Protozoa is an informal term for single-celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microorganisms or organic tissues and debris. The protozoa were regarded as "one-celled animals", because they possess animal-like behaviors, such as motility and predation, lack a cell wall, as found in plants and many algae. Although the traditional practice of grouping protozoa with animals is no longer considered valid, the term continues to be used in a loose way to identify single-celled organisms that can move independently and feed by heterotrophy. In some systems of biological classification, Protozoa is a high-level taxonomic group; when first introduced in 1818, Protozoa was erected as a taxonomic class, but in classification schemes it was elevated to a variety of higher ranks, including phylum and kingdom. In a series of classifications proposed by Thomas Cavalier-Smith and his collaborators since 1981, Protozoa has been ranked as a kingdom; the seven-kingdom scheme presented by Ruggiero et al. in 2015, places eight phyla under Kingdom Protozoa: Euglenozoa, Metamonada, Choanozoa sensu Cavalier-Smith, Percolozoa and Sulcozoa.
Notably, this kingdom excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, dinoflagellates and the parasitic apicomplexans, all of which are classified under Kingdom Chromista. Kingdom Protozoa, as defined in this scheme, does not form a natural group or clade, but a paraphyletic group or evolutionary grade, within which the members of Fungi and Chromista are thought to have evolved; the word "protozoa" was coined in 1818 by zoologist Georg August Goldfuss, as the Greek equivalent of the German Urthiere, meaning "primitive, or original animals". Goldfuss created Protozoa as a class containing; the group included not only single-celled microorganisms but some "lower" multicellular animals, such as rotifers, sponges, jellyfish and polychaete worms. The term Protozoa is formed from the Greek words πρῶτος, meaning "first", ζῶα, plural of ζῶον, meaning "animal"; the use of Protozoa as a formal taxon has been discouraged by some researchers because the term implies kinship with animals and promotes an arbitrary separation of "animal-like" from "plant-like" organisms.
In 1848, as a result of advancements in cell theory pioneered by Theodor Schwann and Matthias Schleiden, the anatomist and zoologist C. T. von Siebold proposed that the bodies of protozoans such as ciliates and amoebae consisted of single cells, similar to those from which the multicellular tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only such unicellular forms, to the exclusion of all metazoa. At the same time, he raised the group to the level of a phylum containing two broad classes of microorganisms: Infusoria, Rhizopoda; the definition of Protozoa as a phylum or sub-kingdom composed of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology"—and the term came into wide use. As a phylum under Animalia, the Protozoa were rooted in the old "two-kingdom" classification of life, according to which all living beings were classified as either animals or plants; as long as this scheme remained dominant, the protozoa were understood to be animals and studied in departments of Zoology, while photosynthetic microorganisms and microscopic fungi—the so-called Protophyta—were assigned to the Plants, studied in departments of Botany.
Criticism of this system began in the latter half of the 19th century, with the realization that many organisms met the criteria for inclusion among both plants and animals. For example, the algae Euglena and Dinobryon have chloroplasts for photosynthesis, but can feed on organic matter and are motile. In 1860, John Hogg argued against the use of "protozoa", on the grounds that "naturalists are divided in opinion—and some will continue so—whether many of these organisms, or living beings, are animals or plants." As an alternative, he proposed a new kingdom called Primigenum, consisting of both the protozoa and unicellular algae, which he combined together under the name "Protoctista". In Hoggs's conception, the animal and plant kingdoms were likened to two great "pyramids" blending at their bases in the Kingdom Primigenum. Six years Ernst Haeckel proposed a third kingdom of life, which he named Protista. At first, Haeckel included a few multicellular organisms in this kingdom, but in work he restricted the Protista to single-celled organisms, or simple colonies whose individual cells are not differentiated into different kinds of tissues.
