In chemistry, pH is a scale used to specify how acidic or basic a water-based solution is. Acidic solutions have a lower pH, while basic solutions have a higher pH. At room temperature, pure water is neither acidic nor basic and has a pH of 7; the pH scale is logarithmic and approximates the negative of the base 10 logarithm of the molar concentration of hydrogen ions in a solution. More it is the negative of the base 10 logarithm of the activity of the hydrogen ion. At 25 °C, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic; the neutral value of the pH depends on the temperature, being lower than 7 if the temperature increases. Contrary to popular belief, the pH value can be less than 0 or greater than 14 for strong acids and bases respectively; the pH scale is traceable to a set of standard solutions whose pH is established by international agreement. Primary pH standard values are determined using a concentration cell with transference, by measuring the potential difference between a hydrogen electrode and a standard electrode such as the silver chloride electrode.
The pH of aqueous solutions can be measured with a glass electrode and a pH meter, or a color-changing indicator. Measurements of pH are important in chemistry, medicine, water treatment, many other applications; the concept of pH was first introduced by the Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909 and revised to the modern pH in 1924 to accommodate definitions and measurements in terms of electrochemical cells. In the first papers, the notation had the "H" as a subscript to the lowercase "p", as so: pH; the exact meaning of the "p" in "pH" is disputed, but according to the Carlsberg Foundation, pH stands for "power of hydrogen". It has been suggested that the "p" stands for the German Potenz, others refer to French puissance. Another suggestion is that the "p" stands for the Latin terms pondus hydrogenii, potentia hydrogenii, or potential hydrogen, it is suggested that Sørensen used the letters "p" and "q" to label the test solution and the reference solution.
In chemistry, the p stands for "decimal cologarithm of", is used in the term pKa, used for acid dissociation constants. Bacteriologist Alice C. Evans, famed for her work's influence on dairying and food safety, credited William Mansfield Clark and colleagues with developing pH measuring methods in the 1910s, which had a wide influence on laboratory and industrial use thereafter. In her memoir, she does not mention how much, or how little and colleagues knew about Sørensen's work a few years prior, she said: In these studies Dr. Clark's attention was directed to the effect of acid on the growth of bacteria, he found that it is the intensity of the acid in terms of hydrogen-ion concentration that affects their growth. But existing methods of measuring acidity determined not the intensity, of the acid. Next, with his collaborators, Dr. Clark developed accurate methods for measuring hydrogen-ion concentration; these methods replaced the inaccurate titration method of determining acid content in use in biologic laboratories throughout the world.
They were found to be applicable in many industrial and other processes in which they came into wide usage. The first electronic method for measuring pH was invented by Arnold Orville Beckman, a professor at California Institute of Technology in 1934, it was in response to local citrus grower Sunkist that wanted a better method for testing the pH of lemons they were picking from their nearby orchards. PH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, aH+, in a solution. PH = − log 10 = log 10 For example, for a solution with a hydrogen ion activity of 5×10−6 we get 1/ = 2×105, thus such a solution has a pH of log10 = 5.3. For a commonplace example based on the facts that the masses of a mole of water, a mole of hydrogen ions, a mole of hydroxide ions are 18 g, 1 g, 17 g, a quantity of 107 moles of pure water, or 180 tonnes, contains close to 1 g of dissociated hydrogen ions and 17 g of hydroxide ions. Note that pH depends on temperature. For instance at 0 °C the pH of pure water is 7.47.
At 25 °C it's 7.00, at 100 °C it's 6.14. This definition was adopted because ion-selective electrodes, which are used to measure pH, respond to activity. Ideally, electrode potential, E, follows the Nernst equation, for the hydrogen ion can be written as E = E 0 + R T F ln = E 0 − 2.303 R T F pH where E is a measured potential, E0 is the standard electrode potential, R is the gas const
Shellac is a resin secreted by the female lac bug, on trees in the forests of India and Thailand. It is processed and sold as dry flakes and dissolved in alcohol to make liquid shellac, used as a brush-on colorant, food glaze and wood finish. Shellac functions as a tough natural primer, sanding sealant, tannin-blocker, odour-blocker and high-gloss varnish. Shellac was once used in electrical applications as it possesses good insulation qualities and it seals out moisture. Phonograph and 78 rpm gramophone records were made of it until they were replaced by vinyl long-playing records from the 1950s onwards. From the time it replaced oil and wax finishes in the 19th century, shellac was one of the dominant wood finishes in the western world until it was replaced by nitrocellulose lacquer in the 1920s and 1930s. Shellac comes from shell and lac, a calque of French laque en écailles, "lac in thin pieces" gomme-laque, "gum lac". Most European languages have borrowed the word for the substance from English or from the German equivalent Schellack.
