DAT Biology: Endosymbiotic Theory

DAT High-Yield Concepts

Origin: Mitochondria evolved from aerobic bacteria; Chloroplasts evolved from photosynthetic cyanobacteria.
Double Membrane: Both organelles have two membranes; the inner is bacterial-like, the outer is eukaryotic-like.
Genetic Material: Both possess their own circular DNA (plasmid-like), independent of the nucleus.
Protein Synthesis: Both contain 70S ribosomes (similar to prokaryotes), distinct from the cell's 80S cytosolic ribosomes.
Replication: They reproduce independently via binary fission, not mitosis.

1. Introduction to Endosymbiosis

The Endosymbiotic Theory describes the evolutionary origin of eukaryotic cells from prokaryotic organisms. It posits that key organelles—specifically mitochondria and chloroplasts—were once free-living prokaryotic bacteria that were engulfed by a larger ancestral eukaryotic cell.

Definition: Endosymbiosis comes from "endo" (within) and "symbiosis" (living together). It describes a relationship where one organism lives inside another for mutual benefit.

The Evolutionary Timeline

Infolding: An ancestral prokaryote’s plasma membrane folded inward to form the nucleus and endomembrane system.
First Endosymbiotic Event (Mitochondria): This ancestral cell engulfed an aerobic heterotrophic prokaryote (likely an alpha-proteobacterium). Instead of being digested, it became a mitochondrion, providing the host with ATP.
Second Endosymbiotic Event (Chloroplasts): A lineage of these heterotrophic eukaryotes then engulfed a photosynthetic prokaryote (likely a cyanobacterium), which became the chloroplast. This lineage evolved into plants and algae.

Diagram showing the steps of serial endosymbiosis: 1. Infolding of plasma membrane. 2. Engulfing of aerobic heterotrophic prokaryote (mitochondrion). 3. Engulfing of photosynthetic prokaryote (chloroplast).

A diagram illustrating the serial endosymbiosis theory. It shows an ancestral eukaryotic cell engulfing an aerobic bacterium (which becomes the mitochondrion) and subsequently a photosynthetic bacterium (which becomes the chloroplast). (Source: Wikimedia Commons)

2. Key Evidence for the Theory

On the DAT, you will be asked to identify evidence supporting this theory. The evidence highlights the similarities between these organelles and modern prokaryotes.

A. Membranes (Double Membrane System)

Mitochondria and chloroplasts have two membranes. This structure supports the theory of phagocytosis (engulfing).

Inner Membrane: Corresponds to the original bacterial membrane. It contains transport proteins and enzymes homologous to those found in living bacteria.
Outer Membrane: Corresponds to the host cell's vesicle that formed when the bacterium was engulfed.

B. Genetic Material (Circular DNA)

Unlike the linear DNA found in the eukaryotic nucleus (which is wrapped around histones), mitochondria and chloroplasts possess their own DNA (mtDNA and cpDNA).

Structure: Single, circular chromosome.
Histones: The DNA is "naked" (not associated with histones), exactly like prokaryotic DNA.

Cross-section of a mitochondrion labeling the outer membrane, inner membrane, cristae, matrix, DNA, ribosomes, and granules.

Detailed diagram of a mitochondrion. Key evidence for endosymbiosis is visible, including the double membrane structure, its own circular DNA, and ribosomes (70S) within the matrix. (Source: Wikimedia Commons)

C. Ribosomes (70S vs. 80S)

Ribosomes are the sites of protein synthesis. They are measured by their sedimentation rate (Svedberg units, S).

Eukaryotic Cytosol: Contains 80S ribosomes (composed of 60S + 40S subunits).
Mitochondria/Chloroplasts: Contain 70S ribosomes (composed of 50S + 30S subunits).

Clinical Relevance: This is why some antibiotics (like tetracyclines) that target bacterial ribosomes can inhibit mitochondrial function, potentially causing side effects in humans.

D. Reproduction (Binary Fission)

Mitochondria and chloroplasts cannot be created de novo (from scratch) by the cell. They must reproduce themselves.

They replicate independently of the cell cycle.
They use a process resembling binary fission, the same method used by bacteria, rather than mitosis.

E. Size

Both organelles are approximately 1–10 microns in size, which is the average size of most bacteria.

Diagram of a chloroplast showing the outer and inner membranes, stroma, thylakoids, granum, plastidial DNA, and ribosomes.

Structure of a chloroplast. Like mitochondria, chloroplasts provide evidence for endosymbiosis through their double membrane envelope, internal thylakoid system, and presence of their own DNA and ribosomes. (Source: Wikimedia Commons)

3. Comparison Summary

Use this table to visualize the relationship between free-living bacteria and these organelles.

Feature	Prokaryotes (Bacteria)	Mitochondria & Chloroplasts	Eukaryotic Host Cell
DNA Structure	Circular, no histones	Circular, no histones	Linear, wrapped in histones
Ribosomes	70S (50S + 30S)	70S (50S + 30S)	80S (60S + 40S)
Division	Binary Fission	Binary Fission-like	Mitosis
Membranes	Plasma membrane + Cell Wall	Double Membrane	Single Plasma Membrane
Electron Transport Chain	Located in Plasma Membrane	Located in Inner Membrane (Mito) or Thylakoid (Chloro)	N/A (Relies on organelles)

Mnemonic: "DR. D"
To remember the evidence for Endosymbiosis:
D - DNA (Circular)
R - Ribosomes (70S)
D - Division (Binary Fission)
D - Double Membrane

Critical Thinking Questions

1. Why is mitochondrial DNA (mtDNA) almost exclusively inherited from the mother?
The egg cell is large and contains many mitochondria, while the sperm contributes almost no cytoplasm or mitochondria to the zygote during fertilization.

2. If a scientist removed all mitochondria from a eukaryotic cell, could the cell regenerate them?
No. Mitochondria must come from pre-existing mitochondria via binary fission. The nucleus contains genes for some mitochondrial proteins, but not the machinery to build the organelle from scratch.

3. Why do antibiotics that target bacterial protein synthesis sometimes cause fatigue in humans?
Because mitochondria contain 70S ribosomes (similar to bacteria), high doses of these antibiotics can inadvertently inhibit protein synthesis within the mitochondria, reducing ATP production.

Endosymbiotic Theory

Summary

Practice Questions

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