Despite these proposals, Protozoa emerged as the preferred taxonomic placement for heterotrophic microorganisms such as amoebae and ciliates, remained so for more than a century. In the course of the 20th century, the old "two kingdom" system began to weaken, with the growing awareness that fungi did not belong among the plants, that most of the unicellular protozoa were no more related to the animals than they were to the plants. By mid-century, some biologists, such as Herbert Copeland, Robert H. Whittaker and Lynn Margulis, advocated the revival of Haeckel's Protista or Hogg's Protoctista as a kingdom-level eukaryotic group, alongside Plants and Fungi. A variety of multi-kingdom systems were proposed, Kingdoms Protista and Protoctista became well est
An autotroph or primary producer, is an organism that produces complex organic compounds from simple substances present in its surroundings using energy from light or inorganic chemical reactions. They are the producers such as plants on land or algae in water, they do not need a living source of energy or organic carbon. Autotrophs can reduce carbon dioxide to make organic compounds for biosynthesis and create a store of chemical energy. Most autotrophs use water as the reducing agent, but some can use other hydrogen compounds such as hydrogen sulfide; some autotrophs, such as green plants and algae, are phototrophs, meaning that they convert electromagnetic energy from sunlight into chemical energy in the form of reduced carbon. Autotrophs can be chemoautotrophs. Phototrophs use light as an energy source, while chemotrophs use electron donors as a source of energy, whether from organic or inorganic sources; such chemotrophs are lithotrophs. Lithotrophs use inorganic compounds, such as hydrogen sulfide, elemental sulfur and ferrous iron, as reducing agents for biosynthesis and chemical energy storage.
Photoautotrophs and lithoautotrophs use a portion of the ATP produced during photosynthesis or the oxidation of inorganic compounds to reduce NADP+ to NADPH to form organic compounds. The Greek term autotroph was coined by the German botanist Albert Bernhard Frank in 1892, it stems from the ancient Greek word τροφή, meaning "nourishment" or "food". Some organisms rely on organic compounds as a source of carbon, but are able to use light or inorganic compounds as a source of energy; such organisms are not defined rather as heterotrophic. An organism that obtains carbon from organic compounds but obtains energy from light is called a photoheterotroph, while an organism that obtains carbon from organic compounds but obtains energy from the oxidation of inorganic compounds is termed a chemoheterotroph, chemolithoheterotroph, or lithoheterotroph. Evidence suggests that some fungi may obtain energy from radiation; such radiotrophic fungi were found growing inside a reactor of the Chernobyl nuclear power plant.
Autotrophs are fundamental to the food chains of all ecosystems in the world. They take energy from the environment in the form of sunlight or inorganic chemicals and use it to create energy-rich molecules such as carbohydrates; this mechanism is called primary production. Other organisms, called heterotrophs, take in autotrophs as food to carry out functions necessary for their life. Thus, heterotrophs — all animals all fungi, as well as most bacteria and protozoa — depend on autotrophs, or primary producers, for the energy and raw materials they need. Heterotrophs obtain energy by breaking down organic molecules obtained in food. Carnivorous organisms rely on autotrophs indirectly, as the nutrients obtained from their heterotroph prey come from autotrophs they have consumed. Most ecosystems are supported by the autotrophic primary production of plants that capture photons released by the sun. Plants can only use a fraction of this energy for photosynthesis 1% is used by autotrophs; the process of photosynthesis splits a water molecule, releasing oxygen into the atmosphere, reducing carbon dioxide to release the hydrogen atoms that fuel the metabolic process of primary production.
Plants convert and store the energy of the photon into the chemical bonds of simple sugars during photosynthesis. These plant sugars are polymerized for storage as long-chain carbohydrates, including other sugars and cellulose; when autotrophs are eaten by heterotrophs, i.e. consumers such as animals, the carbohydrates and proteins contained in them become energy sources for the heterotrophs. Proteins can be made using nitrates and phosphates in the soil. Electrolithoautotroph Organotroph Electrotroph Primary nutritional groups Heterotrophic nutrition
Glycerol is a simple polyol compound. It is a colorless, viscous liquid, sweet-tasting and non-toxic; the glycerol backbone is found in many lipids which are known as glycerides. It is used in the food industry as a sweetener and humectant in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. Although achiral, glycerol is prochiral with respect to reactions of one of the two primary alcohols. Thus, in substituted derivatives, the stereospecific numbering labels each carbon as either sn-1, sn-2, or sn-3. Glycerol is obtained from plant and animal sources where it occurs in triglycerides, esters of glycerol with long-chain carboxylic acids; the hydrolysis, saponification, or transesterification of these triglycerides produces glycerol as well as the fatty acid derivative: Triglycerides can be saponified with sodium hydroxide to give glycerol and fatty sodium salt or soap. Typical plant sources include soybeans or palm.
