Enzymes are macromolecular biological catalysts. Enzymes accelerate chemical reactions; the molecules upon which enzymes may act are called substrates and the enzyme converts the substrates into different molecules known as products. All metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life. Metabolic pathways depend upon enzymes to catalyze individual steps; the study of enzymes is called enzymology and a new field of pseudoenzyme analysis has grown up, recognising that during evolution, some enzymes have lost the ability to carry out biological catalysis, reflected in their amino acid sequences and unusual'pseudocatalytic' properties. Enzymes are known to catalyze more than 5,000 biochemical reaction types. Most enzymes are proteins; the latter are called ribozymes. Enzymes' specificity comes from their unique three-dimensional structures. Like all catalysts, enzymes increase the reaction rate by lowering its activation energy; some enzymes can make their conversion of substrate to product occur many millions of times faster.
An extreme example is orotidine 5'-phosphate decarboxylase, which allows a reaction that would otherwise take millions of years to occur in milliseconds. Chemically, enzymes are like any catalyst and are not consumed in chemical reactions, nor do they alter the equilibrium of a reaction. Enzymes differ from most other catalysts by being much more specific. Enzyme activity can be affected by other molecules: inhibitors are molecules that decrease enzyme activity, activators are molecules that increase activity. Many therapeutic drugs and poisons are enzyme inhibitors. An enzyme's activity decreases markedly outside its optimal temperature and pH, many enzymes are denatured when exposed to excessive heat, losing their structure and catalytic properties; some enzymes are used commercially, in the synthesis of antibiotics. Some household products use enzymes to speed up chemical reactions: enzymes in biological washing powders break down protein, starch or fat stains on clothes, enzymes in meat tenderizer break down proteins into smaller molecules, making the meat easier to chew.
By the late 17th and early 18th centuries, the digestion of meat by stomach secretions and the conversion of starch to sugars by plant extracts and saliva were known but the mechanisms by which these occurred had not been identified. French chemist Anselme Payen was the first to discover an enzyme, diastase, in 1833. A few decades when studying the fermentation of sugar to alcohol by yeast, Louis Pasteur concluded that this fermentation was caused by a vital force contained within the yeast cells called "ferments", which were thought to function only within living organisms, he wrote that "alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells."In 1877, German physiologist Wilhelm Kühne first used the term enzyme, which comes from Greek ἔνζυμον, "leavened" or "in yeast", to describe this process. The word enzyme was used to refer to nonliving substances such as pepsin, the word ferment was used to refer to chemical activity produced by living organisms.
Eduard Buchner submitted his first paper on the study of yeast extracts in 1897. In a series of experiments at the University of Berlin, he found that sugar was fermented by yeast extracts when there were no living yeast cells in the mixture, he named the enzyme that brought about the fermentation of sucrose "zymase". In 1907, he received the Nobel Prize in Chemistry for "his discovery of cell-free fermentation". Following Buchner's example, enzymes are named according to the reaction they carry out: the suffix -ase is combined with the name of the substrate or to the type of reaction; the biochemical identity of enzymes was still unknown in the early 1900s. Many scientists observed that enzymatic activity was associated with proteins, but others argued that proteins were carriers for the true enzymes and that proteins per se were incapable of catalysis. In 1926, James B. Sumner crystallized it; the conclusion that pure proteins can be enzymes was definitively demonstrated by John Howard Northrop and Wendell Meredith Stanley, who worked on the digestive enzymes pepsin and chymotrypsin.
These three scientists were awarded the 1946 Nobel Prize in Chemistry. The discovery that enzymes could be crystallized allowed their structures to be solved by x-ray crystallography; this was first done for lysozyme, an enzyme found in tears and egg whites that digests the coating of some bacteria. This high-resolution structure of lysozyme marked the beginning of the field of structural biology and the effort to understand how enzymes work at an atomic level of detail. An enzyme's name is derived from its substrate or the chemical reaction it catalyzes, with the word ending in -ase. Examples are alcohol dehydrogenase and DNA polymerase. Different enzymes that catalyze the same chemical reaction are called isozymes; the International Union of Biochemistry and Molecular Biology have developed a nomenclature for enzymes, the EC numbers. The first number broadly classifies the enzyme based on its mechanism; the top-level classification is: EC 1, Oxidoreductases: catalyze oxidation/reducti
Astrocytes known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. The proportion of astrocytes in the brain is not well defined. Depending on the counting technique used, studies have found that the astrocyte proportion varies by region and ranges from 20% to 40% of all glia, they perform many functions, including biochemical support of endothelial cells that form the blood–brain barrier, provision of nutrients to the nervous tissue, maintenance of extracellular ion balance, a role in the repair and scarring process of the brain and spinal cord following traumatic injuries. Research since the mid-1990s has shown that astrocytes propagate intercellular Ca2+ waves over long distances in response to stimulation, similar to neurons, release transmitters in a Ca2+-dependent manner. Data suggest that astrocytes signal to neurons through Ca2+-dependent release of glutamate; such discoveries have made astrocytes an important area of research within the field of neuroscience.