Shellac is scraped from the bark of the trees where the female lac bug, Kerria lacca, secretes it to form a tunnel-like tube as it traverses the branches of the tree. Though these tunnels are sometimes referred to as "cocoons", they are not cocoons in the entomological sense; this insect is in the same superfamily as the insect. The insects suck the sap of the tree and excrete "sticklac" constantly; the least coloured shellac is produced. The number of lac bugs required to produce 1 kilogram of shellac has variously been estimated as 50,000, 200,000, or 300,000; the root word lakh is a unit in Indian numbering system for 100,000 and refers to the huge numbers of insects that swarm on host trees, up to 150 per square inch. The raw shellac, which contains bark shavings and lac bugs removed during scraping, is placed in canvas tubes and heated over a fire; this causes the shellac to liquefy, it seeps out of the canvas, leaving the bark and bugs behind. The thick, sticky shellac is dried into a flat sheet and broken into flakes, or dried into "buttons" bagged and sold.
The end-user crushes it into a fine powder and mixes it with ethyl alcohol before use, to dissolve the flakes and make liquid shellac. Liquid shellac has a limited shelf life. Liquid shellac sold in hardware stores is marked with the production date, so the consumer can know whether the shellac inside is still good; some manufacturers have ceased labeling shellac with the production date, but the production date may be discernible from the production lot code. Alternatively, old shellac may be tested to see if it is still usable: a few drops on glass should dry to a hard surface. Shellac that remains tacky for a long time is no longer usable. Storage life depends on peak temperature, so refrigeration extends shelf life; the thickness of shellac is measured by the unit "pound cut", referring to the amount of shellac flakes dissolved in a gallon of denatured alcohol. For example: a 1-lb. Cut of shellac is the strength obtained by dissolving one pound of shellac flakes in a gallon of alcohol. Most pre-mixed commercial preparations come at a 3-lb.
Cut. Multiple thin layers of shellac produce a better end result than a few thick layers. Thick layers of shellac do not adhere to the substrate or to each other well, thus can peel off with relative ease. Shellac dries to a high-gloss sheen. For applications where a flatter sheen is desired, products containing amorphous silica, such as "Shellac Flat", may be added to the dissolved shellac. Shellac contains a small amount of wax, which comes from the lac bug. In some preparations, this wax is removed; this is done for applications where the shellac will be coated with something else, so the topcoat will adhere. Waxy shellac appears dries clear. Shellac comes in many warm colours, ranging from a light blonde to a dark brown, with many varieties of brown, yellow and red in between; the colour is influenced by the sap of the tree the lac bug is living by the time of harvest. The most sold shellac is called "orange shellac", was used extensively as a combination stain and protectant for wood panelling and cabinetry in the 20th century.
Shellac was once common anywhere paints or varnishes were sold. However and more abrasion- and chemical-resistant finishes, such as polyurethane, have completely replaced it in decorative residential wood finishing such as hardwood floors, wooden wainscoting plank panelling, kitchen cabinets; these alternative products, must be applied over a stain if the user wants the wood to be coloured. "Wax over shellac" is regarded as a beautiful, if fragile, finish for hardwood floors. Luthiers still use shellac to French polish fine acoustic stringed instruments, but it has been replaced by synthetic plastic lacquers and varnishes in many workshops high-volume prod
In the manufacture of pharmaceuticals, encapsulation refers to a range of dosage forms—techniques used to enclose medicines—in a stable shell known as a capsule, allowing them to, for example, be taken orally or be used as suppositories. The two main types of capsules are: Hard-shelled capsules, which contain dry, powdered ingredients or miniature pellets made by e.g. processes of extrusion or spheronization. These are made in two halves: a smaller-diameter “body”, filled and sealed using a larger-diameter “cap”. Soft-shelled capsules used for oils and for active ingredients that are dissolved or suspended in oil. Both of these classes of capsules are made from aqueous solutions of gelling agents, such as animal protein or plant polysaccharides or their derivatives. Other ingredients can be added to the gelling agent solution including plasticizers such as glycerin or sorbitol to decrease the capsule's hardness, coloring agents, disintegrants and surface treatment. Since their inception, capsules have been viewed by consumers as the most efficient method of taking medication.
