A carbohydrate is a biomolecule consisting of carbon and oxygen atoms with a hydrogen–oxygen atom ratio of 2:1 and thus with the empirical formula Cmn. This formula holds true for monosaccharides; some exceptions exist. The carbohydrates are technically hydrates of carbon; the term is most common in biochemistry, where it is a synonym of saccharide, a group that includes sugars and cellulose. The saccharides are divided into four chemical groups: monosaccharides, disaccharides and polysaccharides. Monosaccharides and disaccharides, the smallest carbohydrates, are referred to as sugars; the word saccharide comes from the Greek word σάκχαρον, meaning "sugar". While the scientific nomenclature of carbohydrates is complex, the names of the monosaccharides and disaccharides often end in the suffix -ose, as in the monosaccharides fructose and glucose and the disaccharides sucrose and lactose. Carbohydrates perform numerous roles in living organisms. Polysaccharides serve as structural components; the 5-carbon monosaccharide ribose is an important component of coenzymes and the backbone of the genetic molecule known as RNA.
The related deoxyribose is a component of DNA. Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, preventing pathogenesis, blood clotting, development, they are found in a wide variety of processed foods. Starch is a polysaccharide, it is abundant in cereals and processed food based on cereal flour, such as bread, pizza or pasta. Sugars appear in human diet as table sugar, lactose and fructose, both of which occur in honey, many fruits, some vegetables. Table sugar, milk, or honey are added to drinks and many prepared foods such as jam and cakes. Cellulose, a polysaccharide found in the cell walls of all plants, is one of the main components of insoluble dietary fiber. Although it is not digestible, insoluble dietary fiber helps to maintain a healthy digestive system by easing defecation. Other polysaccharides contained in dietary fiber include resistant starch and inulin, which feed some bacteria in the microbiota of the large intestine, are metabolized by these bacteria to yield short-chain fatty acids.
In scientific literature, the term "carbohydrate" has many synonyms, like "sugar", "saccharide", "ose", "glucide", "hydrate of carbon" or "polyhydroxy compounds with aldehyde or ketone". Some of these terms, specially "carbohydrate" and "sugar", are used with other meanings. In food science and in many informal contexts, the term "carbohydrate" means any food, rich in the complex carbohydrate starch or simple carbohydrates, such as sugar. In lists of nutritional information, such as the USDA National Nutrient Database, the term "carbohydrate" is used for everything other than water, fat and ethanol; this includes chemical compounds such as acetic or lactic acid, which are not considered carbohydrates. It includes dietary fiber, a carbohydrate but which does not contribute much in the way of food energy though it is included in the calculation of total food energy just as though it were a sugar. In the strict sense, "sugar" is applied for sweet, soluble carbohydrates, many of which are used in food.
The name "carbohydrate" was used in chemistry for any compound with the formula Cm n. Following this definition, some chemists considered formaldehyde to be the simplest carbohydrate, while others claimed that title for glycolaldehyde. Today, the term is understood in the biochemistry sense, which excludes compounds with only one or two carbons and includes many biological carbohydrates which deviate from this formula. For example, while the above representative formulas would seem to capture the known carbohydrates and abundant carbohydrates deviate from this. For example, carbohydrates display chemical groups such as: N-acetyl, carboxylic acid and deoxy modifications. Natural saccharides are built of simple carbohydrates called monosaccharides with general formula n where n is three or more. A typical monosaccharide has the structure H–x–y–H, that is, an aldehyde or ketone with many hydroxyl groups added one on each carbon atom, not part of the aldehyde or ketone functional group. Examples of monosaccharides are glucose and glyceraldehydes.
However, some biological substances called "monosaccharides" do not conform to this formula and there are many chemicals that do conform to this formula but are not considered to be monosaccharides. The open-chain form of a monosaccharide coexists with a closed ring form where the aldehyde/ketone carbonyl group carbon and hydroxyl group react forming a hemiacetal with a new C–O–C bridge. Monosaccharides can be linked togeth
De Havilland Albatross
The de Havilland DH.91 Albatross was a four-engine British transport aircraft in the 1930s. A total of seven aircraft were built in 1938–39; the DH.91 was designed in 1936 by A. E. Hagg to Air Ministry specification 36/35 for a transatlantic mail plane; the aircraft was notable for the ply-balsa-ply sandwich construction of its fuselage used in the de Havilland Mosquito bomber. Another unique feature was a cooling system for the air-cooled engines that allowed nearly ideal streamlining of the engine mounting; the first Albatross flew on 20 May 1937. The second prototype was repaired with minor reinforcement; the first and second prototypes were operated by Imperial Airways. Although designed as a mailplane, a version to carry 22 passengers was developed. Five examples formed the production order delivered in 1938/1939; when war was declared all seven aircraft were operating from Bristol/Whitchurch to Lisbon and Shannon. As normal for the Imperial Airways fleet of the time, all were given names starting with the same letter, the first aircraft's name was used as a generic description for the type overall, as "Frobisher Class".