Astrocytes are a sub-type of glial cells in the central nervous system. They are known as astrocytic glial cells. Star-shaped, their many processes envelop synapses made by neurons. Astrocytes are classically identified using histological analysis. Several forms of astrocytes exist in the central nervous system including fibrous and radial; the fibrous glia are located within white matter, have few organelles, exhibit long unbranched cellular processes. This type has "vascular feet" that physically connect the cells to the outside of capillary walls when they are in proximity to them; the protoplasmic glia are the most prevalent and are found in grey matter tissue, possess a larger quantity of organelles, exhibit short and branched tertiary processes. The radial glial cells are disposed in planes perpendicular to the axes of ventricles. One of their processes abuts the pia mater, while the other is buried in gray matter. Radial glia are present during development, playing a role in neuron migration.
Müller cells of the retina and Bergmann glia cells of the cerebellar cortex represent an exception, being present still during adulthood. When in proximity to the pia mater, all three forms of astrocytes send out processes to form the pia-glial membrane. Astrocytes are macroglial cells in the central nervous system. Astrocytes are derived from heterogeneous populations of progenitor cells in the neuroepithelium of the developing central nervous system. There is remarkable similarity between the well known genetic mechanisms that specify the lineage of diverse neuron subtypes and that of macroglial cells. Just as with neuronal cell specification, canonical signaling factors like Sonic hedgehog, Fibroblast growth factor, WNTs and bone morphogenetic proteins, provide positional information to developing macroglial cells through morphogen gradients along the dorsal–ventral, anterior–posterior and medial–lateral axes; the resultant patterning along the neuraxis leads to segmentation of the neuroepithelium into progenitor domains for distinct neuron types in the developing spinal cord.
On the basis of several studies it is now believed that this model applies to macroglial cell specification. Studies carried out by Hochstim and colleagues have demonstrated that three distinct populations of astrocytes arise from the p1, p2 and p3 domains; these subtypes of astrocytes can be identified on the basis of their expression of different transcription factors and cell surface markers. The three populations of astrocyte subtypes which have been identified are 1) dorsally located VA1 astrocytes, derived from p1 domain, express PAX6 and reelin 2) ventrally located VA3 astrocytes, derived from p3, express NKX6.1 and SLIT1 and 3) and intermediate white-matter located VA2 astrocyte, derived from the p2 domain, which express PAX6, NKX6.1, reelin and SLIT1. After astrocyte specification has occurred in the developing CNS, it is believed that astrocyte precursors migrate to their final positions within the nervous system before the process of terminal differentiation occurs. In medical science, the neuronal network was considered the only important function of astrocytes, they were looked upon as gap fillers.
More the function of astrocytes has been reconsidered, they are now thought to play a number of active roles in the brain, including the secretion or absorption of neural transmitters and maintenance of the blood–brain barrier. Following on this idea the concept of a tripartite synapse has been proposed, referring to the tight relationship occurring at synapses among a presynaptic element, a postsynaptic element and a glial element. Structural: They are involved in the physical structuring of the brain. Astrocytes get their name because they are "star-shaped", they are the most abundant glial cells in the brain that are associated with neuronal synapses. They regulate the transmission of electrical impulses within the brain. Glycogen fuel reserve buffer: Astrocytes contain glycogen and are capable of gluconeogenesis; the astrocytes next to neurons in hippocampus store and release glucose. Thus, astrocytes can fuel neurons with glucose during periods of high rate of glucose consumption and glucose shortage.
