Mitochondrion is the name given to a cell organelle that, among other functions, is involved in the cell’s energy supply primarily through adenosine triphosphate metabolism. Mitochondria possess their own genetic material in the form of mitochondrial DNA. Depending on the energy requirements of cell types, a few to several thousand mitochondria may be present in a cell.
What is the mitochondrion?
A mitochondrion is a cell organelle that is present in virtually all human cells, sometimes in large numbers of up to several thousand. An exception is the top layer of skin, the stratum corneum, which consists of dead corneal cells and contains no mitochondrion. Mitochondria are endowed with their own genome, mitochondrial DNA (mtDNA), which supports the assumption that mitochondria were originally independent organisms that entered into an endosymbiosis with the cells of multicellular organisms. Mitochondria are no longer independently viable in their present-day manifestation. Mitochondria are characterized by their double membrane, an outer, nearly smooth membrane and an inner, highly unfolded membrane, which provides a correspondingly large surface area for biochemical metabolic processes. Among other things, the mitochondria are involved in the metabolism of the so-called respiratory chain and the citrate cycle. In the respiratory chain, which takes place in the intermembrane space between the outer and inner membranes, glucose is metabolized to synthesize ATP and made available to the cell as an energy carrier. The citrate cycle brings together the metabolic processes that cause the breakdown of carbohydrates, proteins, and fats.
Anatomy and structure
Characteristic of mitochondrial morphology are two membranes, the outer membrane, which gives the organelle its almost bean-shaped appearance, and the inner membrane, which is highly unfolded and thus has a large surface area. Both membranes are composed of phospholipid bilayers and proteins. However, the outer membrane differs significantly from the inner membrane in its properties. The outer membrane contains protein complexes with channels that allow selective exchange of substances between the mitochondrion and the cytosol of the cell. The inner membrane of Sacculus-type mitochondria contains protein complexes necessary for the “operation” of the respiratory chain. The spaces created by the unfolding of the inner membrane toward the outer membrane are called cristae and accelerate the metabolism of the respiratory chain. The cristae are occupied by tiny corpuscles 8.5 nm in diameter, called F1 particles or ATP synthase particles, which play a role in ATP synthesis. Another type of mitochondria is the so-called tubule type, which is found in cells that synthesize steroid hormones. The numerous tubules serve to selectively transport substances.
Function and tasks
One of the most important functions and tasks of mitochondria is the synthesis of adenosine triphosphate (ATP) and the release of ATP into the cellular matrix, the interior of the cell outside the mitochondria. In a complex reaction chain, the energy obtained via catalytically controlled oxidation processes is stored in the form of ATP for a short time and made available to the cells. The respiratory chain makes use of metabolic products of the so-called citrate cycle – also known as the Krebs cycle after its discoverer Hans A. Krebs. The metabolism of the Krebs cycle takes place in the matrix of the mitochondria, i.e. within the space closed off by the inner membrane. Mitochondria are also involved in part of the urea cycle, which takes place partly within the mitochondrial matrix and partly in the cytosol of the cell. The urea cycle serves to convert nitrogenous breakdown products, e.g., from proteinaceous food, to urea for excretion by the kidneys. Mitochondria are also involved in the process of controlled cell death, apoptosis. This is a type of self-destruction of the cell with orderly disposal of the degradation products. Apoptosis can be “commanded”, for example, by the immune system in the case of detected serious defects or infections, in order to prevent damage and dangerous situations for the organism as a whole. Mitochondria have the ability to absorb calcium ions and make them available to the cell when needed. They thus support the important function of calcium homeostasis of the cell.Another important function of mitochondria is the synthesis of iron–sulfur clusters, which are required for numerous enzymes for the catalytic control of the respiratory chain. The synthesis of iron–sulfur clusters is nonredundant, making it an essential supply for all cells that can be provided exclusively by mitochondria.
Diseases
Mitochondriopathies, malfunctions or disorders of mitochondrial metabolic processes affect weakening of the body’s performance primarily because of reduced ATP synthesis. Basically, mitochondriopathy may be due to inherited genetic defects or it may have been acquired during life. Acquired mitochondriopathies are primarily associated with neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, and ALS, but also with diabetes mellitus, obesity, cardiovascular disease, and certain cancers. Inherited mitochondriopathies show a wide variety of manifestations, depending on where in the metabolic cascades the gene defect affects. If, due to gene defects within the respiratory chain or the citrate cycle, certain enzymes are not available that are only required for certain body tissues, symptoms will only occur in the corresponding organs. Because of the variety of symptoms that a mitochondrial gene defect can cause, diagnosis is not straightforward. Because the citrate cycle is usually involved, there is usually a “buildup” of pyrovate, which the body attempts to break down via the alternative route of lactate, resulting in a greatly increased lactic acid concentration, lactic acidosis.
