: 0.1–5 μm diameter; Eukaryotes: 10–100 μm diameter
Prokaryotes have 70S ribosomes; eukaryotic cytoplasm has 80S ribosomes
Prokaryotic DNA is circular and naked; eukaryotic DNA is linear and histone-bound
Only eukaryotes have membrane-bound organelles
An unknown cell measured at 2 μm diameter shows no membrane-bound organelles under an electron microscope. State if it is prokaryotic or eukaryotic, with two supporting reasons.
- 1
Step 1: Recall the key distinguishing features of each cell type
- 2
Step 2: Match the observations: 2 μm diameter falls within the prokaryotic size range, and prokaryotes lack membrane-bound organelles
- 3
Conclusion: The cell is prokaryotic, based on its small size and absence of membrane-bound organelles
Exam tip:
When asked to compare in IB exams, always state both a similarity and a difference to earn full marks
2. Core Principles of Endosymbiotic Theory★★★☆☆HL only⏱ 5 min
Endosymbiotic Theory
The theory that eukaryotic mitochondria and chloroplasts evolved from free-living prokaryotes engulfed by a larger host prokaryote, forming a permanent symbiotic relationship
Example:
Mitochondria evolved from alpha-proteobacteria; chloroplasts evolved from cyanobacteria
First formally proposed by Lynn Margulis in 1967, this theory explains how complex eukaryotic cells evolved from simpler prokaryotic ancestors. The process led to specialized energy-producing organelles that enabled the evolution of large, multicellular eukaryotic life.
Outline the sequence of events that led to the evolution of mitochondria according to endosymbiosis
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Step 1: A large anaerobic prokaryote host cannot produce much energy via anaerobic respiration
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Step 2: The host engulfs a smaller aerobic alpha-proteobacterium but does not digest it
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Step 3: The aerobic bacterium produces extra energy for the host via aerobic respiration, benefiting both partners
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Step 4: Over generations, the bacterium loses most independent genetic material and becomes a permanent, specialized mitochondrion inherited from parent to daughter cells
3. Evidence Supporting Endosymbiosis★★★☆☆HL only⏱ 5 min
Multiple independent lines of evidence align with predictions from the endosymbiotic theory, matching the expected properties of originally free-living prokaryotes:
Mitochondria and chloroplasts have circular, naked DNA like prokaryotes
They have 70S ribosomes, matching prokaryotic ribosome size
They replicate via binary fission, same as prokaryotes
They have a double membrane, consistent with engulfment by the host
Gene sequencing shows their DNA is closely related to modern prokaryotes
Explain how ribosome size supports the endosymbiotic origin of chloroplasts
- 1
Step 1: Recall that prokaryotes have 70S ribosomes, while eukaryotic cytoplasm has 80S ribosomes
- 2
Step 2: Chloroplasts have their own 70S ribosomes, matching the size found in free-living prokaryotes
- 3
Step 3: This matches the prediction of endosymbiosis: chloroplasts evolved from prokaryotes that retained their original ribosomes after engulfment
4. Common Pitfalls
Wrong move:
Claiming prokaryotes have no ribosomes
Why:
Students confuse 'no membrane-bound organelles' with lack of ribosomes. All cells need ribosomes to make proteins.
Correct move:
State that prokaryotes have 70S ribosomes, while eukaryotic cytoplasmic ribosomes are 80S
Wrong move:
Claiming the nucleus formed via endosymbiosis
Why:
Endosymbiosis only explains mitochondria and chloroplast origin; the nucleus is thought to form from cell membrane infoldings.
Correct move:
Only attribute endosymbiotic origin to mitochondria and chloroplasts in exam answers
Wrong move:
Stating mitochondria have linear DNA like eukaryotic nuclear DNA
Why:
Students mix up prokaryotic and eukaryotic DNA structure, leading to lost marks.
Correct move:
Remember mitochondria and chloroplasts have circular, naked DNA matching prokaryotes
Wrong move:
Only mentioning one organelle (chloroplast or mitochondria) when explaining endosymbiosis
Why:
Exam questions expect reference to both organelles unless explicitly told otherwise, leading to partial marks.
Correct move:
Always mention both mitochondria and chloroplast when discussing endosymbiosis evidence
5. Quick Reference Cheatsheet
Feature | Prokaryote | Eukaryote | Endosymbiosis Evidence (Mito/Chloro) |
|---|---|---|---|
Size | 0.1-5 μm | 10-100 μm | N/A |
Ribosomes | 70S | 80S (cytoplasm) | Have 70S, matches prokaryotes |
DNA | Circular, naked | Linear, histone-bound | Circular, naked, prokaryote-like |
Membrane organelles | Absent | Present | Double membrane from engulfment |
Replication | Binary fission | Mitosis/meiosis | Binary fission, matches prokaryotes |
6. Frequently Asked
Is endosymbiosis still just an unproven theory?
No. While it remains a scientific theory, it is overwhelmingly supported by multiple independent lines of evidence and is the accepted model for the origin of mitochondria and chloroplasts.
Going deeper
What's Next
Understanding cell diversity and endosymbiosis is foundational for all subsequent IB Biology HL topics. This concept connects cell biology to evolution, explaining how the complex eukaryotic cell structure that enables multicellular life evolved from simpler prokaryotic ancestors. Endosymbiosis is also a key example of how symbiotic relationships drive major evolutionary transitions, which you will revisit when studying ecology and macroevolution. This topic is frequently tested in both Paper 1 multiple choice and Paper 2 short answer questions, so regular review of the evidence for endosymbiosis is high-yield for exam preparation.