An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In usage, some of the larger kinds of exoskeletons are known as "shells". Examples of animals with exoskeletons include insects such as grasshoppers and cockroaches, crustaceans such as crabs and lobsters; the shells of certain sponges and the various groups of shelled molluscs, including those of snails, tusk shells and nautilus, are exoskeletons. Some animals, such as the tortoise, have both an exoskeleton. Exoskeletons contain rigid and resistant components that fulfill a set of functional roles in many animals including protection, sensing, support and acting as a barrier against desiccation in terrestrial organisms. Exoskeletons have a role in defense from pests and predators, in providing an attachment framework for musculature. Exoskeletons contain chitin. Ingrowths of the arthropod exoskeleton known as apodemes serve as attachment sites for muscles.
These structures are composed of chitin, are six times as strong and twice as stiff as vertebrate tendons. Similar to tendons, apodemes can stretch to store elastic energy for jumping, notably in locusts. Many different species produce exoskeletons. Bone, cartilage, or dentine turtles. Chitin forms the exoskeleton in arthropods including insects, arachnids such as spiders, crustaceans such as crabs and lobsters, in some fungi and bacteria. Calcium carbonates constitute the shells of molluscs and some tube-building polychaete worms. Silica forms the exoskeleton in the microscopic diatoms and radiolaria. One species of mollusc, the scaly-foot gastropod makes use of the iron sulfides greigite and pyrite; some organisms, such as some foraminifera, agglutinate exoskeletons by sticking grains of sand and shell to their exterior. Contrary to a common misconception, echinoderms do not possess an exoskeleton, as their test is always contained within a layer of living tissue. Exoskeletons have evolved independently many times.
Further, other lineages have produced tough outer coatings analogous to an exoskeleton, such as some mammals. This coating is constructed from bone in the armadillo, hair in the pangolin; the armor of reptiles like turtles and dinosaurs like Ankylosaurs is constructed of bone. Since exoskeletons are rigid, they present some limits to growth. Organisms with open shells can grow by adding new material to the aperture of their shell, as is the case in snails and other molluscans. A true exoskeleton, like that found in arthropods, must be shed. A new exoskeleton is produced beneath the old one; as the old one is shed, the new skeleton is pliable. The animal will pump itself up to expand the new shell to maximal size let it harden; when the shell has set, the empty space inside the new skeleton can be filled up. Failure to shed the exoskeleton once outgrown can result in the animal being suffocated within its own shell, will stop subadults from reaching maturity, thus preventing them from reproducing.
This is the mechanism such as Azadirachtin. Exoskeletons, as hard parts of organisms, are useful in assisting preservation of organisms, whose soft parts rot before they can be fossilized. Mineralized exoskeletons can be preserved "as is", as shell fragments, for example; the possession of an exoskeleton permits a couple of other routes to fossilization. For instance, the tough layer can resist compaction, allowing a mold of the organism to be formed underneath the skeleton, which may decay. Alternatively, exceptional preservation may result in chitin being mineralized, as in the Burgess Shale, or transformed to the resistant polymer keratin, which can resist decay and be recovered. However, our dependence on fossilized skeletons significantly limits our understanding of evolution. Only the parts of organisms that were mineralized are preserved, such as the shells of molluscs, it helps that exoskeletons contain "muscle scars", marks where muscles have been attached to the exoskeleton, which may allow the reconstruction of much of an organism's internal parts from its exoskeleton alone.
The most significant limitation is that, although there are 30-plus phyla of living animals, two-thirds of these phyla have never been found as fossils, because most animal species are soft-bodied and decay before they can become fossilized. Mineralized skeletons first appear in the fossil record shortly before the base of the Cambrian period, 550 million years ago; the evolution of a mineralized exoskeleton is seen by some as a possible driving force of the Cambrian explosion of animal life, resulting in a diversification of predatory and defensive tactics. However, some Precambrian organisms produced tough outer shells while others, such as Cloudina, had a calcified exoskeleton; some Cloudina shells show evidence of predation, in the form of borings. On the whole, the fossil record only contains mineralised exoskeletons, since these are by far the most durable. Since most lineages with exoskeletons are thought to have started out with a non-mineralised exoskeleton which they mineralised, this makes it difficult to comment on the early evolution of each lineage's exoskeleton.
