# AHL: Cell diversity and endosymbiosis

> IB Biology HL · Theme A: Unity and Diversity
> Source: https://www.owlsprep.com/study/ib-biology-hl-u1-ahl-cell-diversity-and-endosymbiosis/

This AHL module explores structural diversity between prokaryotic and eukaryotic cells, and the endosymbiotic theory explaining the evolutionary origin of mitochondria and chloroplasts in eukaryotes. We evaluate key evidence for this widely accepted model.

**Prerequisites:** [Basic prokaryotic cell structure](https://www.owlsprep.com/study/ib-biology-hl-u1-prokaryotic-cell-structure/); [Eukaryotic organelle function](https://www.owlsprep.com/study/ib-biology-hl-u1-eukaryotic-organelles/)

## Learning objectives

- Compare structural diversity between prokaryotic and eukaryotic cells
- Explain the core principles of the endosymbiotic theory
- Evaluate key lines of evidence supporting endosymbiosis
- Connect endosymbiosis to the evolution of eukaryotic life

## Cell Structural Diversity: Prokaryotes vs Eukaryotes

**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

**Worked example:** 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

## Core Principles of Endosymbiotic Theory

**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.

**Worked example:** Outline the sequence of events that led to the evolution of mitochondria according to endosymbiosis

1. Step 1: A large anaerobic prokaryote host cannot produce much energy via anaerobic respiration
2. Step 2: The host engulfs a smaller aerobic alpha-proteobacterium but does not digest it
3. Step 3: The aerobic bacterium produces extra energy for the host via aerobic respiration, benefiting both partners
4. Step 4: Over generations, the bacterium loses most independent genetic material and becomes a permanent, specialized mitochondrion inherited from parent to daughter cells

## Evidence Supporting Endosymbiosis

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

**Worked example:** 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

> **Secondary Endosymbiosis**
>
> Secondary endosymbiosis occurs when a eukaryote engulfs another eukaryote that already has a chloroplast. This produces chloroplasts with 3-4 membrane layers, seen in many algae, providing further evidence for the model.

## Common pitfalls

- **Wrong:** Claiming prokaryotes have no ribosomes
  - Why it fails: Students confuse 'no membrane-bound organelles' with lack of ribosomes. All cells need ribosomes to make proteins.
  - Correct: State that prokaryotes have 70S ribosomes, while eukaryotic cytoplasmic ribosomes are 80S
- **Wrong:** Claiming the nucleus formed via endosymbiosis
  - Why it fails: Endosymbiosis only explains mitochondria and chloroplast origin; the nucleus is thought to form from cell membrane infoldings.
  - Correct: Only attribute endosymbiotic origin to mitochondria and chloroplasts in exam answers
- **Wrong:** Stating mitochondria have linear DNA like eukaryotic nuclear DNA
  - Why it fails: Students mix up prokaryotic and eukaryotic DNA structure, leading to lost marks.
  - Correct: Remember mitochondria and chloroplasts have circular, naked DNA matching prokaryotes
- **Wrong:** Only mentioning one organelle (chloroplast or mitochondria) when explaining endosymbiosis
  - Why it fails: Exam questions expect reference to both organelles unless explicitly told otherwise, leading to partial marks.
  - Correct: Always mention both mitochondria and chloroplast when discussing endosymbiosis evidence

## 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 |

## 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.

- [AHL: Cladistics and classification](https://www.owlsprep.com/study/ib-biology-hl-u1-ahl-cladistics-and-classification/)
- [AHL: Origin of life](https://www.owlsprep.com/study/ib-biology-hl-u1-ahl-origin-of-life/)

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