A recent research on rats suggests there may be a connection between this activity and physical exercise. Metabolic support: They provide neurons with nutrients such as lactate. Glucose sensing: associated w
The placenta is a temporary organ that connects the developing fetus via the umbilical cord to the uterine wall to allow nutrient uptake, thermo-regulation, waste elimination, gas exchange via the mother's blood supply. Placentas are a defining characteristic of placental mammals, but are found in marsupials and some non-mammals with varying levels of development; the placenta functions as a fetomaternal organ with two components: the fetal placenta, which develops from the same blastocyst that forms the fetus, the maternal placenta, which develops from the maternal uterine tissue. It metabolizes a number of substances and can release metabolic products into maternal or fetal circulations; the placenta is expelled from the body upon birth of the fetus. The word placenta comes from the Latin word for a type of cake, from Greek πλακόεντα/πλακοῦντα plakóenta/plakoúnta, accusative of πλακόεις/πλακούς plakóeis/plakoús, "flat, slab-like", in reference to its round, flat appearance in humans; the classical plural is placentae, but the form placentas is common in modern English and has the wider currency at present.
Placental mammals, such as humans, have a chorioallantoic placenta that forms from the chorion and allantois. In humans, the placenta averages 22 cm in length and 2–2.5 cm in thickness, with the center being the thickest, the edges being the thinnest. It weighs 500 grams, it has crimson color. It connects to the fetus by an umbilical cord of 55–60 cm in length, which contains two umbilical arteries and one umbilical vein; the umbilical cord inserts into the chorionic plate. Vessels branch out over the surface of the placenta and further divide to form a network covered by a thin layer of cells; this results in the formation of villous tree structures. On the maternal side, these villous tree structures are grouped into lobules called cotyledons. In humans, the placenta has a disc shape, but size varies vastly between different mammalian species; the placenta takes a form in which it comprises several distinct parts connected by blood vessels. The parts, called lobes, may number two, four, or more.
Such placentas are described as bilobed/bilobular/bipartite, trilobed/trilobular/tripartite, so on. If there is a discernible main lobe and auxiliary lobe, the latter is called a succenturiate placenta. Sometimes the blood vessels connecting the lobes get in the way of fetal presentation during labor, called vasa previa. About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal mature placenta; some 350 of these genes are more expressed in the placenta and fewer than 100 genes are placenta specific. The corresponding specific proteins are expressed in trophoblasts and have functions related to female pregnancy. Examples of proteins with elevated expression in placenta compared to other organs and tissues are PEG10 and the cancer testis antigen PAGE4 expressed in cytotrophoblasts, CSH1and KISS1 expressed in syncytiotrophoblasts, PAPPA2 and PRG2 expressed in extravillous trophoblasts; the placenta begins to develop upon implantation of the blastocyst into the maternal endometrium.
The outer layer of the blastocyst becomes the trophoblast, which forms the outer layer of the placenta. This outer layer is divided into two further layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer; the syncytiotrophoblast is a multinucleated continuous cell layer that covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblast cells, a process that continues throughout placental development; the syncytiotrophoblast, thereby contributes to the barrier function of the placenta. The placenta grows throughout pregnancy. Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy week 14. In preparation for implantation of the blastocyst, the endometrium undergoes decidualization. Spiral arteries in the decidua are remodeled so that they become less convoluted and their diameter is increased; the increased diameter and straighter flow path both act to increase maternal blood flow to the placenta.
There is high pressure as the maternal blood fills intervillous space through these spiral arteries which bathe the fetal villi in blood, allowing an exchange of gases to take place. In humans and other hemochorial placentals, the maternal blood comes into direct contact with the fetal chorion, though no fluid is exchanged; as the pressure decreases between pulses, the deoxygenated blood flows back through the endometrial veins. Maternal blood flow is 600–700 ml/min at term; this begins at day 5 - day 12 Deoxygenated fetal blood passes through umbilical arteries to the placenta. At the junction of umbilical cord and placenta, the umbilical arteries branch radially to form chorionic arteries. Chorionic arteries, in turn, branch into cotyledon arteries. In the villi, these vessels branch to form an extensive arterio-capillary-venous system, bringing the fetal blood close to the maternal blood. Endothelin and prostanoids cause vasoconstriction in placental arteries, while nitric oxide causes vasodilation.
