Every cell needs energy to live , and while photosynthetic organisms, autotrophs , obtain it from light, we, consumers, obtain it by consuming other living beings. We are heterotrophic organisms.
Better to ferment or breathe?
Inside a cell, energy is stored in stable molecules, such as sugars or lipids. But for a cell to function, it requires other molecules that release their energy quickly and efficiently, and the most common energy exchange currency is ATP (adenosine triphosphate). This molecule is highly unstable and cannot be stored, which is why cells have different ways of transforming the energy of storage molecules into ATP, which they can immediately use in their metabolism.
Each type of molecule works in a different way; glycolysis to break down glucose, the reaction called β-oxidation for fatty acids, etc. When a cell cannot respire—there is no oxygen—the way to obtain energy is through fermentation . There are different types of fermentation, which give rise to different waste products: lactic fermentation, which produces lactic acid, or alcoholic fermentation, which produces ethanol and carbon dioxide. For each molecule of glucose that is fermented, 2 molecules of ATP are obtained, which can be used by the cell for its metabolism.
However, when breathing is possible thanks to the presence of oxygen, the scenario changes drastically. For this, the mitochondria , the cellular organelle that is responsible for cellular respiration, comes into play. This organelle, and always in the presence of oxygen, can oxidize a glucose molecule to form, in the end, no less than 38 ATP molecules , and its waste is just water and carbon dioxide.
a history of bacteria
Billions of years ago, no living thing was capable of breathing. The world was dominated by two types of organisms: photosynthetic , primary producers that obtained energy from sunlight and expelled oxygen in their metabolism, and consumers , who fed on the former and obtained energy by fermentation of compounds. organic that they produced.
Without other living beings to consume the oxygen released by photosynthetics, the atmosphere accumulated this gas, which turned out to be toxic to life forms, until some groups of bacteria acquired an adaptation that allowed them to use that oxygen in their metabolism. They turned a powerful toxin into a useful reagent. Natural selection favored this feature, and those organisms capable of performing this function, which we now know as respiration, found a great evolutionary advantage. The world was filled with bacteria capable of breathing.
But we, beings composed of eukaryotic cells, are not bacteria. Our cellular respiration occurs thanks to the mitochondria . What relationship do mitochondria have, then, with those bacteria capable of breathing? The answer, of course, is an evolutionary relationship.
The greatest symbiosis in the history of life
Somewhere relatively early in our evolutionary history, a very particular symbiosis event took place. A lineage of eukaryotic cells—those that enclose their DNA inside a membrane-bound nucleus—came into contact with a type of bacteria capable of breathing.
The process of symbiosis between these two organisms was not sudden, but the result of a gradual and gradual association, in which each of the protagonists acquired new adaptations so that this symbiosis would be more and more intimate.
The bacterium became intracellular , it lived inside the eukaryotic cell and a mutualism relationship was established, according to which the eukaryote protected the bacterium and its greater size and mobility provided it with a greater amount of food. For its part, the bacterium was capable of carrying out cellular respiration —until then, let us remember, the eukaryote could only carry out fermentation— and developed carrier proteins that allowed it to exchange ATP with the host cell. Mitochondria are, after all, bacteria that live inside cells .
This association, called endosymbiosis , was strongly favored by natural selection. In fact, today practically all eukaryotic organisms have mitochondria; the only exceptions are in the species of the genus Monocercomonoides and the giant amoeba Pelomyxa palustris ; but, although they lack mitochondria, they do have populations of archaea and endosymbiotic bacteria of other types.
What evidence do we have for endosymbiosis?
The first works that pointed to the endosymbiotic theory as the origin of mitochondria were those of the American biologist Lynn Margulis .
Among the evidence that confirmed this possible explanation was the type of ribosomes in the mitochondria, their genetic material —own and different from that of the eukaryotic cell in which they are found— and their type of membranes .
Ribosomes are small organelles that are responsible for protein synthesis; Bacteria and eukaryotes have different ribosomes. In this sense, the mitochondria have their own cellular machinery and their ribosomes are of the bacterial type, different from those of the host eukaryotic cell.
Similarly, mitochondrial DNA is arranged in a circular chromosome, typical of bacteria, and different from the linear chromosomes of the eukaryotic cell.
In addition, the mitochondria have a double membrane , and each one is different. The outermost layer—the one in contact with the cytoplasm of the host cell—is of the same type as the outer cell membrane, but the inner layer—the one in contact with the cytoplasm of the mitochondria—is of the bacterial type.
All these tests seemed to confirm that hypothesis, and thanks to genetic analysis, it is now widely accepted. In fact, we know that the bacteria that gave rise to mitochondria were α-proteobacteria from the Rickettsiales group . It is called the serial endosymbiosis theory.
The process is “serial”, because it has happened repeatedly throughout the history of life, and not only with mitochondria, but with other organelles. The chloroplasts of plants and many algae —green and red— have the same bacterial origin —with cyanobacteria, in this case—. And this process of serial endosymbiosis did not stop there. Some algae, such as haptophytes, euglenids and dinoflagellates of the genus Lepidodinium have other algae as endosymbionts, in what is called secondary endosymbiosis —an endosymbiosis of already endosymbiotic organisms—. And even more complexly, many dinoflagellates, and probably diatoms, have a tertiary endosymbiosis .
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Margulis, L. et al. 1998. Endosymbioses: cyclical and permanent in evolution. Trendsin Microbiology, 6(9), 342-345. DOI: 10.1016/S0966-842X(98)01325-0
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Whatley, J. M. 1976. Bacteria and Nuclei in Pelomyxa Palustris: Comments on the Theory of Serial Endosymbiosis. The New Phytologist, 76(1), 111-120.