# What is the evidence that supports endosymbiotic theory?


1. Summary

Learning Objectives

• Briefly describe what is meant by the endosymbiotic theory.
• Give some evidence supporting the theory that mitochondria and chloroplasts may have arisen from prokaryotic organisms.

It is thought that life arose on earth around four billion years ago. The endosymbiotic theory states that some of the organelles in today's eukaryotic cells were once prokaryotic microbes. In this theory, the first eukaryotic cell was probably an amoeba-like cell that got nutrients by phagocytosis and contained a nucleus that formed when a piece of the cytoplasmic membrane pinched off around the chromosomes. Some of these amoeba-like organisms ingested prokaryotic cells that then survived within the organism and developed a symbiotic relationship. Mitochondria formed when bacteria capable of aerobic respiration were ingested; chloroplasts formed when photosynthetic bacteria were ingested. They eventually lost their cell wall and much of their DNA because they were not of benefit within the host cell. Mitochondria and chloroplasts cannot grow outside their host cell.

Evidence for this is based on the following:

1. Chloroplasts are the same size as prokaryotic cells, divide by binary fission, and, like bacteria, have Fts proteins at their division plane. The mitochondria are the same size as prokaryotic cells, divide by binary fission, and the mitochondria of some protists have Fts homologs at their division plane.
2. Mitochondria and chloroplasts have their own DNA that is circular, not linear.
3. Mitochondria and chloroplasts have their own ribosomes that have 30S and 50S subunits, not 40S and 60S.
4. Several more primitive eukaryotic microbes, such as Giardia and Trichomonas have a nuclear membrane but no mitochondria.

Although evidence is less convincing, it is also possible that flagella and cilia may have come from spirochetes.

Figure $$\PageIndex{1}$$: One model for the origin of mitochondria and plastids. This model has an amitochondriate eukaryote engulfing an aerobe and then a cyanobacterium. from Kelvinsong

Example $$\PageIndex{1}$$

1. Briefly describe what is meant by the endosymbiotic theory.
2. Give three points of evidence supporting the theory that mitochondria and chloroplasts may have arisen from prokaryotic organisms.

Solutions

1. The endosymbiotic theory states that some of the organelles in eukaryotic cells were once prokaryotic microbes.
• Mitochondria and chloroplasts are the same size as prokaryotic cells and divide by binary fission.
• Mitochondria and chloroplasts have their own DNA which is circular, not linear.
• Mitochondria and chloroplasts have their own ribosomes which have 30S and 50S subunits, not 40S and 60S.

## Summary

The endosymbiotic theory states that mitochondria and chlopoplasts in today's eukaryotic cells were once separate prokaryotic microbes.

This page titled 7.8: The Endosymbiotic Theory is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Gary Kaiser via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

The gene expression processes in the mitochondria and chloroplast are similar to bacterial processes.

#### Explanation:

Both mitochondria (the organelle that produces energy) and chloroplast (photosynthetic machinery) have their own circular DNA. (The DNA molecules present in the nucleus of eukaryotic cells are in the form of strings and are not circular.) We know that circular DNA is more primitive as seen in all bacteria, linear DNA definitely evolved later.

Mitochondria and chloroplasts are both present in eukaryotic cells, but they can both undergo binary fission as they were prokaryotic cells. This process is schematically shown below.

(

)

Gene expression involves DNA to RNA transcription and translation of RNA into protein. This whole machinery is present within mitochondria and chloroplast in prokaryotic condition (They are thus called autonomous organelles).

Therefore gene expression similar to the bacterial system can be regarded as evidence that supports the endosymbiotic theory of the origin of eukaryotic cells from prokaryotic cells.

### What are 3 evidences of the endosymbiotic theory?

Evidence for endosymbiosis.
Membranes — Mitochondria have their own cell membranes, just like a prokaryotic cell does..
DNA — Each mitochondrion has its own circular DNA genome, like a bacteria's genome, but much smaller. ... .
Reproduction — Mitochondria multiply by pinching in half — the same process used by bacteria..

### What evidence supports the endosymbiotic theory quizlet?

The endosymbiotic hypothesis is supported by the physiology and metabolism of chloroplasts and mitochondria with the sequence and structures of their genomes.

### What is the strongest evidence in support of the endosymbiotic theory?

Protein import is the strongest evidence we have for the single origin of chloroplasts and mitochondria. It is probably also the strongest evidence we have to sort out the number and nature of secondary endosymbiotic events that have occurred in evolution involving the red plastid lineage.