Reflection 3: The Endosymbiotic Theory

The Endo-What? Today we are going to look into what exactly is the Endosymbiotic Theory, who came up with it & the evidence that supports this theory.

In class after Mr. Matthew’s Cell lecture he asked us to do some research on the Endosymbiotic theory, not knowing to us that we would be questioned about it in our tutorial sessions that same week.

What is the Endosymbiotic Theory?

The Endosymbiotic theory proposed that the Mitochondria and Chloroplast were once prokaryotic cells living inside larger hosts cells.

Who came up with the Endosymbiotic Theory?

Konstantin Mereschkowski was the first to put forth the hypothesis for plants/chloroplasts in 1905 based on Schimper’s comment about similarity, and Wallin extended the idea to mitochondria. However, Lynn Margulis popularized the idea by use of the electron microscope and studies in molecular biology.

What evidence is there supporting this theory?

1. Mitochondria and Chloroplast have their own DNA.

2. They reproduce themselves independently of the cell cycle, and in that way it is similar to the binary fission of prokaryotes

3. They have small 70S ribosomes just like prokaryotes. These are much smaller than the 80S ribosomes found in the rest of the eukaryotic cell.

4. They have double membranes (the inner one could represent the original prokaryote cell membrane and the outer one may be an endocytotic vesicle membrane from the host cell when it endocytosed the prokaryote.)

If you found any more evidence please feel free to comment and let me know!!

Lastly, lets’ review the differences between Prokaroytic & Eukaryotic cells.

Feature

Prokaryote

Eukaryote

Size

Small about 0.5 micrometers

Up to 40 micrometers

Genetic Material

Circular DNA (in cytoplasm)

DNA in linear form (in nucleus)

Organelles

Few present, None membrane bound

Many organelles

           Double Membranes e.g. nucleus, mitochondria & chloroplast

            Single Membranes e.g. Golgi Apparatus, Endoplasmic Reticulum, Lysosomes

Cell Walls

Rigid formed from glycoprotein (Murein)

Animals- No cell wall

Plant- Cellulose

Fungi- Chitin

Ribosomes

70S

80S

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