History of the Endosymbiotic Theory
The Endosymbiotic theory, also known as Symbiogenesis, is an evolutionary theory on the origin of eukaryotic cells from prokaryotic organisms.
The theory was first proposed in the years 1905 and 1910 by Konstantin Mereschkowski, a Russian Botanist.
The Endosymbiotic theory was later substantiated in 1976 by Lynn Margulis, a former Boston University Biologist. She later wrote about the Endosymbiotic theory in her book, “Symbiosis in Cell Evolution” in the year 1981.
Despite the fact that the theory is accepted today, back in the days, Margulis was ridiculed by her fellow mainstream biologists. They initially never took the argument seriously.
It is because of her hard work, persistence, and the large volumes of data that she gathered to support her hypotheses, that she got a plausible explanation for the evolution of eukaryotes.
It is from Dr Margulis research regarding eukaryotes, prokaryotes, and organelles, that she proposed that the similarities between organelles and prokaryotes, together with their appearance in the fossil records, could only be well described by “endo-symbiosis.”
Due to the fact that the endosymbiotic theory tries to bring out an understanding of the origin of chloroplasts and mitochondria, it would be better if we start with a brief overview of the meaning of a cell, mitochondria and chloroplasts.
What is a Cell?
A cell is a basic building block of any living thing. Our human bodies are made up of trillions of cells.
Cells provide structure for the body, take nutrients from food, and most importantly, convert the nutrients into energy.
Cells have numerous parts, and each part has its own important function. These cell parts are also known as organelles.
Cells also contain the body’s hereditary material, also known as DNA.
Prokaryotes are small and simple and have rings of circular DNA floating free inside the cell.
Eukaryotes are more complex and large. They have a nucleus, that holds strings of linear DNA within the lipid membrane.
All animals, fungi and plants are made up of eukaryotic cells.
On the other hand, the bacteria, are made up of prokaryotic cells.
The prokaryotic cells were the earliest life forms on earth. They first appeared around 4 billion years ago.
The ancestor of all eukaryotic cells was the prokaryote.
All eukaryotes have an organelle known as the mitochondrion, which makes the required energy to power the cell.
Plants have an organelle known as a plastid.
Plastids harvest energy from the sun.
Mitochondria and Chloroplast
Mitochondria and chloroplasts are two organelles found in eukaryotic cells.
Chloroplast is found in plant cells while most of eukaryotic cells contain mitochondria.
Chloroplasts are a type of plastid.
Despite the fact that both organelles are found in eukaryotic cells, both chloroplast and mitochondria have characteristics similar to those of prokaryotic cells.
Characteristics of prokaryotic Cells include:
- Circular DNA
- Bacteria like ribosomes
- Enclosed double membrane.
Both chloroplasts and mitochondria conduct prokaryotic activities. Chloroplast performs photosynthesis while on the other hand, mitochondria performs respiration.
The above characteristics of both chloroplast and mitochondria are the heart and origin of the endosymbiotic theory.
The Meaning of Endosymbiotic Theory
The endosymbiotic theory deals with the origin of chloroplasts and mitochondria.
The chloroplasts and mitochondria are eukaryotic organelles that also have bacteria characteristics.
They are believed to have developed from symbiotic bacteria specifically, the cyanobacteria and alpha-proteobacteria respectively.
The endosymbiotic theory states that a prokaryotic cell was engulfed or consumed by a larger cell.
However, for an unknown reason, the prokaryotic organelle was not consumed.
The lack of consumption of the prokaryotic organelle by the larger cell resulted to mutualism, whereby the two received survival benefits from each other.
Over time, the newly combined organelle would develop into a complex eukaryotic cell of today.
The theory is widely accepted as one of the possibilities of the origins of chloroplasts and mitochondria, and other eukaryotic cells and organelles.
To have a better understanding of the meaning of “endo-symbiosis” we could break the term into three:
Endo = Within
Symbiosis = Together
Bio = Living
Also take a look at this,
Endocytosis = (cyto = cell) a process of “cell eating” meaning that cells are engulfed, but then eaten as food,
Endosymbiosis = cells are engulfed but not eaten or digested. The cells live together in a mutually benefiting relationship.
Dr. Lynn Margulis’ hypothesis originally proposed that:
- Mitochondria are formed as a result of endocytosis of aerobic bacteria.
- Chloroplasts, on the other hand, are formed as a result of endocytosis of photosynthetic bacteria.
- In both cases by large anaerobic bacteria who would not otherwise be able to exist in an aerobic environment.
- This arrangement became a mutually beneficial relationship for both cells, hence a symbiotic relationship.
The key to the success of eukaryotic cells are two powerful and mutually exclusive organelles; the chloroplasts and the mitochondrion.
The chloroplasts consume carbon dioxide and water, capturing energy from sunlight and funnels it into the chemical energy of glucose, hence releasing oxygen in the process.
The mitochondrion, on the other hand, consumes oxygen to efficiently extract energy from carbon sources such as glucose, hence producing water and carbon dioxide in the process.
The endosymbiotic theory, proposes that these two organelles (chloroplasts and mitochondrion), were once prokaryotic cells, living inside larger host cells.
The prokaryotes may have initially been parasites or even an intended meal for the larger cell, but they somehow escaped digestion.
It is worth noting that whatever the reason for their initial internment, the prokaryotes seem to have become willing prisoners to a grateful warden, the larger host cells.
The prokaryotes may have furnished the larger host cells with necessary nutrients (in the case of the primitive chloroplasts) or helped to exploit oxygen for extracting energy (in the case of the primitive mitochondrion).
In return, it seems the prokaryotes received a steady environment to live in and, most importantly, protection.
In a nutshell, the endosymbiotic theory explains that the chloroplasts and mitochondria in today’s eukaryotic cells were at some point in the past separate prokaryotic microbes.
Diagrammatic Representation of the Endosymbiotic Theory
endosymbiotic theory evidence
There are numerous lines of argument that mitochondria and plastids including chloroplasts arose from bacteria.
- Chloroplasts and mitochondria have an intriguing similarity in structure, biochemistry, reproduction, and genetic makeup to certain prokaryotes.
- The obvious fact that chloroplasts and mitochondria have any genetic makeup of their own supports the endosymbiotic theory.
- New mitochondria and plastids are only formed through binary fission, a form of cell division used by archaea and bacteria.
- If a cell’s chloroplasts or mitochondria are removed, the cell loses the means to create new cells. For example, in Euglena, a form of algae, the plastids can be destroyed through a prolonged absence of sunlight without otherwise affecting the cell. In such a scenario, the plastids will not regenerate.
- Porins are transport proteins. They are found in the outer membranes of the chloroplasts and mitochondria. They are also found in bacterial cell membranes.
- A membrane lipid cardiolipin is exclusively found in the inner mitochondrial membrane and bacterial cell membranes.
- Some plastids and mitochondria contain single circular DNA molecules that take after the DNA of bacteria both in structure and size.
- Genome comparisons suggest a close relationship between mitochondria and Rickettsial bacteria.
- Genome comparisons suggest a close relationship between plastids and cyanobacteria.
- Numerous genes in the genomes of mitochondria and chloroplasts have been lost or transferred to the nucleus of the host cell. Consequently, the chromosomes of many eukaryotes contains genes that originated from the genomes of mitochondria and plastids.