This tradition, which came from a maritime and railway background of classes of ships and locomotives, lasted well into postwar days with BOAC and BEA. The first delivery to Imperial Airways was the 22-passenger DH.91 Frobisher in October 1938. The five passenger-carrying aircraft were operated on routes from Croydon to Paris and Zurich. After test flying was completed, the two prototypes were delivered to Imperial Airways as long-range mail carriers; the only significant season of their operation was the summer of 1939, when they were the main type on the two-hourly London Croydon-to-Paris Le Bourget passenger route. With the onset of World War II, the Royal Air Force considered their range and speed useful for courier flights between Great Britain and Iceland, the two mail planes were pressed into service with 271 Squadron in September 1940, operating between Prestwick and Reykjavik but both were destroyed in landing accidents in Reykjavík within the space of 9 months: Faraday in 1941 and Franklin in 1942.
The five passenger aircraft were used by Imperial Airways, on Bristol–Lisbon and Bristol–Shannon routes from Bristol Airport. Frobisher was destroyed during a German air raid on Whitchurch in 1940, Fingal was destroyed in a crash landing following a fuel pipe failure in 1940 at Pucklechurch and Fortuna crashed near Shannon Airport in 1943; the latter accident was found to be due to deterioration of the aircraft's plywood wing structures. In view of the two surviving aircraft's vulnerability to similar problems, for lack of spares parts and Fiona were scrapped in September 1943. Faraday Mail-carrier variant was delivered to Imperial Airways in August 1939 as Faraday and registered G-AEVV, it was transferred to BOAC when it was formed in 1940 but was impressed into Royal Air Force service with serial number AX903 for operation by No. 271 Squadron RAF. It was destroyed in a landing accident at Reykjavik on 11 August 1941. Franklin Mail-carrier variant was delivered to BOAC as Franklin and registered G-AEVW.
Impressed into Royal Air Force Service with the serial number AX904 for operation by 271 Squadron. It was destroyed when the landing gear collapsed on landing at Reykjavik on 7 April 1942. Frobisher Passenger variant was registered G-AFDI and delivered to Imperial Airways as Frobisher in 1938, it was destroyed on the ground during a German air attack on Whitchurch Airport on 20 December 1940. Falcon Passenger variant was registered G-AFDJ and delivered to Imperial Airways as Falcon in 1938, it was scrapped in September 1943. Fortuna Passenger variant was registered G-AFDK and delivered to Imperial Airways as Fortuna in 1939. Destroyed in a crash landing near Shannon Airport, Ireland on 16 July 1943. Fingal Passenger variant was registered G-AFDL and delivered to Imperial Airways as Fingal in 1939. Destroyed in a crash landing near Pucklechurch, England on 6 October 1940. Fiona Passenger variant was registered G-AFDM and delivered to Imperial Airways as Fiona in 1939, it was scrapped in September 1943.
A 1/10 scale model of the Albatross owned by British Airways was found in a crate at Croydon in the 1990s and is on display in the heritage museum at Speedbird House. United KingdomImperial Airways, reorganised as British Overseas Airways Corporation received all seven aircraft. Royal Air Force No. 271 Squadron RAF operated two aircraft taken over from BOAC. Data from British Civil Aircraft since 1919 General characteristics Crew: four Capacity: 22 passengers Length: 71 ft 6 in Wingspan: 105 ft 0 in Height: 22 ft 3 in Wing area: 1,078 ft² Empty weight: 21,230 lb Loaded weight: 29,500 lb Powerplant: 4 × de Havilland Gipsy Twelve 12-cylinder inverted V piston engine, 525 hp eachPerformance Maximum speed: 195 kn Cruise speed: 183 kn Range: 904 nmi, Service ceiling: 17,900 ft Rate of climb: 700 ft/min Wing loading: 27 lb/ft² Power/mass: 0.07 hp/lb Related development Fairey FC1 de Havilland Mosquito Related lists List of aircraft of World War II Notes Citations Bibliography Kopenhagen, Wolfgang.
