Mitochondria
Mitochondria are unique organelles found in nearly all eukaryotic cells. They “are the sites of cellular respiration, the catabolic process that generates ATP by extracting energy from sugars, fats and other fuels with the help of oxygen (Campbell and Reese 124). Thus, mitochondria are essential to the vitality of a cell as they provide the cell with energy. Unlike most organelles, mitochondria have a double membrane and possess their own DNA and ribosomes.
The structure of mitochondria is similar to that of bacteria and is generally inclusive of mitochondria in all plant, animal and lower eukaryotic cells. Mitochondria are oval in shape, and about 1-10µm long and have both and an outer and an inner membrane. Each membrane is a phospholipid bilayer that contains proteins unique to mitochondria. The enzyme that makes ATP and other proteins that operate in respiration are built into the inner membrane (Campbell and Reese 124). The peripheral, or outer, membrane is a “continuous bag that encloses the entire contents of the organelle” (Tzagoloff 16) and has considerably less surface area than the inner membrane. The inner membrane is also continuous; however, it forms a series of folds that project into the interior of the mitochondria. These folds, called mitochondrial cristae, are the cause of the increased surface area of the inner membrane which in turn heightens the productivity of cellular respiration (Campbell and Reese).
The individual membranes form two internal compartments. The intracristal space is located between the outer and inner membranes and in most cases is small. The intercristal space or matrix is surrounded by the inner membrane and is generally large. Within the matrix are “many different enzymes as well as the mitochondrial DNA and ribosomes” (Campbell and Reese 124). Furthermore, some of the steps of metabolic respiration are made possible by enzymes in the matrix. “The inner membrane can assume different spatial arrangements depending on the physiological state of the cell or the composition of the medium in which isolated mitochondria are suspended” (Tzagoloff 17). In the orthodox conformation, the cristae folds into sheets and the matrix fills most of the internal space. On the other hand, in the condensed conformation, the volume of the matrix decreases as a result of a separation of the cristae surfaces and consequently the intermembrane space increases. “In extremely condensed mitochondria, the inner membrane has the appearance of long tubules which wind through the interior to form a highly complex spaghetti-like network” (Tzagoloff 17). In studying mitochondria in rat liver, Hackenbrock observed that mitochondria condensed during active respiration (the transfer of solutes against a concentration gradient) and oxidative phosphorylation (process by which ATP is formed from ADP and phosphate during electron transport). Studies of other types of mitochondria have yielded the same results.
One of the most unique attributes of mitochondria is the fact that they possess their own genome and ribosomes, independent from that of the cell. “Mitochondrial DNA (mtDNA) is a closed circular duplex molecule with superhelical twists” (Tzagoloff 267). It is important to note that the circular form of mtDNA is characteristic of bacteria, not of eukaryistic DNA. On a whole, plants have the largest mitochondrial genomes and animal cells have the smallest. Due to its small size of about “16,500 nucleotides long” (Olsen 25) and its limited information content, the mitochondrial genome is fairly simple compared to other eukaryotic DNAs.
Mitochondrial DNA, coupled with the similarities in structure of mitochondria and bacteria have led to the theory of serial endosymbiosis which holds that mitochondria (and chloroplasts in plant cells) were originally small prokaryotes living within ancient cells. The ancestors of mitochondria are believed to have been “aerobic heterotrophic prokaryotes” (Campbell and Reese 549) that entered the cell as “undigested prey or internal parasites” (Campbell and Reese 550). Symbiosis eventually became mutually beneficial as the prokaryote was provided with a place to live and the eukaryote, now being an anaerobe, could better function in a steadily increasing aerobic world due to the mitochondria’s ability to use oxygen as an advantage to the cell.
There is a host of evidence supporting the theory of serial endosymbiosis. Endosymbiotic relationships exist in the modern world, suggesting that they would have been possible in the ancient world as well. Mitochondria are similar in size to bacteria and “the inner membrane of mitochondria possesses several enzymes and transport systems found on the plasma membranes of modern prokaryotes” (Campbell and Reese 550). Mitochondria also replicate by a splitting process that seems to have been evolved from that in bacteria and they have a genome structure consisting of a one circular DNA molecule as is common in the majority of prokaryotes. Finally, these organelles “contain transfer RNAs, ribosomes that are more similar to prokaryotic than to eukaryotic ribosomes, and other equipment needed to transcribe and translate their DNA and proteins (Campbell and Reese 550).
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