Endosymbiotic Theory Definition
Endosymbiotic theory is the unified and widely accepted theory of how organelles arose in organisms, differing prokaryotic organisms from eukaryotic organisms. In endosymbiotic theory, consistent with general evolutionary theory, all organisms arose from a single common ancestor. This ancestor probably resembled a bacteria, or prokaryote with a single strand of DNA surrounded by a plasma membrane. Throughout time, these bacteria diverged in form and function. Some bacteria acquired the ability to process energy from the environment in novel ways. Photosynthetic bacteria developed the pathways that enabled the production of sugar from sunlight. Other organisms developed novel ways to use this sugar is oxidative phosphorylation, which produced ATP from the breakdown of sugar with oxygen. ATP can then be used to supply energy to other reactions in the cell.
Both of these novel pathways led to organisms that could reproduce at a higher rate than standard bacteria. Other species, not being able to photosynthesis sugars or break them down through oxidative phosphorylation, decreased in abundance until they developed a novel adaptation of their own. The ability of endocytosis, or to capture other cells through the enfolding of the plasma membrane, is thought to have evolved around this time. These cells now had the ability to phagocytize, or eat, other cells. In some cells, the bacteria that were ingested were not eaten, but utilized. By providing the bacteria with the right conditions, the cells could benefit from their excessive production of sugar and ATP. One cell living inside of another is called endosymbiosis if both organisms benefit, hence the name of the theory. The endosymbiotic theory continues further, stating that genes can be transferred between the host and the symbiont throughout time.
This gives rise to the final part of endosymbiotic theory, which explains the variable DNA and double membranes found in various organelles in eukaryotes. While the majority of cell products start in the nucleus, the mitochondria and chloroplast make many of their own genetic products. The nucleus, chloroplasts, and mitochondria of cells all contain DNA of different types and are also surrounded by double membranes, while other organelles are surrounded by only one membrane. Endosymbiotic theory postulates that these membranes are the residual membranes from the ancestral bacterial endosymbiont. If a bacteria was engulfed via endocytosis, it would be surrounded by two membranes. The theory states that these membranes survived evolutionary time because each organism retained the maintenance of its membrane, even while losing other genes entirely or transferring them to the nucleus. Endosymbiotic theory is supported by a large body of evidence.
Endosymbiotic Theory Evidence
The most convincing evidence supporting endosymbiotic theory has been obtained relatively recently, with the invention of DNA sequencing. DNA sequencing allows us to directly compare two molecules of DNA, and look at their exact sequences of amino acids. Logically, if two organism share a sequence of DNA exactly, it is more likely that the sequence was inherited through common descent than the sequence arose independently. If two unrelated organisms need to complete the same function, the enzyme they evolve does not have to look the same or be from the same DNA to fill the same role. Thus, it is much more likely that organisms who share sequences of DNA inherited them from an ancestor who found them useful.
This can be seen when analyzing the mitochondrial DNA (mtDNA) and chloroplast DNA of different organisms. When compared to known bacteria, the mtDNA from a wide variety of organisms contains a number of sequences also found in Rickettsiaceae bacteria. Fitting with endosymbiotic theory, these bacteria are obligate intracellular parasites. This means they must live within a vesicle of an organism that engulfs them through endocytosis. Like bacterial DNA, mtDNA and the DNA in chloroplasts is circular. Eukaryotic DNA is typically linear. The only genes missing from the mtDNA and those of the bacteria are for nucleotide, lipid, and amino acid biosynthesis. An endosymbiotic organism would lose these functions over time, because they are provided for by the host cell.
Further analysis of the proteins, RNA and DNA left in organelles reveals that some of it is too hydrophobic to cross the external membrane of the organelle, meaning the gene could never get transferred to the nucleus, as the cell would have no way of importing certain hydrophobic proteins into the organelle. In fact, chloroplasts and mitochondria have their own genetic code, and their own ribosomes to produce proteins. These proteins are not exported from the mitochondria or chloroplasts, but are needed for their functions. The ribosomes of mitochondria and chloroplasts also resemble the smaller ribosomes of bacteria, and not the large eukaryotic ribosomes. This is more evidence that the DNA originated inside of the organelles, and is separate completely from the eukaryotic DNA. This is consistent with endosymbiotic theory.
Lastly, the position and structure of these organelles lends to the endosymbiotic theory. The mitochondria, chloroplasts, and nuclei of cells are all surrounded in double membranes. All three contain their DNA in the center of the cytoplasm, much like bacterial cells. Although less evidence exists linking the nucleus to any kind of extant species, both chloroplasts and mitochondria greatly resemble several species of intracellular bacteria, existing in much the same manner. The nucleus is thought to have arisen through enfolding of the cell membrane, as seen in the graphic above. Throughout the world, there are various endosymbiont bacteria, all of which live inside other organisms. Bacteria exist almost everywhere, from the soil to inside our gut. Many have found unique niches within the cells of other organisms, and this is the basis of endosymbiotic theory.