It is known, that in a short course of time, just before the Cambrian period, exoskeletons
Cartilage is a resilient and smooth elastic tissue, a rubber-like padding that covers and protects the ends of long bones at the joints, is a structural component of the rib cage, the ear, the nose, the bronchial tubes, the intervertebral discs, many other body components. It is not as hard and rigid as bone; the matrix of cartilage is made up of glycosaminoglycans, collagen fibers and, elastin. Because of its rigidity, cartilage serves the purpose of holding tubes open in the body. Examples include the rings such as the cricoid cartilage and carina. Cartilage is composed of specialized cells called chondrocytes that produce a large amount of collagenous extracellular matrix, abundant ground substance, rich in proteoglycan and elastin fibers. Cartilage is classified in three types, elastic cartilage, hyaline cartilage and fibrocartilage, which differ in relative amounts of collagen and proteoglycan. Cartilage does not contain blood nerves. Nutrition is supplied to the chondrocytes by diffusion.
The compression of the articular cartilage or flexion of the elastic cartilage generates fluid flow, which assists diffusion of nutrients to the chondrocytes. Compared to other connective tissues, cartilage has a slow turnover of its extracellular matrix and does not repair. In embryogenesis, the skeletal system is derived from the mesoderm germ layer. Chondrification is the process by which cartilage is formed from condensed mesenchyme tissue, which differentiates into chondroblasts and begins secreting the molecules that form the extracellular matrix. Following the initial chondrification that occurs during embryogenesis, cartilage growth consists of the maturing of immature cartilage to a more mature state; the division of cells within cartilage occurs slowly, thus growth in cartilage is not based on an increase in size or mass of the cartilage itself. The articular cartilage function is dependent on the molecular composition of the extracellular matrix; the ECM consists of proteoglycan and collagens.
The main proteoglycan in cartilage is aggrecan, which, as its name suggests, forms large aggregates with hyaluronan. These aggregates hold water in the tissue; the collagen collagen type II, constrains the proteoglycans. The ECM responds to compressive forces that are experienced by the cartilage. Cartilage growth thus refers to the matrix deposition, but can refer to both the growth and remodeling of the extracellular matrix. Due to the great stress on the patellofemoral joint during resisted knee extension, the articular cartilage of the patella is among the thickest in the human body; the mechanical properties of articular cartilage in load-bearing joints such as the knee and hip have been studied extensively at macro and nano-scales. These mechanical properties include the response of cartilage in frictional, compressive and tensile loading. Cartilage displays viscoelastic properties. Lubricin, a glycoprotein abundant in cartilage and synovial fluid, plays a major role in bio-lubrication and wear protection of cartilage.
Cartilage has limited repair capabilities: Because chondrocytes are bound in lacunae, they cannot migrate to damaged areas. Therefore, cartilage damage is difficult to heal; because hyaline cartilage does not have a blood supply, the deposition of new matrix is slow. Damaged hyaline cartilage is replaced by fibrocartilage scar tissue. Over the last years and scientists have elaborated a series of cartilage repair procedures that help to postpone the need for joint replacement. Bioengineering techniques are being developed to generate new cartilage, using a cellular "scaffolding" material and cultured cells to grow artificial cartilage. Several diseases can affect cartilage. Chondrodystrophies are a group of diseases, characterized by the disturbance of growth and subsequent ossification of cartilage; some common diseases that affect the cartilage are listed below. Osteoarthritis: Osteoarthritis is a disease of the whole joint, however one of the most affected tissues is the articular cartilage.
The cartilage covering bones is thinned completely wearing away, resulting in a "bone against bone" within the joint, leading to reduced motion, pain. Osteoarthritis affects the joints exposed to high stress and is therefore considered the result of "wear and tear" rather than a true disease, it is treated by arthroplasty, the replacement of the joint by a synthetic joint made of a stainless steel alloy and ultra high molecular weight polyethylene. Chondroitin sulfate or glucosamine sulfate supplements, have been claimed to reduce the symptoms of osteoarthritis but there is little good evidence to support this claim. Traumatic rupture or detachment: The cartilage in the knee is damaged but can be repaired through knee cartilage replacement therapy; when athletes talk of damaged "cartilage" in their knee, they are referring to a damaged meniscus and not the articular cartilage. Achondroplasia: Reduced proliferation of chondrocytes in the epiphyseal plate of long bones during infancy and childhood, resulting in dwarfism.
Costochondritis: Inflammation of cartilage in the ribs, causing chest pain. Spinal disc herniation: Asymmetrical compression of an intervertebral disc ruptures the sac-like disc, causing a herniation of its soft content; the hernia compresses the adjacent nerves and causes back pain. Relapsing polychondritis: a destruction aut
A cytoskeleton is present in the cytoplasm of all cells, including bacteria, archaea. It is a complex, dynamic network of interlinking protein filaments that extends from the cell nucleus to the cell membrane; the cytoskeletal systems of different organisms are composed of similar proteins. In eukaryotes, the cytoskeletal matrix is a dynamic structure composed of three main proteins, which are capable of rapid growth or disassembly dependent on the cell's requirements; the structure and dynamic behavior of the cytoskeleton can be different, depending on organism and cell type. Within one cell the cytoskeleton can change through association with other proteins and the previous history of the network. A multitude of functions can be performed by the cytoskeleton, its primary function is to give the cell its shape and mechanical resistance to deformation, through association with extracellular connective tissue and other cells it stabilizes entire tissues. The cytoskeleton can contract, thereby deforming the cell and the cell's environment and allowing cells to migrate.