Das große Flugzeug-Typenbuch. Transpress. ISBN 3-344-00162-0. CS1 maint: Extra text: authors list Jackson, A. J.. De Havilland aircraft since 1909. Naval Institute Press. ISBN 0-87021-896-4. Jackson, A. J.. British Civil A
In chemistry, a salt is an ionic compound that can be formed by the neutralization reaction of an acid and a base. Salts are composed of related numbers of cations and anions so that the product is electrically neutral; these component ions can be inorganic, such as organic, such as acetate. Salts can be classified in a variety of ways. Salts that produce hydroxide ions when dissolved in water are called alkali salts. Salts that produce acidic solutions are acidic salts. Neutral salts are those salts that are neither basic. Zwitterions contain an anionic and a cationic centres in the same molecule, but are not considered to be salts. Examples of zwitterions include amino acids, many metabolites and proteins. Solid salts tend to be transparent. In many cases, the apparent opacity or transparency are only related to the difference in size of the individual monocrystals. Since light reflects from the grain boundaries, larger crystals tend to be transparent, while the polycrystalline aggregates look like white powders.
Salts exist in many different colors, which arise either from the cations. For example: sodium chromate is yellow by virtue of the chromate ion potassium dichromate is orange by virtue of the dichromate ion cobalt nitrate is red owing to the chromophore of hydrated cobalt. copper sulfate is blue because of the copper chromophore potassium permanganate has the violet color of permanganate anion. Nickel chloride is green of sodium chloride, magnesium sulfate heptahydrate are colorless or white because the constituent cations and anions do not absorb in the visible part of the spectrumFew minerals are salts because they would be solubilized by water. Inorganic pigments tend not to be salts, because insolubility is required for fastness; some organic dyes are salts, but they are insoluble in water. Different salts can elicit all five basic tastes, e.g. salty, sour and umami or savory. Salts of strong acids and strong bases are non-volatile and odorless, whereas salts of either weak acids or weak bases may smell like the conjugate acid or the conjugate base of the component ions.
That slow, partial decomposition is accelerated by the presence of water, since hydrolysis is the other half of the reversible reaction equation of formation of weak salts. Many ionic compounds exhibit significant solubility in water or other polar solvents. Unlike molecular compounds, salts dissociate in solution into cationic components; the lattice energy, the cohesive forces between these ions within a solid, determines the solubility. The solubility is dependent on how well each ion interacts with the solvent, so certain patterns become apparent. For example, salts of sodium and ammonium are soluble in water. Notable exceptions include potassium cobaltinitrite. Most nitrates and many sulfates are water-soluble. Exceptions include barium sulfate, calcium sulfate, lead sulfate, where the 2+/2− pairing leads to high lattice energies. For similar reasons, most alkali metal carbonates are not soluble in water; some soluble carbonate salts are: potassium carbonate and ammonium carbonate. Salts are characteristically insulators.
Molten salts or solutions of salts conduct electricity. For this reason, liquified salts and solutions containing dissolved salts are called electrolytes. Salts characteristically have high melting points. For example, sodium chloride melts at 801 °C; some salts with low lattice energies are liquid near room temperature. These include molten salts, which are mixtures of salts, ionic liquids, which contain organic cations; these liquids exhibit unusual properties as solvents. The name of a salt starts with the name of the cation followed by the name of the anion. Salts are referred to only by the name of the cation or by the name of the anion. Common salt-forming cations include: Ammonium NH+4 Calcium Ca2+ Iron Fe2+ and Fe3+ Magnesium Mg2+ Potassium K+ Pyridinium C5H5NH+ Quaternary ammonium NR+4, R being an alkyl group or an aryl group Sodium Na+ Copper Cu2+Common salt-forming anions include: Acetate CH3COO− Carbonate CO2−3 Chloride Cl− Citrate HOC2 Cyanide C≡N− Fluoride F− Nitrate NO−3 Nitrite NO−2 Oxide O2− Phosphate PO3−4 Sulfate SO2−4 Salts with varying number of hydrogen atoms, with respect to the parent acid, replaced by cations can be referred to as monobasic, dibasic or tribasic salts: Sodium phosphate monobasic Sodium phosphate dibasic Sodium phosphate tribasic Salts are formed by a chemical reaction between: A base and an acid, e.g. NH3 + HCl → NH4Cl A metal and an acid, e.g. Mg + H2SO4 → MgSO4 + H2 A metal and a non-metal, e.g. Ca + Cl2 → CaCl2 A base and an a
Water is a transparent, tasteless and nearly colorless chemical substance, the main constituent of Earth's streams and oceans, the fluids of most living organisms. It is vital for all known forms of life though it provides no calories or organic nutrients, its chemical formula is H2O, meaning that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds. Water is the name of the liquid state of H2O at standard ambient pressure, it forms precipitation in the form of rain and aerosols in the form of fog. Clouds are formed from suspended droplets of its solid state; when finely divided, crystalline ice may precipitate in the form of snow. The gaseous state of water is water vapor. Water moves continually through the water cycle of evaporation, condensation and runoff reaching the sea. Water covers 71% of the Earth's surface in seas and oceans. Small portions of water occur as groundwater, in the glaciers and the ice caps of Antarctica and Greenland, in the air as vapor and precipitation.
