Chloroplasts and mitochondria: structure and function

Biology

The Invader Within Eukaryotic organisms are defined by the existence of organelles within the cell format. Two of the most important of these organelles are chloroplasts and mitochondria. Both organelles serve to manufacture energy within the cell; the chloroplast uses sunlight in this reaction, while the mitochondria does not. Together these organelles are unique in that they, of all the organelles, have their own DNA separate from the nucleic DNA, and are confined within double (triple, or quad) membranes, which show a unique inversion. Because of the unique structure and purpose of these organelles, several theories have arisen considering how they may have developed. The best of these, the endosymbiosis hypothesis, suggests that mitochondria and choloroplasts were originally independent single cell organisms that were incorporated into larger cells where they survived in a symbiotic fashion before eventually becoming so integrated into cell functioning that they were neither independent nor disposable.

Before analyzing the most successful theory, one ought to quickly analyze less respectable theories. Apart from the research originating with Margulis or the deistic claims of creationists, it appears the next-best theory is that mitochondria and chloroplasts evolved "step-wise" (Armstrong) by elaborating on existing structures within the cell. This is the autogenesis hypothesis. Such a hypothesis appears to be feasible for many organelles, such as the cell nucleus. However, when tested against the evidence, this hypothesis has significant flaws. For example, if organelle developed within the cell, then the cytoplasm of the organelle should be similar to that of the cell, its membranes should be compatible with other cell membranes, and there should be evidence of intermediate forms. All of these things appear in the case of the nucleus, and there are a number of organisms with "intermediate nuclear organizations." (Armstrong) However, in the case of mitochondria and chloroplasts, none of these signs are found. Mitochondria not only have different ribosome sizes than surrounding cytoplast, they also can reproduce independently, cannot be reproduced by the cell spontaneously, and are never found in intermediate stages. This seems a major flaw to the autogenesis hypothesis.

The endosymbiosis theory, on the other hand, seems to stand up to further research and experimentation. This theory points to the fact that bacteria are capable of entirely encompassing other bacteria, forming symbiotic relationships with them, and maintaining them alive within their own structures. This has been shown in laboratory tests, and witnessed in nature. In one laboratory experiment, bacteria exposed to high levels of pathogenic bacteria over several hundred generations eventually adapted "their progeny became dependent on having the formerly pathogenic bacteria in food vacuoles...(Jeon, 1991)" (Armstrong) There are several ways in which bacteria may subsume other bacteria, including ingesting them and maintaining them in food vacuoles as in the Jeon experiment, or they may become infected by bacteria that are acting as parasites. Mitochondria, for example, could have been parasitic and fed off the host at the same time that they proved useful to it. Chloroplasts, because they are significantly self-supporting, are more likely to have been introduced as food. This theory continues to suggest that after many generations of true symbiosis, the mitochondria and chloroplasts lost their independence.