Animal-derived tallow is another source. 950,000 tons per year are produced in the United States and Europe. The EU directive 2003/30/EC set a requirement that 5.75% of petroleum fuels are to be replaced with biofuel sources across all member states by 2010. It was projected in 2006 that by the year 2020, production would be six times more than demand, creating an excess of glycerol. Glycerol from triglycerides is produced on a large scale, but the crude product is of variable quality, with a low selling price of as low as 2-5 U. S. cents per kilogram in 2011. It can be purified, but the process is expensive; some glycerol is burned for energy, but its heat value is low. Crude glycerol from the hydrolysis of triglycerides can be purified by treatment with activated carbon to remove organic impurities, alkali to remove unreacted glycerol esters, ion exchange to remove salts. High purity glycerol is obtained by multi-step distillation. Although not cost-effective, glycerol can be produced by various routes from propylene.
The epichlorohydrin process is the most important. This epichlorohydrin is hydrolyzed to give glycerol. Chlorine-free processes from propylene include the synthesis of glycerol from acrolein and propylene oxide; because of the large-scale production of biodiesel from fats, where glycerol is a waste product, the market for glycerol is depressed. Thus, synthetic processes are not economical. Owing to oversupply, efforts are being made to convert glycerol to synthetic precursors, such as acrolein and epichlorohydrin. (See the Chemical intermediate section of this article. In food and beverages, glycerol serves as a humectant and sweetener, may help preserve foods, it is used as filler in commercially prepared low-fat foods, as a thickening agent in liqueurs. Glycerol and water are used to preserve certain types of plant leaves; as a sugar substitute, it has 27 kilocalories per teaspoon and is 60% as sweet as sucrose. It does not feed the bacteria that form plaques and cause dental cavities; as a food additive, glycerol is labeled as E number E422.
It is added to icing to prevent it from setting too hard. As used in foods, glycerol is categorized by the Academy of Nutrition and Dietetics as a carbohydrate; the U. S. Food and Drug Administration carbohydrate designation includes all caloric macronutrients excluding protein and fat. Glycerol has a caloric density similar to table sugar, but a lower glycemic index and different metabolic pathway within the body, so some dietary advocates accept glycerol as a sweetener compatible with low-carbohydrate diets, it is recommended as an additive when using polyol sweeteners such as erythritol and xylitol which have a cooling effect, due to its heating effect in the mouth, if the cooling effect is not wanted. Glycerol is used in medical and personal care preparations as a means of improving smoothness, providing lubrication, as a humectant. Ichthyosis and xerosis have been relieved by the topical use glycerin, it is found in allergen immunotherapies, cough syrups and expectorants, mouthwashes, skin care products, shaving cream, hair care products and water-based personal lubricants.
In solid dosage forms like tablets, glycerol is used as a tablet holding agent. For human consumption, glycerol is classified by the U. S. FDA among the sugar alcohols as a caloric macronutrient. Glycerol is used in blood banking to preserve red blood cells prior to freezing. Glycerol is a component of glycerin soap. Essential oils are added for fragrance; this kind of soap is used by people with sensitive irritated skin because it prevents skin dryness with its moisturizing properties. It draws moisture up through skin layers and slows or prevents excessive drying and evaporation. Taken rectally, glycerol functions as a laxative by irritating the anal mucosa and inducing a hyperosmotic effect, it may be administered undiluted either as a suppository or as a small-volume enema. Alternatively, it may be administered in a dilute solution, e.g. 5%, as a high volume enema. Taken orally, glycerol can cause a rapid, temporary decrease in the internal pressure of the eye; this can be useful for the initial emergency treatment of elevated eye pressure.
Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+. The free energy released in this process is used to form the high-energy molecules ATP and NADH. Glycolysis is a sequence of ten enzyme-catalyzed reactions. Most monosaccharides, such as fructose and galactose, can be converted to one of these intermediates; the intermediates may be directly useful. For example, the intermediate dihydroxyacetone phosphate is a source of the glycerol that combines with fatty acids to form fat. Glycolysis is an oxygen-independent metabolic pathway; the wide occurrence of glycolysis indicates. Indeed, the reactions that constitute glycolysis and its parallel pathway, the pentose phosphate pathway, occur metal-catalyzed under the oxygen-free conditions of the Archean oceans in the absence of enzymes. In most organisms, glycolysis occurs in the cytosol; the most common type of glycolysis is the Embden–Meyerhof–Parnas, discovered by Gustav Embden, Otto Meyerhof, Jakub Karol Parnas.