Water plays an important role in the world economy. 70% of the freshwater used by humans goes to agriculture. Fishing in salt and fresh water bodies is a major source of food for many parts of the world. Much of long-distance trade of commodities and manufactured products is transported by boats through seas, rivers and canals. Large quantities of water and steam are used for cooling and heating, in industry and homes. Water is an excellent solvent for a wide variety of chemical substances. Water is central to many sports and other forms of entertainment, such as swimming, pleasure boating, boat racing, sport fishing, diving; the word water comes from Old English wæter, from Proto-Germanic *watar, from Proto-Indo-European *wod-or, suffixed form of root *wed-. Cognate, through the Indo-European root, with Greek ύδωρ, Russian вода́, Irish uisce, Albanian ujë; the identification of water as a substance Water is a polar inorganic compound, at room temperature a tasteless and odorless liquid, nearly colorless with a hint of blue.
This simplest hydrogen chalcogenide is by far the most studied chemical compound and is described as the "universal solvent" for its ability to dissolve many substances. This allows it to be the "solvent of life", it is the only common substance to exist as a solid and gas in normal terrestrial conditions. Water is a liquid at the pressures that are most adequate for life. At a standard pressure of 1 atm, water is a liquid between 0 and 100 °C. Increasing the pressure lowers the melting point, about −5 °C at 600 atm and −22 °C at 2100 atm; this effect is relevant, for example, to ice skating, to the buried lakes of Antarctica, to the movement of glaciers. Increasing the pressure has a more dramatic effect on the boiling point, about 374 °C at 220 atm; this effect is important in, among other things, deep-sea hydrothermal vents and geysers, pressure cooking, steam engine design. At the top of Mount Everest, where the atmospheric pressure is about 0.34 atm, water boils at 68 °C. At low pressures, water cannot exist in the liquid state and passes directly from solid to gas by sublimation—a phenomenon exploited in the freeze drying of food.
At high pressures, the liquid and gas states are no longer distinguishable, a state called supercritical steam. Water differs from most liquids in that it becomes less dense as it freezes; the maximum density of water in its liquid form is 1,000 kg/m3. The density of ice is 917 kg/m3. Thus, water expands 9% in volume as it freezes, which accounts for the fact that ice floats on liquid water; the details of the exact chemical nature of liquid water are not well understood. Pure water is described as tasteless and odorless, although humans have specific sensors that can feel the presence of water in their mouths, frogs are known to be able to smell it. However, water from ordinary sources has many dissolved substances, that may give it varying tastes and odors. Humans and other animals have developed senses that enable them to evaluate the potability of water by avoiding water, too salty or putrid; the apparent color of natural bodies of water is determined more by dissolved and suspended solids, or by reflection of the sky, than by water itself.