On the other hand, there is no neural vascular regulation, catecholamines have only little effect. The fetoplacental circulation is vulnerable to persistent hypoxia or intermittent hypoxia and
Catalysis is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst, not consumed in the catalyzed reaction and can continue to act repeatedly. Because of this, only small amounts of catalyst are required to alter the reaction rate in principle. In general, chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternative reaction pathway with a lower activation energy than the non-catalyzed mechanism. In catalyzed mechanisms, the catalyst reacts to form a temporary intermediate, which regenerates the original catalyst in a cyclic process. A substance which provides a mechanism with a higher activation energy does not decrease the rate because the reaction can still occur by the non-catalyzed route. An added substance which does reduce the reaction rate is not considered a catalyst but a reaction inhibitor. Catalysts may be classified as either heterogeneous. A homogeneous catalyst is one whose molecules are dispersed in the same phase as the reactant's molecules.
A heterogeneous catalyst is one whose molecules are not in the same phase as the reactant's, which are gases or liquids that are adsorbed onto the surface of the solid catalyst. Enzymes and other biocatalysts are considered as a third category. In the presence of a catalyst, less free energy is required to reach the transition state, but the total free energy from reactants to products does not change. A catalyst may participate in multiple chemical transformations; the effect of a catalyst may vary due to the presence of other substances known as inhibitors or poisons or promoters. Catalyzed reactions have a lower activation energy than the corresponding uncatalyzed reaction, resulting in a higher reaction rate at the same temperature and for the same reactant concentrations. However, the detailed mechanics of catalysis is complex. Catalysts may bind to the reagents to polarize bonds, e.g. acid catalysts for reactions of carbonyl compounds, or form specific intermediates that are not produced such as osmate esters in osmium tetroxide-catalyzed dihydroxylation of alkenes, or cause dissociation of reagents to reactive forms, such as chemisorbed hydrogen in catalytic hydrogenation.
Kinetically, catalytic reactions are typical chemical reactions. The catalyst participates in this slowest step, rates are limited by amount of catalyst and its "activity". In heterogeneous catalysis, the diffusion of reagents to the surface and diffusion of products from the surface can be rate determining. A nanomaterial-based catalyst is an example of a heterogeneous catalyst. Analogous events associated with substrate binding and product dissociation apply to homogeneous catalysts. Although catalysts are not consumed by the reaction itself, they may be inhibited, deactivated, or destroyed by secondary processes. In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products. Additionally, heterogeneous catalysts can dissolve into the solution in a solid–liquid system or sublimate in a solid–gas system; the production of most industrially important chemicals involves catalysis. Most biochemically significant processes are catalysed.
Research into catalysis is a major field in applied science and involves many areas of chemistry, notably organometallic chemistry and materials science. Catalysis is relevant to many aspects of environmental science, e.g. the catalytic converter in automobiles and the dynamics of the ozone hole. Catalytic reactions are preferred in environmentally friendly green chemistry due to the reduced amount of waste generated, as opposed to stoichiometric reactions in which all reactants are consumed and more side products are formed. Many transition metals and transition metal complexes are used in catalysis as well. Catalysts called. A catalyst works by providing an alternative reaction pathway to the reaction product; the rate of the reaction is increased as this alternative route has a lower activation energy than the reaction route not mediated by the catalyst. The disproportionation of hydrogen peroxide creates oxygen, as shown below. 2 H2O2 → 2 H2O + O2This reaction is preferable in the sense that the reaction products are more stable than the starting material, though the uncatalysed reaction is slow.
In fact, the decomposition of hydrogen peroxide is so slow that hydrogen peroxide solutions are commercially available. This reaction is affected by catalysts such as manganese dioxide, or the enzyme peroxidase in organisms. Upon the addition of a small amount of manganese dioxide, the hydrogen peroxide reacts rapidly; this effect is seen by the effervescence of oxygen. The manganese dioxide is not consumed in the reaction, thus may be recovered unchanged, re-used indefinitely. Accordingly, manganese dioxide catalyses this reaction. Catalytic activity is denoted by the symbol z and measured in mol/s, a unit, called katal and defined the SI unit for catalytic activity since 1999. Catalytic activity is not a kind of reaction rate, but a property of the catalyst under certain conditions, in relation to a specific chemical reaction. Catalytic activity of one katal of a catalyst means one mole of that catalyst will catalyse 1 mole of the reactant to product in one second. A catalyst may and will have different catalytic activity for di
Flavin-containing amine oxidoreductase
Flavin-containing amine oxidoreductases are a family of various amine oxidases, including maize polyamine oxidase, L-amino acid oxidases and various flavin containing monoamine oxidases. The aligned region includes the flavin binding site of these enzymes. In vertebrates, MAO plays an important role in regulating the intracellular levels of amines via their oxidation. In lower eukaryotes such as aspergillus and in bacteria the main role of amine oxidases is to provide a source of ammonium. PAOs in plants and protozoa oxidise spermidine and spermine to an aminobutyral and hydrogen peroxide and are involved in the catabolism of polyamines. Other members of this family include tryptophan 2-monooxygenase, putrescine oxidase, corticosteroid-binding proteins, antibacterial glycoproteins. AOF1.