For this reason, producers of drugs such as OTC analgesics wanting to emphasize the strength of their product developed the “caplet”, a portmanteau of “capsule-shaped tablet”, in order to tie this positive association to more efficiently-produced tablet pills, as well as being an easier-to-swallow shape than the usual disk-shaped tablet. In 1833, Mothes and Dublanc were granted a patent for a method to produce a single-piece gelatin capsule, sealed with a drop of gelatin solution, they used individual iron molds for their process, filling the capsules individually with a medicine dropper. On, methods were developed that used sets of plates with pockets to form the capsules. Although some companies still use this method, the equipment is no longer produced commercially. All modern soft-gel encapsulation uses variations of a process developed by R. P. Scherer in 1933, his innovation used. They were filled by blow molding; this method was high-yield and reduced waste. Softgels can be an effective delivery system for oral drugs poorly soluble drugs.
This is because the fill can contain liquid ingredients that help increase solubility or permeability of the drug across the membranes in the body. Liquid ingredients are difficult to include in any other solid dosage form such as a tablet. Softgels are highly suited to potent drugs, where the reproducible filling process helps ensure each softgel has the same drug content, because the operators are not exposed to any drug dust during the manufacturing process. In 1949, the Lederle Laboratories division of the American Cyanamid Company developed the "Accogel" process, allowing powders to be filled into soft gelatin capsules. James Murdock of London patented the two-piece telescoping gelatin capsule in 1847; the capsules are made in two parts by dipping metal pins in the gelling agent solution. The capsules are supplied as closed units to the pharmaceutical manufacturer. Before use, the two halves are separated, the capsule is filled with powder or more pellets made by the process of Extrusion & Spheronization and the other half of the capsule is pressed on.
With the compressed slug method, weight varies less between capsules. However, the machinery required to manufacture them is more complex; the powder or spheroids inside the capsule contains the active ingredient and any excipients, such as binders, fillers and preservatives. Gelatin capsules, informally called gel caps or gelcaps, are composed of gelatin manufactured from the collagen of animal skin or bone. Vegetable capsules are composed of a polymer formulated from cellulose. Or Pullulan, polysaccharide polymer produced from tapioca starch; the process of encapsulation of hard gelatin capsules can be done on manual, semi-automatic and automatic capsule filling machines. Softgels are filled at the same time as they are produced and sealed on the rotary die of a automatic machine. Capsule fill weight is a critical attribute in encapsulation and various real time fill weight monitoring techniques such as near-infrared spectroscopy and vibrational spectroscopy are used, as well as in-line weight checks, to ensure product quality.
Capsule endoscopy OROS Pharmacy Automation - The Tablet Counter Pharmaceutical formulation Pill splitting Tablet Oblaat L. Lachman. A. Lieberman. L. Kanig; the Theory and Practice of Industrial Pharmacy. Lea & Febiger, Philadelphia. ISBN 0-8121-0977-5
Keratin is one of a family of fibrous structural proteins. It is the key structural material making up hair, horns, claws and the outer layer of human skin. Keratin is the protein that protects epithelial cells from damage or stress. Keratin is insoluble in water and organic solvents. Keratin monomers assemble into bundles to form intermediate filaments, which are tough and form strong unmineralized epidermal appendages found in reptiles, birds and mammals; the only other biological matter known to approximate the toughness of keratinized tissue is chitin. Keratin filaments are abundant in keratinocytes in the cornified layer of the epidermis. In addition, keratin filaments are present in epithelial cells in general. For example, mouse thymic epithelial cells are known to react with antibodies for keratin 5, keratin 8, keratin 14; these antibodies are used as fluorescent markers to distinguish subsets of TECs in genetic studies of the thymus. The α-keratins are found in all vertebrates, they form the hair, stratum corneum, nails and hooves of mammals and the hagfish slime threads.