Moreover, it is involved in many cell signaling pathways: in the uptake of extracellular material, segregates chromosomes during cellular division, is involved in cytokinesis, provides a scaffold to organize the contents of the cell in space and for intracellular transport. Furthermore, it forms specialized structures, such as flagella, cilia and podosomes. A large-scale example of an action performed by the cytoskeleton is muscle contraction; this is carried out by groups of specialized cells working together. A main component in the cytoskeleton that helps show the true function of this muscle contraction is the microfilament. Microfilaments are composed of the most abundant cellular protein known as actin. During contraction of a muscle, within each muscle cell, myosin molecular motors collectively exert forces on parallel actin filaments. Muscle contraction starts from nerve impulses which causes increased amounts of calcium to be released from the sarcoplasmic reticulum. Increases in calcium in the cytosol allows muscle contraction to begin with the help of two proteins and troponin.
Tropomyosin inhibits the interaction between actin and myosin, while troponin senses the increase in calcium and releases the inhibition. This action contracts the muscle cell, through the synchronous process in many muscle cells, the entire muscle. In 1903, Nikolai K. Koltsov proposed that the shape of cells was determined by a network of tubules that he termed the cytoskeleton; the concept of a protein mosaic that dynamically coordinated cytoplasmic biochemistry was proposed by Rudolph Peters in 1929 while the term was first introduced by French embryologist Paul Wintrebert in 1931. When the cytoskeleton was first introduced, it was thought to be an uninteresting gel-like substance that helps organelles stay in place. Much research took place to try to understand the purpose of its components. With the help of Stuart Hameroff and Roger Penrose, they discovered that microtubules vibrate within neurons in the brain which suggest that brain waves come from deeper microtubule vibrations; this discovery showed that the cytoskeleton is not just a gel like substance but it has a purpose.
It was thought that the cytoskeleton was exclusive to eukaryotes but in 1992, it was discovered to be present in prokaryotes as well. This discovery came after the realization that bacteria possess proteins that are homologous to tubulin and actin. Eukaryotic cells contain three main kinds of cytoskeletal filaments: microfilaments and intermediate filaments; each type is formed by the polymerization of a distinct type of protein subunit and has its own characteristic shape and intracellular distribution. Microfilaments are 7 nm in diameter. Microtubules are 25 nm in diameter. Intermediate filaments are composed of various proteins, depending on the type of cell in which they are found; the cytoskeleton provides the cell with structure and shape, by excluding macromolecules from some of the cytosol, it adds to the level of macromolecular crowding in this compartment. Cytoskeletal elements interact intimately with cellular membranes. Research into neurodegenerative disorders such as Parkinson's disease, Alzheimer's disease, Huntington's disease, amyotrophic lateral sclerosis indicate that the cytoskeleton is affected in these diseases.
Parkinson's disease is marked by the degradation of neurons, resulting in tremors and other non-motor symptoms. Research has shown that microtubule assembly and stability in the cytoskeleton is compromised causing the neurons to degrade over time. In Alzheimer's disease, tau proteins which stabilize microtubules, malfunction in the progression of the disease, causing pathology with the cytoskeleton. Excess glutamine in the Huntington protein, involved with linking vesicles to the cytoskeleton is proposed to be a factor in the development of Huntington's disease. Amyotrophic lateral sclerosis which results in a loss of movement caused by the degradation of motor neurons is seen to involve defects in the cytoskeleton. A number of small-molecule cytoskeletal drugs have been discovered that interact with actin and microtubules; these compounds have proven useful in studying the cytoskeleton and several have clinical applications. All filaments interact with accessory prote
Oklahoma City shortened to OKC, is the capital and largest city of the U. S. state of Oklahoma. The county seat of Oklahoma County, the city ranks 27th among United States cities in population; the population grew following the 2010 Census, with the population estimated to have increased to 643,648 as of July 2017. As of 2015, the Oklahoma City metropolitan area had a population of 1,358,452, the Oklahoma City-Shawnee Combined Statistical Area had a population of 1,459,758 residents, making it Oklahoma's largest metropolitan area. Oklahoma City's city limits extend into Canadian and Pottawatomie counties, though much of those areas outside the core Oklahoma County area are suburban or rural; the city ranks as the ninth-largest city in the United States by total area when including consolidated city-counties. Lying in the Great Plains region, Oklahoma City has one of the world's largest livestock markets. Oil, natural gas, petroleum products and related industries are the largest sector of the local economy.