Endothelium refers to cells that line the interior surface of blood vessels and lymphatic vessels, forming an interface between circulating blood or lymph in the lumen and the rest of the vessel wall. It is a thin layer of single-layered, squamous cells called endothelial cells. Endothelial cells in direct contact with blood are called vascular endothelial cells, whereas those in direct contact with lymph are known as lymphatic endothelial cells. Vascular endothelial cells line the entire circulatory system, from the heart to the smallest capillaries; these cells have unique functions in vascular biology. These functions include fluid filtration, such as in the glomerulus of the kidney, blood vessel tone, neutrophil recruitment, hormone trafficking. Endothelium of the interior surfaces of the heart chambers is called endocardium. Endothelium is mesodermal in origin. Both blood and lymphatic capillaries are composed of a single layer of endothelial cells called a monolayer. In straight sections of a blood vessel, vascular endothelial cells align and elongate in the direction of fluid flow.
The foundational model of anatomy makes a distinction between endothelial cells and epithelial cells on the basis of which tissues they develop from, states that the presence of vimentin rather than keratin filaments separate these from epithelial cells. Many considered the endothelium a specialized epithelial tissue. Endothelial cells are involved in many aspects of vascular biology, including: Barrier function - the endothelium acts as a semi-selective barrier between the vessel lumen and surrounding tissue, controlling the passage of materials and the transit of white blood cells into and out of the bloodstream. Excessive or prolonged increases in permeability of the endothelial monolayer, as in cases of chronic inflammation, may lead to tissue edema/swelling. Blood clotting; the endothelium provides a non-thrombogenic surface because it contains, for example, heparan sulfate which acts as a cofactor for activating antithrombin, a protease that inactivates several factors in the coagulation cascade.
Inflammation Formation of new blood vessels Vasoconstriction and vasodilation, hence the control of blood pressure Repair of damaged or diseased organs via an injection of blood vessel cells Angiopoietin-2 works with VEGF to facilitate cell proliferation and migration of endothelial cells Endothelial dysfunction, or the loss of proper endothelial function, is a hallmark for vascular diseases, is regarded as a key early event in the development of atherosclerosis. Impaired endothelial function, causing hypertension and thrombosis, is seen in patients with coronary artery disease, diabetes mellitus, hypercholesterolemia, as well as in smokers. Endothelial dysfunction has been shown to be predictive of future adverse cardiovascular events, is present in inflammatory disease such as rheumatoid arthritis and systemic lupus erythematosus. One of the main mechanisms of endothelial dysfunction is the diminishing of nitric oxide due to high levels of asymmetric dimethylarginine, which interfere with the normal L-arginine-stimulated nitric oxide synthesis and so leads to hypertension.
The most prevailing mechanism of endothelial dysfunction is an increase in reactive oxygen species, which can impair nitric oxide production and activity via several mechanisms. The signalling protein ERK5 is essential for maintaining normal endothelial cell function. A further consequence of damage to the endothelium is the release of pathological quantities of von Willebrand factor, which promote platelet aggregation and adhesion to the subendothelium, thus the formation of fatal thrombi. Anatomy photo: Circulatory/vessels/capillaries1/capillaries3 - Comparative Organology at University of California, Davis, "Capillaries, non-fenestrated" Histology image: 21402ooa – Histology Learning System at Boston University Endothelium Journal of Endothelial Cell Research, Informa Healthcare Endothelium and inflammation Platelet Activation, University of Washington
The mitochondrion is a double-membrane-bound organelle found in most eukaryotic organisms. Some cells in some multicellular organisms may, lack them. A number of unicellular organisms, such as microsporidia and diplomonads, have reduced or transformed their mitochondria into other structures. To date, only one eukaryote, Monocercomonoides, is known to have lost its mitochondria; the word mitochondrion comes from the Greek μίτος, mitos, "thread", χονδρίον, chondrion, "granule" or "grain-like". Mitochondria generate most of the cell's supply of adenosine triphosphate, used as a source of chemical energy. A mitochondrion was thus termed the powerhouse of the cell. Mitochondria are between 0.75 and 3 μm in diameter but vary in size and structure. Unless stained, they are not visible. In addition to supplying cellular energy, mitochondria are involved in other tasks, such as signaling, cellular differentiation, cell death, as well as maintaining control of the cell cycle and cell growth. Mitochondrial biogenesis is in turn temporally coordinated with these cellular processes.