The harder β-keratins are found only in the sauropsids, all living reptiles and birds. They are found in the nails and claws of reptiles, some reptile shells, in the feathers and claws of birds. Additionally, the baleen plates of filter-feeding whales are made of keratin. Keratins are polymers of type I and type II intermediate filaments, which have only been found in the genomes of chordates. Nematodes and many other non-chordate animals seem to only have type VI intermediate filaments, which have a long rod domain; the human genome encodes 54 functional keratin genes which are located in two clusters on chromosomes 12 and 17. This suggests; the keratins include the following proteins of which KRT23, KRT24, KRT25, KRT26, KRT27, KRT28, KRT31, KRT32, KRT33A, KRT33B, KRT34, KRT35, KRT36, KRT37, KRT38, KRT39, KRT40, KRT71, KRT72, KRT73, KRT74, KRT75, KRT76, KRT77, KRT78, KRT79, KRT8, KRT80, KRT81, KRT82, KRT83, KRT84, KRT85 and KRT86 have been used to describe keratins past 20. The first sequences of keratins were determined by Fuchs.
These sequences revealed that there are two distinct but homologous keratin families which were named as Type I keratin and Type II keratins. By analysis of the primary structures of these keratins and other intermediate filament proteins and Fuchs suggested a model that keratins and intermediate filament proteins contain a central ~310 residue domain with four segments in α-helical conformation that are separated by three short linker segments predicted to be in beta-turn conformation; this model has been confirmed by the determination of the crystal structure of a helical domain of keratins. Fibrous keratin molecules supercoil to form a stable, left-handed superhelical motif to multimerise, forming filaments consisting of multiple copies of the keratin monomer; the major force that keeps the coiled-coil structure is hydrophobic interactions between apolar residues along the keratins helical segments. Limited interior space is the reason why the triple helix of the structural protein collagen, found in skin and bone has a high percentage of glycine.
The connective tissue protein elastin has a high percentage of both glycine and alanine. Silk fibroin, considered a β-keratin, can have these two as 75–80% of the total, with 10–15% serine, with the rest having bulky side groups; the chains are antiparallel, with an alternating C → N orientation. A preponderance of amino acids with small, nonreactive side groups is characteristic for structural proteins, for which H-bonded close packing is more important than chemical specificity. In addition to intra- and intermolecular hydrogen bonds, the distinguishing feature of keratins is the presence of large amounts of the sulfur-containing amino acid cysteine, required for the disulfide bridges that confer additional strength and rigidity by permanent, thermally stable crosslinking—in much the same way that non-protein sulfur bridges stabilize vulcanized rubber. Human hair is 14% cysteine; the pungent smells of burning hair and skin are due to the volatile sulfur compounds formed. Extensive disulfide bonding contributes to the insolubility of keratins, except in a small number of solvents such as dissociating or reducing agents.
The more flexible and elastic keratins of hair have fewer interchain disulfide bridges than the keratins in mammalian fingernails and claws, which are harder and more like their analogs in other vertebrate classes. Hair and other α-keratins consist of α-helically coiled single protein strands, which are further twisted into superhelical ropes that may be further coiled; the β-keratins of reptiles and birds have β-pleated sheets twisted together stabilized and hardened by disulfide bridges. It has been proposed that keratins can be divided into'hard' and'soft' forms, or'cytokeratins' and'other keratins'; that model is now understood to be correct. A new nuclear addition in 2006 to describe keratins takes this into account. Keratin filaments are intermediate filaments. Like all intermediate filaments, keratin proteins form filamentous polymers in a series of assembly steps beginning with dimerization.
Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream. Digestion is a form of catabolism, divided into two processes based on how food is broken down: mechanical and chemical digestion; the term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. In chemical digestion, enzymes break down food into the small molecules. In the human digestive system, food enters the mouth and mechanical digestion of the food starts by the action of mastication, a form of mechanical digestion, the wetting contact of saliva. Saliva, a liquid secreted by the salivary glands, contains salivary amylase, an enzyme which starts the digestion of starch in the food. After undergoing mastication and starch digestion, the food will be in the form of a small, round slurry mass called a bolus.