Glycolysis refers to other pathways, such as the Entner–Doudoroff pathway and various heterofermentative and homofermentative pathways. However, the discussion here will be limited to the Embden–Meyerhof–Parnas pathway; the glycolysis pathway can be separated into two phases: The Preparatory/Investment Phase – wherein ATP is consumed. The Pay Off Phase – wherein ATP is produced; the overall reaction of glycolysis is: The use of symbols in this equation makes it appear unbalanced with respect to oxygen atoms, hydrogen atoms, charges. Atom balance is maintained by the two phosphate groups: Each exists in the form of a hydrogen phosphate anion, dissociating to contribute 2 H+ overall Each liberates an oxygen atom when it binds to an ADP molecule, contributing 2 O overallCharges are balanced by the difference between ADP and ATP. In the cellular environment, all three hydroxyl groups of ADP dissociate into −O− and H+, giving ADP3−, this ion tends to exist in an ionic bond with Mg2+, giving ADPMg−.
ATP behaves identically except that it has four hydroxyl groups, giving ATPMg2−. When these differences along with the true charges on the two phosphate groups are considered together, the net charges of −4 on each side are balanced. For simple fermentations, the metabolism of one molecule of glucose to two molecules of pyruvate has a net yield of two molecules of ATP. Most cells will carry out further reactions to'repay' the used NAD+ and produce a final product of ethanol or lactic acid. Many bacteria use inorganic compounds as hydrogen acceptors to regenerate the NAD+. Cells performing aerobic respiration synthesize much more ATP, but not as part of glycolysis; these further aerobic reactions use pyruvate and NADH + H+ from glycolysis. Eukaryotic aerobic respiration produces 34 additional molecules of ATP for each glucose molecule, however most of these are produced by a vastly different mechanism to the substrate-level phosphorylation in glycolysis; the lower-energy production, per glucose, of anaerobic respiration relative to aerobic respiration, results in greater flux through the pathway under hypoxic conditions, unless alternative sources of anaerobically oxidizable substrates, such as fatty acids, are found.
The pathway of glycolysis as it is known today took 100 years to discover. The combined results of many smaller experiments were required in order to understand the pathway as a whole; the first steps in understanding glycolysis began in the nineteenth century with the wine industry. For economic reasons, the French wine industry sought to investigate why wine sometime turned distasteful, instead of fermenting into alcohol. French scientist Louis Pasteur researched this issue during the 1850s, the results of his experiments began the long road to elucidating the pathway of glycolysis, his experiments showed. While Pasteur's experiments were groundbreaking, insight into the component steps of glycolysis were provided by the non-cellular fermentation experiments of Eduard Buchner during the 1890s. Buchner demonstrated that the conversion of glucose to ethanol was possible using a non-living extract of yeast; this experiment not only revolutionized biochemistry, but allowed scientists to analyze this pathway in a more controlled lab setting.
In a series of experiments, scientists Arthur Harden and William Young discovered more pieces of glycolysis. They discovered the regulatory effects of ATP on glucose consumption during alcohol fermentation, they shed light on the role of one compound as a glycolysis intermediate: fructose 1,6-bisphosphate. The elucidation of fructose 1,6-bisphosphate was accomplished by measuring CO2 levels when yeast juice was incubated with glucose. CO2 production increased then slowed down. Harden and Young noted that this process would restart if an inorganic phosphate was added to the mixture. Harden and Young deduced that this process produced organic phosphate esters, further experiments allowed them to extract fructose diphosphate. Arthur Harden and William Young along with Nick Sheppard determined, in a second experiment, that a heat-sensitive high-molecular-weight subcellular fraction and a heat-insensitive low-molecular-weight cytoplasm fraction are required together for fermenta
Archaea constitute a domain of single-celled microorganisms. These microbes are prokaryotes. Archaea were classified as bacteria, receiving the name archaebacteria, but this classification is outdated. Archaeal cells have unique properties separating them from the other two domains of life and Eukarya. Archaea are further divided into multiple recognized phyla. Classification is difficult because most have not been isolated in the laboratory and were only detected by analysis of their nucleic acids in samples from their environment. Archaea and bacteria are similar in size and shape, although a few archaea have shapes quite unlike that of bacteria, such as the flat and square-shaped cells of Haloquadratum walsbyi. Despite this morphological similarity to bacteria, archaea possess genes and several metabolic pathways that are more related to those of eukaryotes, notably for the enzymes involved in transcription and translation. Other aspects of archaeal biochemistry are unique, such as their reliance on ether lipids in their cell membranes, including archaeols.