Light in the visible electromagnetic spectrum can traverse a couple meters of pure water without significant absorption, so that it looks transparent and colorless. Thus aquatic plants and other photosynthetic organisms can live in water up to hundreds of meters deep, because sunlight can reach them. Water vapour is invisible as a gas. Through a thickness of 10 meters or more, the intrinsic color of water is visibly turquoise, as its absorption spectrum has
A calf is the young of domestic cattle. Calves are reared to become adult cattle, or are slaughtered for their meat, called veal, for their calfskin; the term "calf" is used for some other species. See "Other animals" below. "Calf" is the term used from birth to weaning, when it becomes known as a weaner or weaner calf, though in some areas the term "calf" may be used until the animal is a yearling. The birth of a calf is known as calving. A calf that has lost its mother is an orphan calf known as a poddy or poddy-calf in British English. Bobby calves are young calves. A vealer is a fat calf weighing less than about 330 kg, at about eight to nine months of age. A young female calf from birth until she has had a calf of her own is called a heifer. In the American Old West, a motherless or small, runty calf was sometimes referred to as a dogie; the term "calf" is used for some other species. See "Other animals" below. Calves may be produced by natural means, or by artificial breeding using artificial insemination or embryo transfer.
Calves are born after a gestation of nine months. They stand within a few minutes of calving, suckle within an hour. However, for the first few days they are not able to keep up with the rest of the herd, so young calves are left hidden by their mothers, who visit them several times a day to suckle them. By a week old the calf is able to follow the mother all the time; some calves are ear tagged soon after birth those that are stud cattle in order to identify their dams, or in areas where tagging is a legal requirement for cattle. A calf must have the best of everything until it is at least eight months old if it is to reach its maximum potential; when the calves are about two months old they are branded, ear marked and vaccinated. The single suckler system of rearing calves is similar to that occurring in wild cattle, where each calf is suckled by its own mother until it is weaned at about nine months old; this system is used for rearing beef cattle throughout the world. Cows kept on poor forage produce a limited amount of milk.
A calf left with such a mother all the time can drink all the milk, leaving none for human consumption. For dairy production under such circumstances, the calf's access to the cow must be limited, for example by penning the calf and bringing the mother to it once a day after milking her; the small amount of milk available for the calf under such systems may mean that it takes a longer time to rear, in subsistence farming it is therefore common for cows to calve only in alternate years. In more intensive dairy farming, cows can be bred and fed to produce far more milk than one calf can drink. In the multi-suckler system, several calves are fostered onto one cow in addition to her own, these calves' mothers can be used wholly for milk production. More calves of dairy cows are fed formula milk from a bottle or bucket from soon after birth. Purebred female calves of dairy cows are reared as replacement dairy cows. Most purebred dairy calves are produced by artificial insemination. By this method each bull can serve many cows, so only a few of the purebred dairy male calves are needed to provide bulls for breeding.
The remainder of the male calves may be reared for veal. Only a proportion of purebred heifers are needed to provide replacement cows, so some of the cows in dairy herds are put to a beef bull to produce crossbred calves suitable for rearing as beef. Veal calves may be reared on milk formula and killed at about 18 or 20 weeks as "white" veal, or fed on grain and hay and killed at 22 to 35 weeks to produce red or pink veal. A commercial steer or bull calf is expected to put on about 32 to 36 kg per month. A nine-month-old steer or bull is therefore expected to weigh about 250 to 270 kg. Heifers will weigh at least 200 kg at eight months of age. Calves are weaned at about eight to nine months of age, but depending on the season and condition of the dam, they might be weaned earlier, they may be paddock weaned next to their mothers, or weaned in stockyards. The latter system is preferred by some as it accustoms the weaners to the presence of people and they are trained to take feed other than grass.
Small numbers may be weaned with their dams with the use of weaning nose rings or nosebands which results in the mothers rejecting the calves' attempts to suckle. Many calves are weaned when they are taken to the large weaner auction sales that are conducted in the south eastern states of Australia. Victoria and New South Wales have yardings of up to 8,000 weaners for auction sale in one day; the best of these weaners may go to the butchers. Others fatten on grass or as potential breeders. In the United States these weaners may be known as feeders and would be placed directly into feedlots. At about 12 months old a beef heifer reaches puberty. Calves suffer from few congenital abnormalities but the Akabane virus is distributed in temperate to tropical regions of the world; the virus is a teratogenic pathogen which causes abortions, premature births and congenital abnormalities, but occurs only during some years. Calf meat for human consumption is called veal, is produced from the male calves of Dairy cattle.
