Essay 10 07/12/04: Microbiology, Evolutionary Biology
| Endosymbiosis Decades of research and analysis has led microbiologists to conclude that individual animal and plant cells are fundamentally more complex than bacterial cells. Years of inquiry into how this complexity might have arisen has allowed for the development of a single truly comprehensive explanation called endosymbiosis theory. This theory predicts that very simplistic, bacterial (prokaryotic) cells must have physically joined together to develop more complex, more highly functioning cells. One of the most interesting, logical consequences of this theory reveals that neither plants nor animals could have evolved if it were not for endosymbiosis. Before the arrival of plants and animals, over three billion years ago, there was relatively little variety in body plan. Most cells at this time were chemoautotrophs, they gathered energy from the inorganic matter available to them deep within the ocean. These cells were able to engulf specific molecular compounds and break them down chemically in order to release the potential energy that they housed. This method of energy extraction was anaerobic (did not necessitate oxygen) and was therefore very inefficient. It is thought that these organisms might have depleted their chemical resources and that out of this vacuum other more complex forms of cells evolved. One of the first types of cells to evolve in response to the food shortage was another kind of autotroph that used photosynthesis (yes, just like plants) to take energy from sunlight and use it to create chemical bonds between available molecules in order to live and grow. Some of these single celled organisms were very successful, and they proliferated. These photoautotrophs, called cyanobacteria physically and genetically joined with other single-cell life forms. The product of the symbiosis between these two types of bacteria created more complex cells. These more behaviorally versatile (eukaryotic) cells allowed for the development of some of the most evolutionarily successful life forms, including algae and green plants. The chloroplasts (the main cellular component involved in photosynthesis) that we observe within each cell of modern day plants were originally a type of cyanobacteria. But the story does not end here. Before green plants came to be, another type of early cell evolved. This cell was able to burn energy in a much more efficient way, it burned chemicals aerobically (using oxygen to perform oxidation). This type of organism was heterotrophic, meaning that it required organic energy in the form of other living things (you and I are heterotrophic). Most evolutionary biologists think that these types of organisms evolved to eat the proliferating multicellular and unicellular cyanobacteria. Well, this type of energy consumption proved to be wildly efficient and it allowed these heterotrophic cells a great deal of energy that they could then turn into kinetic energy (the energy of movement). Just like the cyanobacteria, these heterotrophic cells began to experiment with multicellularity. Today, within the cells of each animal (including you and me) there exists a mitochondrion which allows our cells to quickly and efficiently burn the organic chemicals that we consume. These mitochondria are contained within our cells because of endosymbiosis. They are symbiotic bacterial cells that have been employed for hundreds of millions of years by animal cells to burn energy. These mitochondria are not aliens though, they are as much a part of us as any of the other bacterial cells that merged to create our eukaryotic, heterotrophic cells. One of the strongest pieces of evidence that supports the theory of endosymbiosis is the fact that mitochondria (and chloroplasts for that matter) have their own DNA, and this DNA is extremely similar to bacterial DNA. You may have observed that plants do not seem to expend a great deal of energy in comparison to animals, this is because they formed an alliance with a photosynthetic symbiont instead of an aerobic one. Animals have more energy, but we have to move, and work to procure it. Plants on the other hand can be sedentary, and they get their food from the water and air, and their energy from the sun. Animals are very much reliant on plants; most animals either eat plants or eat other animals that eat plants. Plants even provide us with the oxygen that we need to burn the food that we get from them. At the same time, plants are also very much reliant on us. Just as we use the oxygen that they produce, plants use the carbon dioxide that we exhale. Plants also use the dead bodies of animals as fertile soil to grow roots in and to extract necessary minerals and nutrients from. For me, the most amazing relationship to arise out of the endosymbiosis between bacterial cells is a distinctly different form of symbiosis, one between animals and plants. Autotroph: noun An organism capable of synthesizing its own food from inorganic substances, using either light or chemical energy. Green plants, algae, and certain bacteria are autotrophs. Chemoautotroph: noun An organism, such as a bacterium or protozoan, that obtains its nourishment through inorganic chemical compounds. Endosymbiosis: noun Symbiosis in which a symbiotic organism lives within the body of its partner. Also the widely held theory that eukaryotic organelles were originally acquired by endosymbiosis. Heterotroph: noun An organism that cannot synthesize its own food and is dependent on complex organic substances for nutrition. Photoautotroph: noun An organism capable of synthesizing its own food from inorganic substances using light as an source of energy. Green plants and photosynthetic bacteria are photoautotrophs. Symbiosis: noun A close association between two or more different organisms of different species that may, but does not necessarily, benefit each member. |
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