Archaea use more energy sources than eukaryotes: these range from organic compounds, such as sugars, to ammonia, metal ions or hydrogen gas. Salt-tolerant archaea use sunlight as an energy source, other species of archaea fix carbon, but unlike plants and cyanobacteria, no known species of archaea does both. Archaea reproduce asexually by budding; the first observed archaea were extremophiles, living in harsh environments, such as hot springs and salt lakes with no other organisms, but improved detection tools led to the discovery of archaea in every habitat, including soil and marshlands. They are part of the microbiota of all organisms, in the human microbiota they are important in the gut, on the skin. Archaea are numerous in the oceans, the archaea in plankton may be one of the most abundant groups of organisms on the planet. Archaea are a major part of Earth's life, may play roles in the carbon cycle and the nitrogen cycle. No clear examples of archaeal pathogens or parasites are known. Instead they are mutualists or commensals, such as the methanogens that inhabit the gastrointestinal tract in humans and ruminants, where their vast numbers aid digestion.
Methanogens are used in biogas production and sewage treatment, biotechnology exploits enzymes from extremophile archaea that can endure high temperatures and organic solvents. For much of the 20th century, prokaryotes were regarded as a single group of organisms and classified based on their biochemistry and metabolism. For example, microbiologists tried to classify microorganisms based on the structures of their cell walls, their shapes, the substances they consume. In 1965, Emile Zuckerkandl and Linus Pauling proposed instead using the sequences of the genes in different prokaryotes to work out how they are related to each other; this phylogenetic approach is the main method used today. Archaea – at that time only the methanogens were known – were first classified separately from bacteria in 1977 by Carl Woese and George E. Fox based on their ribosomal RNA genes, they called these groups the Urkingdoms of Archaebacteria and Eubacteria, though other researchers treated them as kingdoms or subkingdoms.
Woese and Fox gave the first evidence for Archaebacteria as a separate "line of descent": 1. Lack of peptidoglycan in their cell walls, 2. Two unusual coenzymes, 3. Results of 16S ribosomal RNA gene sequencing. To emphasize this difference, Otto Kandler and Mark Wheelis proposed reclassifying organisms into three natural domains known as the three-domain system: the Eukarya, the Bacteria and the Archaea, in what is now known as "The Woesian Revolution"; the word archaea comes from the Ancient Greek ἀρχαῖα, meaning "ancient things", as the first representatives of the domain Archaea were methanogens and it was assumed that their metabolism reflected Earth's primitive atmosphere and the organisms' antiquity, but as new habitats were studied, more organisms were discovered. Extreme halophilic and hyperthermophilic microbes were included in Archaea. For a long time, archaea were seen as extremophiles that only exist in extreme habitats such as hot springs and salt lakes, but by the end of the 20th century, archaea had been identified in non-extreme environments as well.
Today, they are known to be a large and diverse group of organisms abundantly distributed throughout nature. This new appreciation of the importance and ubiquity of archaea came from using polymerase chain reaction to detect prokaryotes from environmental samples by multiplying their ribosomal genes; this allows the detection and identification of organisms that have not been cultured in the laboratory. The classification of archaea, of prokaryotes in general, is a moving and contentious field. Current classification systems aim to organize archaea into groups of organisms that share structural features and common ancestors; these classifications rely on the use of the sequence of ribosomal RNA genes to reveal relationships between organisms. Most of the culturable and well-investigated species of archaea are members of two main phyla, the Euryarchaeota and Crenarchaeota. Other groups have been tentatively created. For example, the peculiar species Nanoarchaeum equitans, discovered in 2003, has been given its own phylum, the Nanoarchaeota.
A new phylum Korarchaeota has been proposed. It contains a sm
Acetic acid, systematically named ethanoic acid, is a colourless liquid organic compound with the chemical formula CH3COOH. When undiluted, it is sometimes called glacial acetic acid. Vinegar is no less than 4% acetic acid by volume, making acetic acid the main component of vinegar apart from water. Acetic acid has pungent smell. In addition to household vinegar, it is produced as a precursor to polyvinyl acetate and cellulose acetate, it is classified as a weak acid since it only dissociates in solution, but concentrated acetic acid is corrosive and can attack the skin. Acetic acid is the second simplest carboxylic acid, it consists of a methyl group attached to a carboxyl group. It is an important chemical reagent and industrial chemical, used in the production of cellulose acetate for photographic film, polyvinyl acetate for wood glue, synthetic fibres and fabrics. In households, diluted acetic acid is used in descaling agents. In the food industry, acetic acid is controlled by the food additive code E260 as an acidity regulator and as a condiment.
