History
Back in the 1840s, the presence of granule-like structures within muscle cells and other cell types were being recognized by several scientists. (Ernster and Schatz 1981) In 1890, Richard Altmann, who was a cytologist, used a dye technique to detect the granules and termed them as “bioblasts.” He speculated that they were the basic units of cellular activity. (Ernster and Schatz 1981)
It was in 1898 when Carl Benda gave these bioblasts a new the Greek name “mitochondria” meaning thread granules. Discovery of the mitochondrion however, cannot be limited to just a few people. Over decades of time, many contributions have been made in relation to the properties and functions of the mitochondria. (Ernster and Schatz 1981)
This organelle is the “power house” of the eukaryotic cell and is located in the cytoplasm. The mitochondrion requires transcription of several genes associated with the organelle along with translocation, targeting and assembly of proteins. (Hood and Joseph 2004) Mitochondria’s main function is to convert energy into forms that can be used by the cell. Along with generating fuel for the cell’s activities, the mitochondrion functions in a range of other processes including, cell signaling, cell division, cell growth, and cell death. Structure
The mitochondrion can have different overall structures depending on the cell type. Most mitochondria appear as rod-like shaped organelles although sometimes they can appear like a branched interconnected tubular network. Mitochondria are very dynamic organelles that are consistently changing their shape. The mitochondrion occupies between 15 and 20 percent of the mammalian liver cell and contains thousands of different proteins. (Alberts et al 2008) Its structure contains two membranes, an outer and an inner, and also two internal compartments.
The outer membrane encloses the mitochondria completely. It is composed mainly of lipids and contains a mixture of enzymes. (Karp 2008) It also contains several transport proteins called porins. (Alberts et al 2008) These proteins consist of a barrel of β sheets and form large aqueous channels across the lipid bilayer which allows molecules to penetrate and undergo diffusion. (Alberts et al 2008) The porins of the outer membrane are able to undergo reversible closure as a response to cell conditions. (Karp 2008) When the porin channels are open the outer membrane is very permeable to ATP, NAD, and coenzyme A. (Karp 2008) Most molecules, depending on size, are able to pass through the outer membrane easily.
The inner membrane of the mitochondria is impermeable and highly specialized. (Karp 2008) In order for molecules or ions to gain access to the matrix of the mitochondria, special membrane transporters are required. It contains several different polypeptides, lipids, and phospholipids, including cardiolipin. (Karp 2008) The inner membrane has evolved from bacterial plasma membranes (Hood and Joseph 2204) and contains a series of double-layered sheets called cristae which meet with the inner membrane at the boundary of the organelle. These cristae project into the matrix and contain a large amount of the membrane surface. (Karp 2008)
Cristae consume one third of the liver cell and are said to be three times greater in the mitochondria of the cardiac muscle due to a larger demand for ATP in the heart cells. (Perkins et al 2009) Up until recently, cristae were believed to consist of invaginations of the inner membrane but it is now agreed that cristae and the inner membrane are functionally distinct even though they are linked to one another by narrow connections. (Karp 2008)
These two membranes of the mitochondria divide the organelle into two aqueous compartments. The first compartment is found within the interior of the mitochondria and is called the matrix. This and the inner membrane are the two parts of the mitochondria that do most of the work. (Karp 2008) The matrix contains several different enzymes, ribosomes, tRNAs, and DNA molecules. The other aqueous compartment formed by the membranes of the mitochondria is called the intermembrane space. This is the region between the inner and outer membranes. This space plays a primary role in oxidative phosphorylation. (Alberts et al 2008)
Studies that have been done on mitochondrial assembly along with protein import are important because if the appropriate combinations of gene products are not incorporated properly then it can lead to reduction of ATP synthesis and enhanced reactive oxygen species production which can lead to mitochondrial disease and cell death. (Hood and Joseph 2004)
Mitochondria also carry their own set of DNA. (Shock et al 2010) Mitochondrial DNA performs aerobic respiration in the cell and contains 5-methylcytosine at cpG dinucleotides in the nuclear genome. (Shock et al 2011) Mitochondrial DNA is solely inherited from the mother. (Schwartz and Vissing 2002) Sperm mitochondrial DNA are targeted for destruction by nuclear-encoded proteins (Shitara et al 2000) and disappear during early embryogenesis by dilution of the paternal sperm which is much smaller than the maternal egg. (Nishimura et al 2005) The mechanism for generating mtDNA methylation and its functional significance are unknown. (Shock et al 2011)
Function
The mitochondrion plays an important role in several different processes in the cell. Its main function is to produce ATP through oxidative phosphorylation. (Karp 2008) In order to achieve this, the cell must first undergo glycolysis.
The cell gets its energy from glucose. When the cells glucose levels fall, it triggers the breakdown of fats in order to produce energy. (Karp 2008) Glycolysis is found under aerobic and anaerobic conditions and occurs in the cytoplasm.