Mitochondria have been implicated in several human diseases, including mitochondrial disorders, cardiac dysfunction, heart failure and autism. The number of mitochondria in a cell can vary by organism and cell type. For instance, red blood cells have no mitochondria, whereas liver cells can have more than 2000; the organelle is composed of compartments. These compartments or regions include the outer membrane, the intermembrane space, the inner membrane, the cristae and matrix. Although most of a cell's DNA is contained in the cell nucleus, the mitochondrion has its own independent genome that shows substantial similarity to bacterial genomes. Mitochondrial proteins vary depending on the species. In humans, 615 distinct types of protein have been identified from cardiac mitochondria, whereas in rats, 940 proteins have been reported; the mitochondrial proteome is thought to be dynamically regulated. The first observations of intracellular structures that represented mitochondria were published in the 1840s.
Richard Altmann, in 1890, established them as cell organelles and called them "bioblasts". The term "mitochondria" was coined by Carl Benda in 1898. Leonor Michaelis discovered that Janus green can be used as a supravital stain for mitochondria in 1900. In 1904, Friedrich Meves, made the first recorded observation of mitochondria in plants in cells of the white waterlily, Nymphaea alba and in 1908, along with Claudius Regaud, suggested that they contain proteins and lipids. Benjamin F. Kingsbury, in 1912, first related them with cell respiration, but exclusively based on morphological observations. In 1913, particles from extracts of guinea-pig liver were linked to respiration by Otto Heinrich Warburg, which he called "grana". Warburg and Heinrich Otto Wieland, who had postulated a similar particle mechanism, disagreed on the chemical nature of the respiration, it was not until 1925, when David Keilin discovered cytochromes, that the respiratory chain was described. In 1939, experiments using minced muscle cells demonstrated that cellular respiration using one oxygen atom can form two adenosine triphosphate molecules, and, in 1941, the concept of the phosphate bonds of ATP being a form of energy in cellular metabolism was developed by Fritz Albert Lipmann.
In the following years, the mechanism behind cellular respiration was further elaborated, although its link to the mitochondria was not known. The introduction of tissue fractionation by Albert Claude allowed mitochondria to be isolated from other cell fractions and biochemical analysis to be conducted on them alone. In 1946, he concluded that cytochrome oxidase and other enzymes responsible for the respiratory chain were isolated to the mitochondria. Eugene Kennedy and Albert Lehninger discovered in 1948 that mitochondria are the site of oxidative phosphorylation in eukaryotes. Over time, the fractionation method was further developed, improving the quality of the mitochondria isolated, other elements of cell respiration were determined to occur in the mitochondria; the first high-resolution electron micrographs appeared in 1952, replacing the Janus Green stains as the preferred way of visualising the mitochondria. This led to a more detailed analysis of the structure of the mitochondria, including confirmation that they were surrounded by a membrane.
It showed a second membrane inside the mitochondria that folded up in ridges dividing up the inner chamber and that the size and shape of the mitochondria varied from cell to cell. The popular term "powerhouse of the cell" was coined by Philip Siekevitz in 1957. In 1967, it was discovered. In 1968, methods were developed for mapping the mitochondrial genes, with the genetic and physical map of yeast mitochondrial DNA being completed in 1976. There are two hypotheses about the origin of mitochondria: autogenous; the endosymbiotic hypothesis suggests that mitochondria were prokaryotic cells, capable of implementing oxidative mechanisms that were not possible for eukaryotic cells. In the autogenous hypothesis, mitochondria were born by splitting off a portion of DNA from the nucleus of the eukaryotic cell at the time of divergence with the prokaryotes. Since mitochondria have many features in common with bacteria, the endosymbiotic hypothesis is more accepted. A mitochondrion contains DNA, which i