There are many similarities that exist between semi-autonomous organelles, such as mitochondria and chloroplasts, and forms of prokaryotes present today. These similarities help support the Endosymbiotic Hypothesis, since the theory states that early prokaryotes, with traits similar to these semi-autonomous organelles, were engulfed by a much larger heterotrophic organism and formed a symbiotic relationship with it.

  • Size

One of the more superficial similarities between the semi-autonomous organelles and bacterial species is the relative size of the two. On average, eukaryotes range from sizes of approximately 50-500 microns, dwarfing the average prokaryote that only has a rough size of 1-10 microns. However, these prokaryotes have the exact same size range as that of the semi-autonomous organelles. While this observation does not directly support the theory, if there existed a significant size difference between prokaryotes and these organelles, then there would exist a major hole within the theory.

  • Actions performed by bacteria as compared to:

1.   Mitochondria

Current mitochondria exhibit characteristics that are very similar to a class of bacteria known as alphaproteobacteria, more specifically the order of Rickettsiales. They are similar to this order in that they produce ATP in the exact same manner. Both mitochondrion and Rickettsiales bacteria use the Krebs Cycle and Oxidative Phosphorylation in order produce ATP. Also a genus with the Rickettsiales order, known as Rickettsia, must also depend on entry and replication within a host cell. These aerobic bacterial cells, informally known as “purple cells”, are believed to have developed into what we know as mitochondria today.

File:Rickettsia rickettsii.jpg

An example of gram stained Rickettsia rickettsi in tick hemolymph cell

2.  Chloroplasts

Chloroplasts exhibit traits that closely resemble photosynthetic cyanobacteria. Both are able to harness light energy, store it in molecules like ATP, and use it to make organic molecules which can be used for energy later.

3. Both

Mitochondria and chloroplasts are both able to replicate through a process similar to binary fission independent of the host cell. They do not undergo mitosis like eukaryotic cells, but through means that are strikingly similar to that of modern pro karyotic cells.

  • DNA and Ribosomes

Another similarity that exists between semi-autonomous organelles and bacterial cells is the DNA that is contained within them. Within every mitochondria and chloroplast is a single, circular band of genetic material that is unlike any of the linear DNA that is within the nucleus of the host eukaryotic cell. However  these circular chromosomes are more similar to the single, circular chromosomes of DNA found in prokaryotic cells.
The proteins and enzymes that are present within the organelles perform work that is used only within these organelles and no other place within the cell. When compared to proteins found within prokaryotes, they were shown to be similar in size and even nucleotide sequence. Also, within a typical eukaryotic cell, the specific type of ribosome found is referred to as an “80 S” ribosome. The ribosome that is commonly found in semi-autonomous organelle, and consequently prokaryotes, is a type of ribosome known as a “70 S” ribosome. All of these each provided substantial evidence in favor of the theory.

An example of Mitochondria Replicating, Independently of the Host Cell

Presence of Double Phospholipid Bilayer

Another substantial piece of evidence that supports likelyhood of the endosymbiotic hypothesis is the presence of a double phospholipid bilayer encapsulating both the mitochondria and chloroplasts. The innermost of these layers is like nothing that is found present in any other membranes in the cell. This further supports the idea that a smaller organism was engulfed by a larger heterotrophic cell via endocytosis. This would place the smaller organism within a vesicle and bonding with it to form a symbiotic relationship with the host cell. This would othe innermost layer of both mitochondria and chloroplasts being unlike the membranes present in the host cells, while the outermost layer is more similar to the host cell. This concept is simplified and represented in the following video:

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