During glycolysis one molecule of glucose is converted to produce a net total of two pyruvates, two NADH, and two ATP. (Karp 2008) In anaerobic conditions, glycolysis is the only pathway for ATP production. NAP+ has to be regenerated to continue making ATP. In aerobic conditions, molecules of pyruvate and fatty acids are actively transported across the inner mitochondrial membrane and into the mitochondrial matrix where the pyruvate undergoes oxidation. (Alberts et al 2008)
Pyruvate oxidation takes place within the matrix of the mitochondria. Oxygen and two pyruvate go into pyruvate oxidation. The pyruvate goes from three carbons to two carbon acetyl. (Alberts et al 2008) As a result, CO2 is released as waste. In the next step, NAD+ is reduced to NADH. Lastly, the pyruvate dehydrogenase complex attaches the CoA to the acetyl. (Alberts et al 2008) No ATP is produced during pyruvate oxidation. During this entire process, two NADH’s are used for energy and the net products include two acetyl-CoA and two NADH. These two acetyl-CoA molecules become oxidized by the citric acid cycle. Acetyl-CoA can also be produced by breaking down fats and amino acids.
Fatty acid oxidation occurs in the matrix, when a fatty acid is linked to the thiol group of coenzyme A. (Karp 2008) The acetyl CoA is removed from the fatty acid chain during each round and is fed into the citric acid cycle. (Karp 2008) This leads to the disassembly of the fatty acyl CoA molecule. Fatty acid oxidation produces one NADH and one FADH2. (Karp 2008)
Once acetyl Co-A is formed, it is fed into the citric acid cycle, which is also referred to as the Krebs Cycle or the tricarboxylic acid cycle. Here, carbon atoms are oxidized and released one by one as waste product and the energy is conserved. (Karp 2008) This process occurs in the mitochondrial matrix and occurs twice per molecule of glucose.
The two carbon acetyl group from acetyl-CoA is transferred to the four carbon oxaloacetate compound and forms the six carbon molecule citrate. (Karp 2008) The citrate molecule undergoes isomerization which removes water and then adds water back in to move the hydroxyl group from one carbon atom to another. This produces an isometric form isocitrate. (Karp 2008)
This six carbon isocitrate is oxidized and one molecule of carbon dioxide is removed and the five carbon alpha-ketoglutarate molecule is formed. (Karp 2008) Also in this step, NAD+ is reduced to NADH and H+. Next, alpha-ketoglutarate becomes oxidized as carbon dioxide is removed. Coenzyme A is added and the four carbon succinyl-CoA compound is formed. (Karp 2008) Here also, NAD+ is reduced to NADH and H+.
In the next step of the citric acid cycle, CoA is removed from the succinyl-CoA compound to produce succinate. (Karp 2008) This forms a high-energy phosphate linkage to succinate. This phosphate is passed to GDP. (Alberts et al 2008) GDP and Pi undergo substrate-level phosphorylation to create GTP. This GTP can then be used to make ATP. (Karp 2008)
During the third oxidation step of the citric acid cycle, FAD removes two atoms of hydrogen from succinate to produce fumarate. (Karp 2008) Also, during this oxidation, FAD is reduced to FADH2. The addition of water to fumarate forms malate which is oxidized to produce oxaloacetate and NAD+ is reduced to NADH and H+. (Karp 2008) Oxaloacetate is the starting compound of the citric acid cycle and once it is regenerated, it begins a second turn of the cycle. The end products of the citric acid cycle are 6 NADH, 2 FADH2, and 2 GTP. (Alberts et al 2008) The reduced NADH and H+ molecules along with the FADH2 molecules carry protons and electrons to the electron transport chain which then generates additional ATP through oxidative phosphorylation.
Oxidative phosphorylation occurs through chemiosmosis in the inner mitochondrial membrane. (Karp 2008) Chemiosmosis occurs in two stages. In the first stage, electron transport drives the pump that pumps protons across the membrane. High-energy electrons are passed from FADH2 or NADH to the electron carriers that make up the electron transport chain. These electrons are passed along the electron-transport chain in energy releasing reactions which are coupled to energy requiring conformational changes in electron carriers that move protons outward across the membrane. (Karp 2008) As a result, energy being released during electron transport is stored in the form of an electrochemical gradient of protons across the membrane. Eventually, the low energy electrons are transferred to the terminal electron acceptor, O2, which is reduced to water. (Karp 2008)
In the second stage, the proton gradient is harnessed by ATP synthase to make ATP. (Alberts et al 2008) The movement of protons back across the membrane is done through an ATP-synthesizing enzyme that provides energy which is required to phosphorylate ADP to ATP. (Karp 2008)
The mitochondrion not only functions as the powerhouse of the cell but is capable of releasing molecules to the cell when needed. Mitochondria also play a pivotal role in cell death pathways involved with different forms of neurodegeneration. (Perkins et al 2009)
