Endosymbiosis Theory

What is the endosymbiosis theory?

• Explains origin of mitochondria and chloroplasts

• Proposes they were once free-living prokaryotes

• Engulfed by ancestral eukaryotic cells

• Formed a mutualistic relationship.

Which two organelles are primarily explained by the endosymbiosis theory?

• Mitochondria

• Chloroplasts.

How does the endosymbiosis theory explain the origin of mitochondria?

Evolved from aerobic bacteria
Engulfed by anaerobic host cell
Provided efficient ATP production
Became permanent organelles.

What kind of organisms were the ancestors of mitochondria and chloroplasts?

Mitochondria: aerobic proteobacteria
Chloroplasts: photosynthetic cyanobacteria.

What evidence supports the endosymbiosis theory?

Own circular DNA
Reproduce by binary fission
Double membranes
70S ribosomes (like prokaryotes)
Genetic similarities to bacteria.

Why do mitochondria and chloroplasts have their own DNA?

Inherited from ancestral bacteria
Retained some of their original genes.

How do the membranes support the theory?

Double membranes
Inner: from engulfed bacteria
Outer: from host’s engulfing vesicle.

Similarities with prokaryotes:

Circular DNA
70S ribosomes
Binary fission reproduction
Similar enzymes and transport systems.

Ribosomes comparison:

Organelle ribosomes: 70S (like bacteria)
Cytoplasmic ribosomes: 80S (eukaryotic).

Why is binary fission significant?

Same reproduction method as bacteria
Supports bacterial ancestry.

Evolutionary advantage of chloroplasts:

Enabled photosynthesis
Produced glucose from sunlight
Energy independence for host cell.

Double membranes explanation:

Inner membrane: from engulfed prokaryote
Outer membrane: from host’s vesicle during engulfing.

What if mitochondria lost DNA?

Could weaken the theory
But many genes already transferred to nucleus
Still supports long-term integration.

Gene transfer to nucleus relevance:

Shows co-evolution and integration
Suggests organelles became dependent on host.

Modern examples of endosymbiosis:

Paramecium and Euglena with internal algae
Coral and photosynthetic zooxanthellae
Suggests endosymbiosis can still occur.