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|  | Eucaryotic Cells Depend on Mitochondria for Their Oxidative Metabolism15
Mitochondria show many similarities to free-living procaryotic organisms: for example, they often resemble bacteria in size and shape, they contain DNA, they make protein, and they reproduce by dividing in two. By breaking up eucaryotic cells and separating their component parts, it is possible to show that mitochondria are responsible for respiration and that this process occurs nowhere else in the eucaryotic cell. Without mitochondria the cells of animals and fungi would be anaerobic organisms, depending on the relatively inefficient and antique process of glycolysis for their energy. Many present-day bacteria respire like mitochondria, and it seems probable that eucaryotic cells are descendants of primitive anaerobic organisms that survived, in a world that had become rich in oxygen, by engulfing aerobic bacteria - keeping them in symbiosis for the sake of their capacity to consume atmospheric oxygen and produce energy. Certain present-day microorganisms offer strong evidence of the feasibility of such an evolutionary sequence. There are several hundred species of single-celled eucaryotes that resemble the hypothetical ancestral eucaryote in that they live in oxygen-poor conditions (in the guts of animals, for example) and lack mitochondria altogether. Comparative nucleotide sequence analyses have revealed that at least two groups of these organisms, the diplomonads and the microsporidia, diverged very early from the line leading to other eucaryotic cells (Figure 1-21). There is another eucaryote, the amoeba Pelomyxa palustris, that, while lacking mitochondria, nevertheless carries out oxidative metabolism by harboring aerobic bacteria in its cytoplasm in a permanent symbiotic relationship. Diplomonads and microsporidia, on the one hand, and Pelomyxa, on the other, therefore resemble two proposed stages in the evolution of eucaryotes such as ourselves.
Acquisition of mitochondria must have had many repercussions. The plasma membrane, for example, is heavily committed to energy metabolism in procaryotic cells but not in eucaryotic cells, where this crucial function has been relegated to the mitochondria. It seems likely that the separation of functions left the eucaryotic plasma membrane free to evolve important new features. In particular, because eucaryotic cells need not maintain a large H+ gradient across their plasma membrane, as required for ATP production in procaryotes, it became possible to use controlled changes in the ion permeability of the plasma membrane for cell-signaling purposes. Thus, a variety of ion channels appeared in the eucaryotic plasma membrane. Today, these channels mediate the elaborate electrical signaling processes in higher organisms - notably in the nervous system -
and they control much of the behavior of single-celled free-living eucaryotes such as protozoa (see below).
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