Biologist Lynn Margulis first made the case for endosymbiosis in the 1960s, but for many years other biologists were skeptical. Although Jeon watched his amoebae become infected with the x-bacteria and then evolve to depend upon them, no one was around over a billion years ago to observe the events of endosymbiosis. Why should
we think that a mitochondrion used to be a free-living organism in its own right? It turns out that many lines of evidence support this idea. Most important are the many striking similarities between prokaryotes (like bacteria) and mitochondria: When you look at it this way, mitochondria really resemble tiny bacteria making their livings inside eukaryotic cells! Based on decades of accumulated evidence, the scientific community supports Margulis’s ideas: endosymbiosis is the best
explanation for the evolution of the eukaryotic cell. What’s more, the evidence for endosymbiosis applies not only to mitochondria, but to other cellular organelles as well. Chloroplasts are like tiny green factories within plant cells that help convert energy from sunlight into sugars, and they have many similarities to mitochondria. The evidence suggests that these chloroplast
organelles were also once free-living bacteria. The endosymbiotic event that generated mitochondria must have happened early in the history of eukaryotes, because all eukaryotes have them. Then, later, a similar event brought chloroplasts into some eukaryotic cells, creating the lineage that led to plants. Despite their many similarities, mitochondria (and chloroplasts) aren’t free-living bacteria anymore. The first eukaryotic cell evolved more than a billion years ago. Since then, these organelles have become completely dependent on their host cells. For example, many of the key proteins needed by the mitochondrion are imported from the rest of the cell. Sometime during their long-standing relationship, the genes that code for these proteins were transferred from the mitochondrion to its host’s genome. Scientists consider this mixing of genomes to be the irreversible step at which the two independent organisms become a single individual. Grabbing take-out: Paramecium bursaria packs a lunch  Paramecium bursaria, a single-celled eukaryote that swims around in pond water, may not have its own chloroplasts, but it does manage to “borrow” them in a rather unusual way. P. bursaria swallows photosynthetic green algae, but it stores them instead of digesting them. In fact, the normally clear paramecium can pack so many algae into its body that it even looks green! When P. bursaria swims into the light, the algae photosynthesize sugar, and both cells share lunch on the go. But P. bursaria doesn’t exploit its algae. Not only does the agile paramecium chauffeur its algae into well-lit areas, it also shares the food it finds with its algae if they are forced to live in the dark. Endosymbiotic theory
A eukaryotic cell is distinct from a prokaryotic cell by the presence of membrane-bound cellular structures called organelles. And based on this theory, the organelles mitochondria and chloroplasts are supposedly the early prokaryotic endosymbionts that had been taken in. They stayed inside the host cell for so long that they transitioned into those semi-autonomous organelles we know today. Endosymbiotic theory is one of the theories that are still prevalent to this day. It is a presumption that an endosymbiosis occurred between the early life forms. This form of symbiosis involves a larger cell that serves as a host and a smaller cell that is referred to as an endosymbiont. In endosymbiotic theory, it posited that the larger cell engulfed or took in the smaller cell. The larger cell represents the eukaryotic cell of today whereas the smaller cell is the prokaryotic cell. Watch this vid about endosymbiotic theory: EndosymbiosisEndosymbiosis is one of the many forms of symbiotic relationships (symbioses) that occur between or among organisms. In endosymbiosis, the endosymbiont lives within the body of its host. Endosymbiosis naturally occurs to this day. An example is a biological interaction between Rhizobium and the plant legumes. Rhizobium is the endosymbiont that occurs within the roots of legumes and fixes atmospheric nitrogen into a form that is ready for use by the legume. The legume, in turn, provides Rhizobium metabolites such as malate and succinate from photosynthesis. Endosymbiosis is the precept of the Endosymbiotic Theory, which was first conceptualized by botanist Konstantin Mereschkowski (4 August 1855 – 9 January 1921), and then backed up by scientific evidence by Lynn Margulis 1938–2011. According to the Endosymbiotic Theory, endosymbiosis became the means by which organelles such as mitochondria and chloroplasts within eukaryotic cells came about.1 Proponent of this theory posited that about 1.5 billion years ago a larger cell took in smaller free-living prokaryotes (bacteria) and inside the cell the prokaryotes lived as endosymbionts. Research findings that seem to back up this theory implicate that the mitochondria arose from proteobacteria (such as SAR11 clade) 2 whereas the chloroplasts arose from cyanobacteria (particularly the nitrogen-fixing cyanobacteria). 