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Mitochondrion is a long and oval shaped organelle ranging from 1 to 4 micrometers in length and from 0.2 to 1.0 micrometers in diameter. It is rod-shaped and is enclosed in two membranes: inner and outer membranes. Both membranes are composed of proteins and phospholipids. There exists a space between the two membranes known as inter-membrane space. The composition of an inter-membrane space resembles that of the cell’s cytoplasm (Cutler 452).
The mitochondrion is vital for energy production within the cell. It is also referred to as the cell’s powerhouse. It is one of the most important organelles within the cell producing about 90% of the energy used by the cell (How Healthy Nutrition Builds Health 2012). It aids in a range of processes such as cellular differentiation, signaling, and cell death. It is also involved in the control of cell growth and cell cycle. It has also been associated with the development of certain human diseases such as mitochondrial disorders, cardiac dysfunction. Finally, mitochondria play an important role in the aging process.
Mitochondrion is located within the cytoplasm of a cell. Fifteen to twenty percent of the total volume of the cell is occupied by the mitochondria. The number of mitochondria found in a cell differs according to the tissue and organism type. Several cells have a single mitochondrion, while others can have thousands of mitochondria. For instance, the muscle tissue and heart cells contain more mitochondria than other less-active tissues. They contain up to forty percent of cell space. The human body has an overall amount of over one quadrillion mitochondria constantly producing energy.
Composition and Functions of the Various Structures of the Mitochondrion
The Outer Membrane. Unlike the inner membrane, the outer membrane is smooth and contains almost equal amount of proteins and phospholipids. It has numerous special proteins known as porins, whose function is to allow molecules with a weight of five thousands daltons or less pass through. The membrane is totally permeable to ions, nutrient molecules, ADP, and ATP molecules.
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The inner membrane. It has a more complex structure than the outer one since it constitutes of the ATP synthetase complex and the electron transport chain. The membrane is not completely permeable. It is only permeable to water, oxygen, and carbon dioxide. A large number of proteins make up the inner membrane. These proteins play a vital role in the production of ATP and regulation of the transfer of metabolites across the inner membrane. This membrane consists of cristae.
Cristae. The cristae are shelf-like infoldings in the inner membrane. They increase the surface area of the membrane structure, where more space is required for the protein molecules to aid ATP production.
The intermembrane space. It aids in oxidative phosphorylation.
Matrix. The cytoplasmic matrix contains a mixture of enzymes important in synthesis of ATP molecules, mitochondrial ribosomes, mitochondrial DNA molecules, enzymes and RNAs. In addition, it has dissolved gases: oxygen, carbon dioxide, recyclable intermediates which serve as energy, and water. Therefore, the mitochondrion possesses its own genetic material and the capability to manufacture its own RNAs and proteins.
The general functions of the mitochondrion are described further in the paper.
Production of energy by oxidative phosphorylation. This involves the breakdown of the food a person eats into simpler molecules such as carbohydrates and fats. The molecules are then sent to the mitochondrion and processed further to produce the charged molecules that combine with oxygen to produce the ATP molecules (Cutler and Rondriguez 455). Secondly, the mitochondrion is vital for maintaining a proper concentration of calcium ions throughout the cell. This is achieved by the mitochondrion acting as a storehouse of the calcium ions. Thirdly, it helps build several parts of the blood and hormones such as estrogen and testosterone. Fourth, the mitochondria found in cells of the liver have special enzymes for detoxifying ammonia. Lastly, it plays a vital role in the process of cell death. The excess of unwanted cells is regulated during organism’s development, a process called apoptosis. However, abnormal cell death resulting from mitochondrial dysfunction may affect the organ’s function.
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Energy production. The main function of the mitochondrion is production of ATP. There exist numerous proteins in its inner membrane for this purpose. The production of ATP is done by the process of oxidation. The key products of glucose, pyruvate, and NADH produced in the cytosol are oxidized. This process known as cellular respiration or aerobic respiration relies on the existence of oxygen.
The process of aerobic respiration has several steps. The first step is glycolysis.
Glycolysis means ‘splitting sugar’. Glucose is a 6-carbon sugar. Glycolysis is initiated by the enzymes found in the cytoplasmic matrix where one glucose molecule is oxidized to two molecules of 3-carbon sugar. This produces two ATP molecules, two molecules of pyruvic acid and 2 NADH electron-carrying molecules. The second step is Krebs cycle, which is also known as citric acid cycle. It is a series of chemical reactions that take place in every organism for generating energy though oxidation of acetate. Acetate is derived from proteins, carbohydrates, and fats and is oxidized into carbon dioxide and water. The cycle consumes acetate in form of acetyl-CoA and reduces NAD to NADH producing carbon dioxide. The NADH generated by the tricarboxylic acid (TCA) cycle is then fed to the oxidative phosphorylation pathway. The last step is electron transport. Its chain of transport consists of a chain of electron carriers generated by the membrane of mitochondrion via the Krebs’ cycle. Further, the ATP molecules are produced by the chemical reactions involving these electron-carrier molecules (Gropper, Smith and Groff 85).
Mitochondrial disease is the condition in which the ability of mitochondria to produce energy reduces due to various defects. This reduced production may lead to brain dysfunction, weak muscles, vision problems, restricted movement of limbs among others. According to Cutler and Rondriguez (453) more than fifty million people in the United States have chronic degenerative disorders involving mitochondrial dysfunction. Mitochondrial dysfunction is likely to affect the production of specific cell products essential for proper functioning of the cell and production of energy. Eventually, this can cause cell death and organ system failure. It can also be fatal in other cases.
The disease is capable of destroying the health of any system or organ of the body. The disease can affect any person regardless of gender or age. While the symptoms of the mitochondrial disease are progressive, they may vary from one patient to another. The symptoms of the disease include a weak immune system, recurring infections, reduced heart capacity, muscle fatigue, seizures, strokes, diabetes, obesity, deafness, blindness, and liver problems. Adulthood diseases attributed to aging include heart disease, diabetes, stroke, osteoporosis, neurodegenerative diseases like Alzheimer’s, Parkinson, cancer, and Huntington’s disease (Cutler and Rondriguez 453). Cutler and Rodriguez (453) also report that there exists more than fifty inheritable metabolic childhood diseases that involve mitochondrial dysfunction. They made an estimate that four thousands out of four million children born in the United States each year have mitochondrial disease at birth. Finally, the function of the mitochondria can be adversely affected by various medicines or environmental factors. The dysfunction of this organelle also plays a key role in the premature aging symptoms.