In biochemistry, the acetyl group, derived from acetic acid, is fundamental to all forms of life. When bound to coenzyme A, it is central to the metabolism of fats; the global demand for acetic acid is about 6.5 million metric tons per year, of which 1.5 Mt/a is met by recycling. Vinegar is dilute acetic acid produced by fermentation and subsequent oxidation of ethanol; the trivial name acetic acid is the most used and preferred IUPAC name. The systematic name ethanoic acid, a valid IUPAC name, is constructed according to the substitutive nomenclature; the name acetic acid derives from acetum, the Latin word for vinegar, is related to the word acid itself. Glacial acetic acid is a name for water-free acetic acid. Similar to the German name Eisessig, the name comes from the ice-like crystals that form below room temperature at 16.6 °C. A common symbol for acetic acid is AcOH, where Ac is the pseudoelement symbol representing the acetyl group CH3−C−. To better reflect its structure, acetic acid is written as CH3–COH, CH3−COH, CH3COOH, CH3CO2H.
In the context of acid-base reactions, the abbreviation HAc is sometimes used, where Ac in this case is a symbol for acetate. Acetate is the ion resulting from loss of H+ from acetic acid; the name acetate can refer to a salt containing this anion, or an ester of acetic acid. The hydrogen centre in the carboxyl group in carboxylic acids such as acetic acid can separate from the molecule by ionization: CH3CO2H ⇌ CH3CO2− + H+Because of this release of the proton, acetic acid has acidic character. Acetic acid is a weak monoprotic acid. In aqueous solution, it has a pKa value of 4.76. Its conjugate base is acetate. A 1.0 M solution has a pH of 2.4, indicating that 0.4% of the acetic acid molecules are dissociated. However, in dilute solution acetic acid is >90% dissociated. In solid acetic acid, the molecules form chains, individual molecules being interconnected by hydrogen bonds. In the vapour at 120 °C, dimers can be detected. Dimers occur in the liquid phase in dilute solutions in non-hydrogen-bonding solvents, a certain extent in pure acetic acid, but are disrupted by hydrogen-bonding solvents.
The dissociation enthalpy of the dimer is estimated at 65.0–66.0 kJ/mol, the dissociation entropy at 154–157 J mol−1 K−1. Other carboxylic acids engage in similar intermolecular hydrogen bonding interactions. Liquid acetic acid is a hydrophilic protic similar to ethanol and water. With a moderate relative static permittivity of 6.2, it dissolves not only polar compounds such as inorganic salts and sugars, but non-polar compounds such as oils as well as polar solutes. It is miscible with polar and non-polar solvents such as water and hexane. With higher alkanes, acetic acid is not miscible, its miscibility declines with longer n-alkanes; the solvent and miscibility properties of acetic acid make it a useful industrial chemical, for example, as a solvent in the production of dimethyl terephthalate. At physiological pHs, acetic acid is fully ionised to acetate; the acetyl group, formally derived from acetic acid, is fundamental to all forms of life. When bound to coenzyme A, it is central to the metabolism of fats.
Unlike longer-chain carboxylic acids, acetic acid does not occur in natural triglycerides. However, the artificial triglyceride triacetin is a common food additive and is found in cosmetics and topical medicines. Acetic acid is produced and excreted by acetic acid bacteria, notably the genus Acetobacter and Clostridium acetobutylicum; these bacteria are found universally in foodstuffs and soil, acetic acid is produced as fruits and other foods spoil. Acetic acid is a component of the vaginal lubrication of humans and other primates, where it appears to serve as a mild antibacterial agent. Acetic acid is produced industrially both synthetically and by bacterial fermentation. About 75% of acetic acid made for use in the chemical industry is made by the carbonylation of methanol, explained below; the biological route accounts for only a
Phagocytosis is the process by which a cell uses its plasma membrane to engulf a large particle, giving rise to an internal compartment called the phagosome. It is one type of endocytosis. In a multicellular organism's immune system, phagocytosis is a major mechanism used to remove pathogens and cell debris; the ingested material is digested in the phagosome. Bacteria, dead tissue cells, small mineral particles are all examples of objects that may be phagocytized; some protozoa use phagocytosis. Phagocytosis was first noted by Canadian physician William Osler, studied and named by Élie Metchnikoff. Phagocytosis is one of the main mechanisms of the innate immune defense, it is one of the first processes responding to infection, is one of the initiating branches of an adaptive immune response. Although most cells are capable of phagocytosis, some cell types perform it as part of their main function; these are called'professional phagocytes.' Phagocytosis is old in evolutionary terms, being present in invertebrates.
