We have all been taught that mitochondria are the powerhouse of the eukaryotic cell (cells containing membrane bound organelles such as the nucleus, golgi apparatus, mitochondria …). It is within this membrane bound organelle where most of the energy needed for our cells to survive is produced. What exactly is this energy, and how did mitochondria come about?
The energy, quite simply, is ATP (adenosine triphosphate). ATP is produced by a series of pathways involving many enzymes, cofactors, and of course glucose. With every glucose molecule that enters a cell, roughly 36-38 molecules of ATP are produced. Glucose begins its breakdown in the cytoplasm of the cell through the anaerobic pathway of glycolysis. The products of glycolysis are then transferred into the mitochondria where they are shuttled into the Krebs cycle and then on to oxidative phosphorylation. In respect to cellular respiration, the difference between the cytoplasm and the mitochondria besides the enzymes that are present, is the use of oxygen. Aerobic respiration, which consumes oxygen in the reaction, only occurs in the mitochondria. It is the addition of this extraordinary organelle that aided in the evolution of the eukaryotic cell and therefore the higher species.
It is believed that mitochondria arose through a process called endosymbiosis nearly 2.1 billion years ago. This theory suggests that a bacterium entered a cell, lived in a mutually beneficial relationship with its host and hence evolved into the mitochondria that we are familiar with today. The diagram at the left illustrates how this could have happened. In A, the cell membrane surrounds a bacterial cell and engulfs it. In B, we see that the bacteria now contains an outer membrane, compliments of the host cell. The original bacterial plasma membrane becomes highly folded and becomes the inner mitochondrial membrane that we see in C. This folded structure increases the surface area where oxidative phosphorylation takes place, therefore many of these reactions can occur simultaneously, optimizing the amount of ATP produced. Quite an efficient system for something that had once been a lowly bacterium.
The evidence for this theory is overwhelming. First, mitochondria are similar in size and shape to some strains of bacteria. They are between 0.5-1 μm in diameter and 1-10 μm long. The inner membrane of the mitochondria contain enzymes similar to those found in some bacterial cells. Mitochondria also contain their own genome. It is a circular molecule like that found in bacterial cells. mtDNA containing ~16 kilobases. Thirty-seven genes have been identified. Thirteen of these genes encode proteins used in cellular respiration that takes place within the mitochondria, and 24 code for RNA molecules involved in mitochondrial protein synthesis. There is a greater degree of homology between mitochondria RNA and the RNA found in bacteria than there is between mtRNA and the RNA found in the cytoplasm of eukaryotic cells. Mitochondria reproduce within the cell by way of fission, as do bacteria.
Mitochondria are inherited by the mother. They are found in the egg, and any mitochondria that might have been inside the sperm and entered the egg during conception are destroyed. Because of this, maternal linages can be easily traced.
The number of mitochondria within a cell type varies considerably depending on the energy needs of that cell. As you may imagine, muscle cells and brain cells contain thousands because their energy needs are greatest. Red blood cells do not contain any mitochondria, as their life is short and their energy requirements are produced in their progeny cells.
Without mitochondria, the organism would not be able to meet its energy needs and we might still be one cell organisms swimming in an oxygen deficient bog.