3 The indication that this theory is plausible is based upon the same features shared by these organelles and their prokaryotic ancestors. Some of the characteristics common to them are as follows:
Other Thoughts
The age of the Earth is estimated to be around 4.54 billion years and life eventually existed and began about 3.5 billion years ago or earlier. The modern theory of abiogenesis holds that life on Earth began when the earliest living entities took in non-living materials. They used these organic compounds to produce biomolecules and other building blocks of life. Biochemical processes, e.g. self-replication, self-assembly, autocatalysis, and cell membrane formation, probably led to the emergence of living entities. These processes were believed to be gradual and comprised of multiple events. In the Miller-Urey experiment, the results indicated that the simulated-primitive Earth favored the chemical syntheses of the fundamental structures of the cell membrane. Mixing gases methane, ammonia, hydrogen, and water and then electrically-sparking them resulted in the formation of amino acids.
Around four billion years ago, the Earth was hostile to life. No life forms could exist due to the harsh conditions. Eventually, simple organic compounds formed. The hypothetical model of the early Earth with conditions that led to the synthesis of simple organic compounds is called the prebiotic (primordial) soup. Alexander Oparin 1894–1980 and John Burdon Sanderson Haldane 1892–1964 were the ones to conceive this idea and independently formulated theories that collectively became the heterotrophic origin of life theory. Both of them theorized that the early Earth’s atmosphere was a chemically reducing atmosphere. It aided in producing such organic compounds. As these compounds were produced, they accumulated and formed a so-called prebiotic soup. Through time, these simple organic compounds transformed into more complex organic polymers. In the long run, life came about. The first life entities took in and used organic molecules to thrive and survive in the prebiotic soup. They theorized that the first forms of life were heterotrophic. Recent evidence, though, suggests that autotrophs are likely the first organisms.
The four major biomolecules essential to life are nucleic acids (e.g. RNA, DNA), carbohydrates (various sugars), lipids (fats), and amino acids (constituents of proteins). Primitive life is hypothesized to be RNA-based since RNA could be both genetic material and a catalyst. The transitioning of primitive life forms into single-celled living things occurred gradually for many million years. Take the Quiz on Endosymbiotic Theory! Further Reading
References
© Biology Online. Content provided and moderated by Biology Online Editors What are the ancestors of the mitochondria according to the endosymbiotic theory?According to the endosymbiotic theory, mitochondria derive from archeo bacteria, which built a structural and functional association with the cell.
What were mitochondria ancestors?Mitochondria and chloroplasts likely evolved from engulfed prokaryotes that once lived as independent organisms. At some point, a eukaryotic cell engulfed an aerobic prokaryote, which then formed an endosymbiotic relationship with the host eukaryote, gradually developing into a mitochondrion.
What were the ancestors of mitochondria according to the endosymbiotic theory quizlet?The endosymbiotic theory hypothesizes that the ancestors of modern-day eukaryotic cells were actually two or more prokaryotic cells in a symbiotic relationship.
What does the theory of endosymbiosis say about mitochondria?Explanation: The Endosymbiotic Theory states that the mitochondria and chloroplast in eukaryotic cells were once aerobic bacteria (prokaryote) that were ingested by a large anaerobic bacteria (prokaryote).
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What were the ancestors of mitochondria according to the endosymbiotic theory
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