The Endosymbiont Hypothesis and the evolution of the Chloroplast and Mitochondria
Dr Lynn Margulis is seen as the first person to have put forward the Endosymbiont Hypothesis which is based on a theory which explains the likely origin of the mitochondria and chloroplast (plants) in eukaryote organisms which we observe today. Dr Margulis received evidence from all over the world and from many scientific researchers and experiments, Margulis simply had to put all the evidence together to form her hypothesis. Margulis states that the eukaryote cell organelles, mitochondria as well as the chloroplast in plants, arose from simple prokaryote organisms which formed an endosymbiotic relationship with the early eukaryote cells. Prokaryotes are believed to be the first inhabitants of the earth, they are primitive organisms that do not possess a true nucleus(Starr et al, 2009). It is thought that the prokaryotes gave rise to the complex eukaryotes of today (Trager, 1970). These cells which have a membrane bound nucleus which also possesses the endomembrane system, parts of this system were thought to have differentiated to form the mitochondria and plastids, this particular idea being the Autogenous theory (Trager, 1970). When two organisms live with one another and form a bond where both benefit from the interaction we refer to the relationship as a mutualistic symbiotic relationship; however, when one of the organisms lives and reproduces inside the other to the benefit of both, it is referred to as an endosymbiotic relationship (Starr et al, 2009). Life on earth was probably not likely without the presence of photosynthetic organisms, namely the characteristic prokaryotic cyanobacteria which is a simple photosynthetic prokaryote which has existed on earth since the prehistoric eras. Trager explains that it is believed that the cyanobacteria formed an endosymbiotic relationship with a eukaryotic plant cell, essentially allowing an early eukaryotic cell to possess a photosynthetic bacteria which the eukaryote could use to its advantage and gain extra nutrients from the photosynthetic products. The bacteria developed into the plastid organelle known as the chloroplast, which gave the eukaryote photosynthetic properties. The plant, which the eukaryote cells belonged to, had now gained an advantage over other organisms and could now colonize the earth because plants were now autotrophs, can produce their own food(Starr et al,2009). The chlorophyll pigment in the cyanobacteria could be used to harness the light energy and convert it into chemical energy. Margulis hypothesized that chloroplast are derived from cyanobacteria and that mitochondria are derived from a aerobic bacteria. About 3.8 billion years ago there were only anaerobic prokaryotes on earth due to the lack of oxygen in the immature atmosphere surrounding the earth. However, due to variation in the population, photosynthetic bacteria arose around 3.2 billion years ago, these bacteria could harness the abundant light energy (cyanobacteria). As a result of the photosynthetic prokaryotes in the environment, the Oxygen levels in the atmosphere began to rise. Oxygen is a biproduct of photosynthesis (Starr et al, 2009). The rise in Oxygen levels caused a change in the atmosphere composition, this was not ideal for the primitive anaerobic prokaryotes as they were not used to the new habitat. Only those that were more adaptive to the environment could survive and those prokaryotes became more and more aerobic over time. This is a summary of the explanation from the website, Endosymbiotic Hypothesis. It was believed that the aerobic prokaryotes existed on earth, but they were relatively inefficient in getting food (nutrients) from the environment by means of phagocytosis (Mauseth, 2009). Phagocytosis is a process by which the cell engulfs food particles, also known as" cell eating" (Starr et al, 2009). They were however; very effective in producing energy by means of aerobic...
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