Neutrophils, monocytes, dendritic cells and eosinophils can be classified as professional phagocytes. The first three have the greatest role in immune response to most infections; the role of neutrophils is patrolling the bloodstream and rapid migration to the tissues in large numbers only in case of infection. There they have direct microbicidal effect by phagocytosis. After ingestion, neutrophils are efficient in intracellular killing of pathogens. Neutrophils phagocytose via the Fcγ receptors and complement receptors 1 and 3; the microbicidal effect of neutrophils is due to a large repertoire of molecules present in pre-formed granules. Enzymes and other molecules prepared in these granules are proteases, such as collagenase, gelatinase or serine proteases, myeloperoxidase and antibiotic proteins. Degranulation of these into the phagosome, accompanied by high reactive oxygen species production is microbicidal. Monocytes, the macrophages that mature from them, leave blood circulation to migrate through tissues.
There they form a resting barrier. Macrophages initiate phagocytosis by mannose receptors, scavenger receptors, Fcγ receptors and complement receptors 1, 3 and 4. Macrophages can continue phagocytosis by forming new lysosomes. Dendritic cells reside in tissues and ingest pathogens by phagocytosis, their role is not killing or clearance of microbes, but rather breaking them down for antigen presentation to the cells of the adaptive immune system. Receptors for phagocytosis can be divided into two categories by recognised molecules; the first, opsonic receptors, are dependent on opsonins. Among these are receptors that recognise the Fc part of bound IgG antibodies, deposited complement or receptors, that recognise other opsonins of cell or plasma origin. Non-opsonic receptors include Dectin receptor, or scavenger receptors; some phagocytic pathways require a second signal from pattern recognition receptors activated by attachment to pathogen-associated molecular patterns, which leads to NF-κB activation.
Fcγ receptors recognise IgG coated targets. The main recognised part is the Fc fragment; the molecule of the receptor contain an intracellular ITAM domain or associates with an ITAM-containing adaptor molecule. ITAM domains transduce the signal from the surface of the phagocyte to the nucleus. For example activating receptors of human macrophages are FcγRI, FcγRIIA, FcγRIII. Fcγ receptor mediated phagocytosis includes formation of protrusions of the cell called a'phagocytic cup' and activates an oxidative burst in neutrophils; these receptors recognise targets coated in C4b and C3bi from plasma complement. The extracellular domain of the receptors contains a lectin-like complement-binding domain. Recognition by complement receptors is not enough to cause internalisation without additional signals. In macrophages, the CR1, CR3 and CR4 are responsible for recognition of targets. Complement coated targets are internalised by'sinking' into the phagocyte membrane, without any protrusions. Mannose and other pathogen-associated sugars, such as fucose, are recognised by the mannose receptor.
Eight lectin-like domains form the extracellular part of the receptor. The ingestion mediated by the mannose receptor is distinct in molecular mechanisms from Fcγ receptor or complement receptor mediated phagocytosis. Engulfment of material is facilitated by the actin-myosin contractile system; the phagosome is the organelle formed by phagocytosis of material. It moves toward the centrosome of the phagocyte and is fused with lysosomes, forming a phagolysosome and leading to degradation. Progressively, the phagolysosome is acidified, activating degradative enzymes. Degradation can be oxygen-independent. Oxygen-dependent degradation depends on the production of reactive oxygen species. Hydrogen peroxide and myeloperoxidase activate a halogenating system, which leads to the creation of hypochlorite and the destruction of bacteria. Oxygen-independent degradation depends on the release of granules, containing proteolytic enzymes such as lysozymes, cationic proteins such as defensins. Other antimicrobial peptides are present in these granules, including lactoferrin, which sequesters iron to provide unfavourable growth conditions for bacteria.
Other enzymes like hyaluronidase, collagenase, ribonuclease, deoxyribonuclease play an important role in preventing the spread of infection and degradation of essential microbial biomolecules leading to cell death. Leukocytes generate hydrog