Origin of Mitochondria in Eukaryotic CellsBy Kristina Penniston, 7/97 The inner organization of the living cell, consisting of specialized organelles, makes complex forms of life possible. It is indisputable, given the fossil records, that single-celled organisms with little or no intercellular organization once dominated the earth. At what many believe was the start of life (as we define it), blue-green algae ruled the planet but went into decline after about 1.6 billion years. Sowing the seeds of their own demise by producing oxygen which, at a critical mass point, could no longer be absorbed by the oceans and accumulated in the earth's atmosphere, the blue-green algae gave way to other cell-based organisms which could grow in an oxygenated environment. These new organisms marked the origins of the eukaryotic cell, estimated to have occurred when the oxygen level rose to about 3% of its present atmospheric level (Crawford and Marsh, 1995, p. 69). While this general chronology is fairly well accepted as fact, there is debate about how the eukaryotes arrived on the scene, specifically, how the eukaryotic mitochondrion originated. Strict Darwinists or proponents of natural selection theory would argue that the blue-green algae mutated and made selectively advantageous mutations over millions of years until, through competition and selection, a winning combination of traits allowed the algae to survive in the presence of oxygen, albeit as a changed life form. Others believe that symbiosis, more specifically, endosymbiosis, was the basis for the first eukaryote. This theory, popularized by Lynn Margulis in her 1981 book, Symbiosis in Cell Evolution, advocates the following chronology:
How did both species benefit? The ingested aerobic bacterium received nutrients from the host while the host received energy from the aerobic activity of the bacterium. While there are many applications of the symbiosis theory of cell evolution, the subject of this discussion is the origin and function of mitochondria in eukaryotic cells. In the scenario presented above, the aerobic bacterium which was ingested by the anaerobic bacterium was proto mitochondrion, in other words, the organism which made possible the production of energy from oxygen. Symbiosis, the relationship of mutual benefit between two species such as the one described above, is believed by some to have been the process by which mitochondria became organelles in eukaryotes. Before delving into the evidence for the theory, however, the functions and structure of the present-day mitochondrion must be addressed. Mitochondria are the eukaryotic organelles which carry out oxidative respiration, the final step in cellular respiration. Oxidative respiration breaks down the pyruvate formed from glycolysis to form carbon dioxide and produces the majority of the cell's ATP. Oxygen is necessary for the eukaryotic cell because mitochondria use it as the terminal electron acceptor in the electron transport chain, ultimately resulting in a proton gradient which drives ATP synthesis. Mitochondria are present to varying degrees in different eukaryotic cells. Cells requiring lots of energy such as in muscle tissue and liver have proportionately more mitochondria than cells requiring less energy such as bone. What is the structure of the mitochondrion? The features examined here are size, membrane structure, protein status, and genetic information. Mitochondria are one of the larger organelles in the eukaryotic cell, ranging from 0.3-1.0 &m by 5-10 &m. It has two membranes, the innermost of which folds in at many points along its perimeter in a formation resembling a maze. These infoldings, called cristae, are the launch pad of enzymes and electron carriers (cytochromes) responsible for electron transport and oxidative phosphorylation. The manner in which the cristae are arranged keeps these enzymes segregated according to their utility, an example of the high level of organization within the mitochondrion. Mitochondria are unique among all other organelles because they contain their own DNA, i.e., separate from the cell nucleus' DNA, and its configuration is circular. Ribosomes in the mitochondrion produce some of the organelle's proteins according to its own, independent DNA. What is the evidence supporting the endosymbiotic theory of the mitochondrion and the eukaryotic cell? Some of the most convincing evidence supporting the symbiosis theory was just outlined in the preceding paragraph on mitochondrial structure. If mitochondria were once free-living bacteria, the thinking goes, they could be expected to exhibit some vestigial remnants of their former condition even though they are organelles today. Along these lines, examination of six issues follows. In general, mitochondria and bacteria are basically the same size. This cannot be said about the other eukaryotic organelles.