It will travel down the esophagus and into the stomach by the action of peristalsis. Gastric juice in the stomach starts protein digestion. Gastric juice contains hydrochloric acid and pepsin, it contains rennin in case of infants and toddlers. As the first two chemicals may damage the stomach wall, mucus is secreted by the stomach, providing a slimy layer that acts as a shield against the damaging effects of the chemicals. At the same time protein digestion is occurring, mechanical mixing occurs by peristalsis, waves of muscular contractions that move along the stomach wall; this allows the mass of food to further mix with the digestive enzymes. After some time, the resulting thick liquid is called chyme; when the pyloric sphincter valve opens, chyme enters the duodenum where it mixes with digestive enzymes from the pancreas and bile juice from the liver and passes through the small intestine, in which digestion continues. When the chyme is digested, it is absorbed into the blood. 95% of absorption of nutrients occurs in the small intestine.
Water and minerals are reabsorbed back into the blood in the colon where the pH is acidic about 5.6 ~ 6.9. Some vitamins, such as biotin and vitamin K produced by bacteria in the colon are absorbed into the blood in the colon. Waste material is eliminated from the rectum during defecation. Digestive systems take many forms. There is a fundamental distinction between external digestion. External digestion developed earlier in evolutionary history, most fungi still rely on it. In this process, enzymes are secreted into the environment surrounding the organism, where they break down an organic material, some of the products diffuse back to the organism. Animals have a tube in which internal digestion occurs, more efficient because more of the broken down products can be captured, the internal chemical environment can be more efficiently controlled; some organisms, including nearly all spiders secrete biotoxins and digestive chemicals into the extracellular environment prior to ingestion of the consequent "soup".
In others, once potential nutrients or food is inside the organism, digestion can be conducted to a vesicle or a sac-like structure, through a tube, or through several specialized organs aimed at making the absorption of nutrients more efficient. Bacteria use several systems to obtain nutrients from other organisms in the environments. In a channel transupport system, several proteins form a contiguous channel traversing the inner and outer membranes of the bacteria, it is a simple system, which consists of only three protein subunits: the ABC protein, membrane fusion protein, outer membrane protein. This secretion system transports various molecules, from drugs, to proteins of various sizes; the molecules secreted vary in size from the small Escherichia coli peptide colicin V, to the Pseudomonas fluorescens cell adhesion protein LapA of 900 kDa. A type III secretion system means that a molecular syringe is used through which a bacterium can inject nutrients into protist cells. One such mechanism was first discovered in Y. pestis and showed that toxins could be injected directly from the bacterial cytoplasm into the cytoplasm of its host's cells rather than be secreted into the extracellular medium.
The conjugation machinery of some bacteria is capable of transporting proteins. It was discovered in Agrobacterium tumefaciens, which uses this system to introduce the Ti plasmid and proteins into the host, which develops the crown gall; the VirB complex of Agrobacterium tumefaciens is the prototypic system. The nitrogen fixing Rhizobia are an interesting case, wherein conjugative elements engage in inter-kingdom conjugation; such elements as the Agrobacterium Ti or Ri plasmids contain elements that can transfer to plant cells. Transferred genes enter the plant cell nucleus and transform the plant cells into factories for the production of opines, which the bacteria use as carbon and energy sources. Infected plant cells form crown root tumors; the Ti and Ri plasmids are thus endosymbionts of the bacteria, which are in turn endosymbionts of the infected plant. The Ti and Ri plasmids are themselves conjugative. Ti and Ri transfer between bacteria uses an inde
Omega-3 fatty acid
Omega−3 fatty acids called ω−3 fatty acids or n−3 fatty acids, are polyunsaturated fatty acids characterized by the presence of a double bond three atoms away from the terminal methyl group in their chemical structure. They are distributed in nature, being important constituents of animal lipid metabolism, they play an important role in the human diet and in human physiology; the three types of omega−3 fatty acids involved in human physiology are α-linolenic acid, found in plant oils, eicosapentaenoic acid and docosahexaenoic acid, both found in marine oils. Marine algae and phytoplankton are primary sources of omega−3 fatty acids. Common sources of plant oils containing ALA include walnut, edible seeds, clary sage seed oil, algal oil, flaxseed oil, Sacha Inchi oil, Echium oil, hemp oil, while sources of animal omega−3 fatty acids EPA and DHA include fish, fish oils, eggs from chickens fed EPA and DHA, squid oils, krill oil. Omega−3 fatty acids are important for normal metabolism. Mammals are unable to synthesize the essential omega−3 fatty acid ALA and must obtain it through diet, which they can use to form the long-chain omega−3 fatty acids, EPA and from EPA make DHA.