Calcium hydroxide is an inorganic compound with the chemical formula Ca2. It is a colorless crystal or white powder and is obtained when quicklime is mixed, or slaked with water, it has many names including hydrated lime, caustic lime, builders' lime, slack lime, cal, or pickling lime. Calcium hydroxide is used in many applications, including food preparation, where it has been identified as E number E526. Limewater is the common name for a saturated solution of calcium hydroxide. Calcium hydroxide is insoluble in water, with a solubility product Ksp of 5.5 × 10−6. It is large enough that its solutions are basic according to the following reaction: Ca2 → Ca2+ + 2 OH−At ambient temperature. Calcium hydroxide dissolves in pure water to produce an alkaline solution with a pH of about 12.4. Calcium hydroxide solutions can cause chemical burns. At high pH value, its solubility drastically decreases; this behavior is relevant to cement pastes. Aqueous solutions of calcium hydroxide are called limewater and are medium strength bases that reacts with acids and can attack some metals such as aluminium while protecting other metals from corrosion such as iron and steel by passivation of their surface.
Limewater turns milky in the presence of carbon dioxide due to formation of calcium carbonate, a process called carbonatation:for example lime water Ca2 + CO2 → CaCO3 + H2OWhen heated to 512 °C, the partial pressure of water in equilibrium with calcium hydroxide reaches 101 kPa, which decomposes calcium hydroxide into calcium oxide and water. Ca2 → CaO + H2O Calcium hydroxide adopts a polymeric structure; the structure is identical to that of Mg2. Strong hydrogen bonds exist between the layers. Calcium hydroxide is produced commercially by treating lime with water: CaO + H2O → Ca2In the laboratory it can be prepared by mixing aqueous solutions of calcium chloride and sodium hydroxide; the mineral form, portlandite, is rare but can be found in some volcanic and metamorphic rocks. It has been known to arise in burning coal dumps; the positively charged. The solubility of calcium hydroxide at 70 °C is about half of its value at 25 °C; the reason for this rather uncommon phenomenon is that the dissolution of calcium hydroxide in water is an exothermic process, adheres to Le Chatelier's principle.
A lowering of temperature thus favours the elimination of the heat liberated through the process of dissolution and increases the equilibrium constant of dissolution of Ca2, so increase its solubility at low temperature. This counter-intuitive temperature dependence of the solubility is referred to as "retrograde" or "inverse" solubility; the variably hydrated phases of calcium sulfate exhibit a retrograde solubility for the same reason because their dissolution reactions are exothermic. One significant application of calcium hydroxide is in water and sewage treatment, it forms a fluffy charged solid that aids in the removal of smaller particles from water, resulting in a clearer product. This application is enabled by the low cost and low toxicity of calcium hydroxide, it is used in fresh water treatment for raising the pH of the water so that pipes will not corrode where the base water is acidic, because it is self-regulating and does not raise the pH too much. It is used in the preparation of ammonia gas, using the following reaction: Ca2 + 2NH4Cl → 2NH3 + CaCl2 + 2H2OAnother large application is in the paper industry, where it is an intermediate in the reaction in the production of sodium hydroxide.
This conversion is part of the causticizing step in the Kraft process for making pulp. In the causticizing operation, burned lime is added to green liquor, a solution of sodium carbonate and sodium sulfate produced by dissolving smelt, the molten form of these chemicals from the recovery furnace; because of its low toxicity and the mildness of its basic properties, slaked lime is used in the food industry to: clarify raw juice from sugarcane or sugar beets in the sugar industry, process water for alcoholic beverages and soft drinks pickle cucumbers and other foods make Chinese century eggs in maize preparation: removes the cellulose hull of maize kernels clear a brine of carbonates of calcium and magnesium in the manufacture of salt for food and pharmaceutical uses fortify fruit drinks, such as orange juice, infant formula aid digestion substitute for baking soda in making papadam. Remove carbon dioxide from controlled atmosphere produce storage rooms. In Spanish, calcium hydroxide is called cal.