A closer look at mitochondrial ribosomes reveals even more supportive evidence for symbiosis theory. Mitochondrial ribosomes have more similar antibiotic sensitivities with bacterial ribosomes than with eukaryotic ribosomes. For example, cycloheximide blocks eukaryotic ribosomes by affecting tRNA transfer but affects neither mitochondria or bacteria. On the other hand, drugs that block prokaryotic synthesis but not eukaryotic synthesis block mitochondrial protein synthesis as well, e.g., erythromycin and tetracycline (Margulis, 1981, p. 217-218). The structural analogies between mitochondria and bacteria are compelling but not at all conclusive. Many have questioned the plausibility of the symbiotic theory -- that there even existed a free-living proto mitochondrion at the advent of oxygen (fossil records aren't, of course, conclusive), that it somehow entered a proto eukaryote, was a partner in a symbiotic relationship that eventually resulted in proto mitochondrion yielding its autonomy to the larger cell, giving way to the eukaryote. Is this too much to swallow? Because evolution is, in many ways, the history of various chemical reactions from the earthís formation to the biochemical reactions in living cells, let us examine the plausibility of symbiotic theory. First, it isn't inconceivable that free-living aerobic bacteria, producing high-energy molecules such as ATP, would enter into a relationship in which that energy, the likes of which had not existed prior to the advent of oxygen (aerobic respiration represented a new level of efficiency), could be used. Second, the production of all that energy must have required an enormous input of energy, i.e., a plentiful, available, efficient nutrient source. Third, the advent of oxygen, a poisonous gas to the vast majority of the inhabitants of the earth at that point, required new forms of metabolism based on a new chemistry. Finally, without a means to metabolize the oxygen in the atmosphere, proto eukaryote would have a more difficult time surviving. The basis for the symbiosis is clear: the larger, anaerobic organism provided a steady source of nutrients as well as phospholipids for the mitochondrial membranes (Crawford and Marsh, 1995, p. 71-72) to the smaller, aerobic bacterium. In return, the energy provided by the respiration of oxygen allowed the host to survive and further adapt to the world's new conditions. The symbiotic theory is bolstered by natural, observed examples of symbiotic relationships. Certain marine fish are able to emit light due to the presence of luminous bacteria in their interior. These luminous bacteria also live freely in seawater, but do not present luminosity (Dyer and Obar, 1985, p. 127). Other categorical examples of symbiosis are represented by the relationships between various fungi and cyanobacteria, algae and plants, and bacteria and mammals (Margulis, 1981, p. 165). While the transformation of these symbionts to organelles is quite a leap if provided only with examples of present-day symbiotic relationships such as those above, it must be remembered that this transformation, if indeed it transpired, occurred over millions of years. Moreover, it is doubtful that we humans, during the brief tick of the evolutionary clock in which we have existed, have experienced the kind of change to our environment that the advent of oxygen represented in its time. We cannot possibly know, with our fleeting existence on Earth, what kinds of evolutionary leaps and bounds are possible with such monumental change. For the academic/philosophical reasons just described and with the physical and chemical facts gleaned through scientific experimentation in mind, one cannot dismiss the theory that mitochondria evolved as organelles in the eukaryotic cell from free-living, aerobic bacteria in a newly-oxygenated world. Questions do remain, however, and should provide for interesting study. How (and from what) did proto mitochondrion evolve alongside the blue-green algae? By what means did proto mitochondrion enter the larger, anaerobic cell? Did examples of free-living proto mitochondrion survive? Do they or their progeny exist today? Are other eukaryotic organelles also derived from free-living organisms? But for now, the evidence demonstrates that the symbiosis theory of cell evolution is on sound footing. REFERENCE CITATIONS 1. Crawford, Michael and David Marsh, 1995. Nutrition and Evolution, p. 65-83. Keats Publishing, Inc., New Canaan, CT. 2. Dyer, Betsey Dexter and Robert Obar (editors), 1985. The Origin of Eukaryotic Cells, Van Nostrand Reinhold Company, Inc., NY. Papers by: a.) Goksøyr, J., 1967, "Evolution of Eucaryotic Cells;" b.) Schwartz, R.M. and M.O. Dayhoff, 1978, "Origins of Prokaryotes, Eukaryotes, Mitochondria, and Chloroplasts;" c.) Raven, P.H., 1970, "A Multiple Origin for Plastids and Mitochondria;" d.) Doolittle, W.F., 1980, "Revolutionary Concepts in Evolutionary Cell Biology;" and e.) Smith, D.C., 1979, "From Extracellular to Intracellular: The Establishment of a Symbiosis." 3. Margulis, Lynn, 1981. Symbiosis in Cell Evolution, p. 206-227. W. H. Freeman and Company, San Francisco. 4. Prescott, L.M., J.P. Harley and D.A. Klein, 1996. Microbiology, third edition. Wm. C. Brown Publishers, Dubuque, IA. |