The ability to make the longer-chain omega−3 fatty acids from ALA may be impaired in aging. In foods exposed to air, unsaturated fatty acids are vulnerable to rancidity. Dietary supplementation with omega−3 fatty acids does not appear to affect the risk of death, cancer or heart disease. Furthermore, fish oil supplement studies have failed to support claims of preventing heart attacks or strokes or any vascular disease outcomes; the terms ω–3 fatty acid and n–3 fatty acid are derived from organic nomenclature. One way in which a fatty acid is named is determined by the location of the first double bond, counted from the methyl end, that is, the omega or the n- end. In general terminology using either n–x or ω–x, the dash is a minus sign, the number n–x refers to the locant of the double bond linking two carbon atoms. Thus, in omega–3 fatty acids in particular, there is a double bond located on the carbon numbered 3 lower than the highest carbon number. By example, α-linolenic acid is an 18-carbon chain having three double bonds, the first being located at the third carbon from the methyl end of the fatty acid chain at carbon 15.
Α-Linolenic acid is polyunsaturated and is described by a lipid number, 18:3, meaning that there are 18 carbon atoms and 3 double bonds. Although n and ω are synonymous, the IUPAC recommends that n be used to identify the highest carbon number of a fatty acid; the more common name – omega–3 fatty acid – is used in both the lay media and scientific literature. Supplementation does not appear to be associated with a lower risk of all-cause mortality; the evidence linking the consumption of marine omega−3 fats to a lower risk of cancer is poor. With the possible exception of breast cancer, there is insufficient evidence that supplementation with omega−3 fatty acids has an effect on different cancers; the effect of consumption on prostate cancer is not conclusive. There is a decreased risk with higher blood levels of DPA, but an increased risk of more aggressive prostate cancer was shown with higher blood levels of combined EPA and DHA. In people with advanced cancer and cachexia, omega−3 fatty acids supplements may be of benefit, improving appetite and quality of life.
Evidence in the population does not support a beneficial role for omega−3 fatty acid supplementation in preventing cardiovascular disease or stroke. A 2018 meta-analysis found no support that daily intake of one gram of omega-3 fatty acid in individuals with a history of coronary heart disease prevents fatal coronary heart disease, nonfatal myocardial infarction or any other vascular event. However, omega−3 fatty acid supplementation greater than one gram daily for at least a year may be protective against cardiac death, sudden death, myocardial infarction in people who have a history of cardiovascular disease. No protective effect against the development of stroke or all-cause mortality was seen in this population. Eating a diet high in fish that contain long chain omega−3 fatty acids does appear to decrease the risk of stroke. Fish oil supplementation has not been shown to benefit revascularization or abnormal heart rhythms and has no effect on heart failure hospital admission rates. Furthermore, fish oil supplement studies have failed to support claims of preventing heart attacks or strokes.
Evidence suggests that omega−3 fatty acids modestly lower blood pressure in people with hypertension and in people with normal blood pressure. Some evidence suggests that people with certain circulatory problems, such as varicose veins, may benefit from the consumption of EPA and DHA, which may stimulate blood circulation and increase the breakdown of fibrin, a protein involved in blood clotting and scar formation. Omega−3 fatty acids reduce blood triglyceride levels but do not change the level of LDL cholesterol or HDL cholesterol in the blood; the American Heart Association position is that borderline elevated triglycerides, defined as 150–199 mg/dL, can be lowered by 0.5-1.0 grams of EPA and DHA per day. ALA does not confer the cardiovascular health benefits of DHAs; the effect of omega−3 polyunsaturated fatty
In animal anatomy, the mouth known as the oral cavity, buccal cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is the cavity lying at the upper end of the alimentary canal, bounded on the outside by the lips and inside by the pharynx and containing in higher vertebrates the tongue and teeth; this cavity is known as the buccal cavity, from the Latin bucca. Some animal phyla, including vertebrates, have a complete digestive system, with a mouth at one end and an anus at the other. Which end forms first in ontogeny is a criterion used to classify animals into protostomes and deuterostomes. In the first multicellular animals, there was no mouth or gut and food particles were engulfed by the cells on the exterior surface by a process known as endocytosis; the particles became enclosed in vacuoles into which enzymes were secreted and digestion took place intracellularly. The digestive products were diffused into other cells; this form of digestion is used nowadays by simple organisms such as Amoeba and Paramecium and by sponges which, despite their large size, have no mouth or gut and capture their food by endocytosis.