Maize cooked with cal becomes hominy, which increases the bioavailability of niacin, it is considered tastier and easier to digest. In chewing coca leaves, calcium hydroxide is chewed alongside to keep the alkaloid stimulants chemically available for absorption by the body. Native Americans traditionally chewed tobacco leaves with calcium hydroxide derived from burnt mollusc shells to enhance the effects, it has been used by some indigenous American tribes as an ingredient in yopo, a psychedelic snuff prepared from the beans of some Anadenanthera species. Calcium hydroxide is added to a bundle of areca nut and
Peptides are short chains of amino acid monomers linked by peptide bonds. The covalent chemical bonds are formed when the carboxyl group of one amino acid reacts with the amino group of another; the shortest peptides are dipeptides, consisting of 2 amino acids joined by a single peptide bond, followed by tripeptides, etc. A polypeptide is a long and unbranched peptide chain. Hence, peptides fall under the broad chemical classes of biological oligomers and polymers, alongside nucleic acids and polysaccharides, etc. Peptides are distinguished from proteins on the basis of size, as an arbitrary benchmark can be understood to contain 50 or fewer amino acids. Proteins consist of one or more polypeptides arranged in a biologically functional way bound to ligands such as coenzymes and cofactors, or to another protein or other macromolecule, or to complex macromolecular assemblies. While aspects of the lab techniques applied to peptides versus polypeptides and proteins differ, the size boundaries that distinguish peptides from polypeptides and proteins are not absolute: long peptides such as amyloid beta have been referred to as proteins, smaller proteins like insulin have been considered peptides.
Amino acids that have been incorporated into peptides are termed "residues" due to the release of either a hydrogen ion from the amine end or a hydroxyl ion from the carboxyl end, or both, as a water molecule is released during formation of each amide bond. All peptides except cyclic peptides have an N-terminal and C-terminal residue at the end of the peptide. Many kinds of peptides are known, they have been categorized according to their sources and function. According to the Handbook of Biologically Active Peptides, some groups of peptides include plant peptides, bacterial/antibiotic peptides, fungal peptides, invertebrate peptides, amphibian/skin peptides, venom peptides, cancer/anticancer peptides, vaccine peptides, immune/inflammatory peptides, brain peptides, endocrine peptides, ingestive peptides, gastrointestinal peptides, cardiovascular peptides, renal peptides, respiratory peptides, opiate peptides, neurotrophic peptides, blood–brain peptides; some ribosomal peptides are subject to proteolysis.
These function in higher organisms, as hormones and signaling molecules. Some organisms produce peptides as antibiotics, such as microcins. Peptides have posttranslational modifications such as phosphorylation, sulfonation, palmitoylation and disulfide formation. In general, peptides are linear. More exotic manipulations do occur, such as racemization of L-amino acids to D-amino acids in platypus venom. Nonribosomal peptides are assembled by enzymes, not the ribosome. A common non-ribosomal peptide is glutathione, a component of the antioxidant defenses of most aerobic organisms. Other nonribosomal peptides are most common in unicellular organisms and fungi and are synthesized by modular enzyme complexes called nonribosomal peptide synthetases; these complexes are laid out in a similar fashion, they can contain many different modules to perform a diverse set of chemical manipulations on the developing product. These peptides are cyclic and can have complex cyclic structures, although linear nonribosomal peptides are common.
Since the system is related to the machinery for building fatty acids and polyketides, hybrid compounds are found. The presence of oxazoles or thiazoles indicates that the compound was synthesized in this fashion. Peptide fragments refer to fragments of proteins that are used to identify or quantify the source protein; these are the products of enzymatic degradation performed in the laboratory on a controlled sample, but can be forensic or paleontological samples that have been degraded by natural effects. Use of peptides received prominence in molecular biology for several reasons; the first is that peptides allow the creation of peptide antibodies in animals without the need of purifying the protein of interest. This involves synthesizing antigenic peptides of sections of the protein of interest; these will be used to make antibodies in a rabbit or mouse against the protein. Another reason is that techniques such as mass spectrometry enable the identification of proteins based on the peptide masses and sequence that result from their fragmentation.
Peptides have been used in the study of protein structure and function. For example, synthetic peptides can be used as probes to see where protein-peptide interactions occur- see the page on Protein tags. Inhibitory peptides are used in clinical research to examine the effects of peptides on the inhibition of cancer proteins and other diseases. For example, one of the most promising application is through peptides that target LHRH; these particular peptides act as an agonist, meaning that they bind to a cell in a way that regulates LHRH receptors. The process of inhibiting the cell receptors suggests that peptides could be beneficial in treating prostate cancer, but additional investigations and experiments are required before their cancer-fighting attributes can be considered definitive; the peptide families in this section are ribosomal peptides with hormonal activity. All of these peptides are synthesized by cells as longer "propeptides" or "proproteins" and truncated prior to exiting the cell.
They are released into the bloodstream. Magainin family Cecropin famil