Current Research
In 2010, a group of researchers in Spain were able to discover a new protein in the Drosophila melanogaster fly, also known as the fruit fly, which is vital to the mitochondrion. The protein is called SLIMP which stands for seryl-tRNA synthetase-like insect mitochondrial protein. (Guitart et al 2010) The SLIMP protein stems from a seryl-tRNA synthetase. These are enzymes used for the synthesis of new proteins. (Guitart et al 2010) The duplication of a mitochondrial SRS gene was fixed in Insecta which resulted in seryl-tRNA synthetase-like insect mitochondrial protein. The SLIMP protein is then localized in the mitochondria. In the mitochondria it carries out an essential function that is unrelated to the aminoacylation of tRNA. (Guitart et al 2010) RNA interference removes SLIMP in the flies resulting in a decrease in metabolic capacity and an increase in mass in the form of aberrant mitochondria. (Guitart et al 2010) Future studies with this new protein will hopefully help determine its unknown biological function. (Guitart et al 2010)
Another popular research topic involving mitochondria is that children with autism have mitochondrial dysfunction. Mitochondrial DNA diseases are caused by dysfunction and increased cell death as mutations accumulate. (James and Murphy 2002) Researchers have found that children with autism have trouble producing cellular energy due to collective damage and oxidative stress in the mitochondrion. (Giulivi et al 2010) If the brain is unable to get ATP to fuel the brains neurons it leads to cognitive damages that have been linked with autism. (Giulivi et al 2010)
This study was led by the Cecilia Giulivi and blood samples of 10 children, ages 2 to 5, diagnosed with autism were taken and the metabolic pathways of mitochondria in the lymphocytes were analyzed. In earlier studies mitochondria was obtained from muscle cells. (Giulivi et al 2010) Muscle cells can generate energy through anaerobic glycolysis so mitochondrial dysfunction is not always expressed. (Giulivi et al 2010) Lymphocytes use aerobic respiration that is conducted by mitochondria which allow researchers to make more accurate conclusions. (Giulivi et al 2010) They found that the mitochondria from children with autism contained less oxygen than children without autism who were used as a control group. This indicates lower mitochondrial activity which means less ATP production. (Giulivi et al 2010)
The conclusion of this study indicated that oxidative stress in the mitochondrion could be contributing to the onset of autism in children. (Giulivi et al 2010) Although these findings do not completely establish a cause for autism, they do provide us with useful information to help uncover new information regarding autism and what new approaches can be taken in treating the disease.

Works Cited

1.) Andrew James, Michael Murphy. 2002. How Mitochondrial Damage Affects Cell Function. J Biomed Sci 9: 475-487. 2.) Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts. 2008. Molecular Biology of the Cell. Garland Science: New York. p815-817. 3.) Cecilia Giulivi, PhD, Yi-Fan Zhang, BS. Alicja Omanska-Klusek, MS, Catherine Ross-Inta, BS, Sarah Wong, BS, Irva Hertz-Picciotto, PhD, Flora Tassone, PhD, Isaac N. Pessah, PhD. 2010. Mitochondrial Dysfunction in Autism. JAMA 304(21):2389-2396 4.) David Hood, Anna-Maria Joseph.2004. Mitochondrial Assembly: protein import. Proc Nutr Soc. 63(2): 293-300. 5.) Gerald Karp. 2008. Cell and Molecular Biology. John Wiley & Sons, Inc. New Jersey. p180-213. 6.) Guy Perkins, Ella Boddy-Wetzel, Mark Ellisman. 2009. New Insights into Mitochondrial Structure During Cell Death. Experimental Neurology 218: 183-192 7.) Hiromichi Yonekawa, Jun-Ichi Hayashi. 2000. Selective and Continuous Elimination of Mitochondria Microinjected Into Mouse Eggs From Spermatids, but Not From Liver Cells, Occurs Throughout Embryogenesis. Genetics 156(3): 1277–1284. 8.) Hiroshi Shitara, Hideki Kaneda, Akitsugu Sato, Kimiko Inoue, Atsuo Ogura, Lars Ernster, Gottfried Schatz. 1981. Mitochondria: A Historical Review. J Cell Biol. 91 (3 pt 2) 227s-255s. 9.) Lisa Shock, Prashant Thakkar, Erica Peterson, Richard Moran, Shirley Taylor. 2011. DNA Methyltransferase 1, Cytosine Methylation, and Cytosine Hydroxymethylation in Mammalian Mitochondria. PNAS vol. 108 no. 9 3630-3635. 10.) Marianne Schwartz, PhD, John Vissing, MD, PhD. 2002. Paternal Inheritance of Mitochondrial DNA. N Engl J Med 347:576-580 11.) Tanit Guitart, Teresa Leon Bernardo, Jessica Sagales, Thomas Stratmann, Jordi Bernues, Lluís Ribas de Pouplana. 2010. New Aminoacyl-tRNA Synthetase-like Protein in Insecta with an Essential Mitochondrial Function. J Biol 285(49): 38157–38166. 12.) Yoshiki Nishimura, Tomoya Yoshinari, Kiyoshi Naruse, Takeshi Yamada, Kazuyoshi Sumi, Hiroshi Mitani, Tetsuya Higashiyama, Tsuneyoshi Kuroiwa. 2005. Active digestion of sperm mitochondrial DNA in single living sperm revealed by optical tweezers. PNAS vol. 103 no. 5 1382-1387.