The vast majority of other multicellular organisms have a mouth and a gut, the lining of, continuous with the epithelial cells on the surface of the body. A few animals which live parasitically had guts but have secondarily lost these structures; the original gut of multicellular organisms consisted of a simple sac with a single opening, the mouth. Many modern invertebrates have such a system, food being ingested through the mouth broken down by enzymes secreted in the gut, the resulting particles engulfed by the other cells in the gut lining. Indigestible waste is ejected through the mouth. In animals at least as complex as an earthworm, the embryo forms a dent on one side, the blastopore, which deepens to become the archenteron, the first phase in the formation of the gut. In deuterostomes, the blastopore becomes the anus while the gut tunnels through to make another opening, which forms the mouth. In the protostomes, it used to be thought that the blastopore formed the mouth while the anus formed as an opening made by the other end of the gut.
More recent research, shows that in protostomes the edges of the slit-like blastopore close up in the middle, leaving openings at both ends that become the mouth and anus. Apart from sponges and placozoans all animals have an internal gut cavity, lined with gastrodermal cells. In less advanced invertebrates such as the sea anemone, the mouth acts as an anus. Circular muscles around the mouth are able to contract in order to open or close it. A fringe of tentacles thrusts food into the cavity and it can gape enough to accommodate large prey items. Food passes first into a pharynx and digestion occurs extracellularly in the gastrovascular cavity. Annelids have simple tube-like gets and the possession of an anus allows them to separate the digestion of their foodstuffs from the absorption of the nutrients. Many molluscs have a radula, used to scrape microscopic particles off surfaces. In invertebrates with hard exoskeletons, various mouthparts may be involved in feeding behaviour. Insects have a range of mouthparts suited to their mode of feeding.
These include mandibles and labium and can be modified into suitable appendages for chewing, piercing and sucking. Decapods have six pairs of mouth appendages, one pair of mandibles, two pairs of maxillae and three of maxillipeds. Sea urchins have a set of five sharp calcareous plates which are used as jaws and are known as Aristotle's lantern. In vertebrates, the first part of the digestive system is the buccal cavity known as the mouth; the buccal cavity of a fish is separated from the opercular cavity by the gills. Water flows in through passes over the gills and exits via the operculum or gill slits. Nearly all fish have jaws and may seize food with them but most feed by opening their jaws, expanding their pharynx and sucking in food items; the food may be held or chewed by teeth located in the jaws, on the roof of the mouth, on the pharynx or on the gill arches. Nearly all amphibians are carnivorous as adults. Many catch their prey by flicking out an elongated tongue with a sticky tip and drawing it back into the mouth where they hold the prey with their jaws.
They swallow their food whole without much chewing. They have many small hinged pedicellate teeth, the bases of which are attached to the jaws while the crowns break off at intervals and are replaced. Most amphibians have one or two rows of teeth in both jaws but some frogs lack teeth in the lower jaw. In many amphibians there are vomerine teeth attached to the bone in the roof of the mouth; the mouths of reptiles are similar to those of mammals. The crocodilians are the only reptiles to have teeth anchored in sockets in their jaws, they are able to replace each of their 80 teeth up to 50 times during their lives. Most reptiles are either carnivorous or insectivorous but turtles are herbivorous. Lacking teeth that are suitable for efficiently chewing of their food, turtles have gastroliths in their stomach to further grind the plant material. Snakes have a flexible lower jaw, the two halves of which are not rigidly attached, numerous other joints in their skull; these modifications allow them to open their mouths wide enough to swallow their prey whole if it is wider than they are.
Birds do not have teeth, macerating their food. Their beaks have a range of sizes and shapes according to their diet and are compose