The city is in the middle of an active oil field and oil derricks dot the capitol grounds. The federal government employs large numbers of workers at Tinker Air Force Base and the United States Department of Transportation's Mike Monroney Aeronautical Center. Oklahoma City is on the I-35 Corridor, one of the primary travel corridors south into neighboring Texas and Mexico and north towards Wichita and Kansas City. Located in the state's Frontier Country region, the city's northeast section lies in an ecological region known as the Cross Timbers; the city was founded during the Land Run of 1889 and grew to a population of over 10,000 within hours of its founding. The city was the scene of the April 19, 1995 bombing of the Alfred P. Murrah Federal Building, in which 168 people died, it was the deadliest terror attack in the history of the United States until the attacks of September 11, 2001, remains the deadliest act of domestic terrorism in U. S. history. Since the time weather records have been kept, Oklahoma City has been struck by thirteen strong tornadoes.
Since 2008, Oklahoma City has been home to the National Basketball Association's Oklahoma City Thunder, who play their home basketball games at the Chesapeake Energy Arena. Oklahoma City was settled on April 22, 1889, when the area known as the "Unassigned Lands" was opened for settlement in an event known as "The Land Run"; some 10,000 homesteaders settled the area. The town grew quickly. Early leaders of the development of the city included Anton Classen, John Shartel, Henry Overholser and James W. Maney. By the time Oklahoma was admitted to the Union in 1907, Oklahoma City had surpassed Guthrie, the territorial capital, as the new state's population center and commercial hub. Soon after, the capital was moved from Guthrie to Oklahoma City. Oklahoma City was a major stop on Route 66 during the early part of the 20th century. Before World War II, Oklahoma City developed major stockyards, attracting jobs and revenue in Chicago and Omaha, Nebraska. With the 1928 discovery of oil within the city limits, Oklahoma City became a major center of oil production.
Post-war growth accompanied the construction of the Interstate Highway System, which made Oklahoma City a major interchange as the convergence of I-35, I-40, I-44. It was aided by federal development of Tinker Air Force Base. In 1950, the Census Bureau reported city's population as 8.6 % 90.7 % white. Patience Latting was elected Mayor of Oklahoma City in 1971. Latting was the first woman to serve as mayor of a U. S. city with over 350,000 residents. Like many other American cities, center city population declined in the 1970s and 1980s as families followed newly constructed highways to move to newer housing in nearby suburbs. Urban renewal projects in the 1970s, including the Pei Plan, removed older structures but failed to spark much new development, leaving the city dotted with vacant lots used for parking. A notable exception was the city's construction of the Myriad Gardens and Crystal Bridge, a botanical garden and modernistic conservatory in the heart of downtown. Architecturally significant historic buildings lost to clearances were the Criterion Theater, the Baum Building, the Hales Building, the Biltmore Hotel.
In 1993, the city passed a massive redevelopment package known as the Metropolitan Area Projects, intended to rebuild the city's core with civic projects to establish more activities and life to downtown. The city added a new baseball park. Water taxis transport passengers within the district, adding activity along the canal. MAPS has become one of the most successful public-private partnerships undertaken in the U. S. exceeding $3 billion in private investment as of 2010. As a result of MAPS, the population living in downtown housing has exponentially increased, together with demand for additional residential and retail amenities, such as grocery and shops. Since the MAPS projects' completion, the downtown area has seen continued
An arthropod is an invertebrate animal having an exoskeleton, a segmented body, paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids and crustaceans; the term Arthropoda as proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora. Arthropods are characterized by their jointed limbs and cuticle made of chitin mineralised with calcium carbonate; the arthropod body plan consists of each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthopods are bilaterally symmetrical and their body possesses an external skeleton; some species have wings. Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments, they have over a million described species, making up more than 80 per cent of all described living animal species, some of which, unlike most other animals, are successful in dry environments. Arthropods range in size from the microscopic crustacean Stygotantulus up to the Japanese spider crab.
Arthropods' primary internal cavity is a haemocoel, which accommodates their internal organs, through which their haemolymph – analogue of blood – circulates. Like their exteriors, the internal organs of arthropods are built of repeated segments, their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, their brains are formed by fusion of the ganglia of these segments and encircle the esophagus; the respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong. Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey.
Arthropods have a wide range of chemical and mechanical sensors based on modifications of the many setae that project through their cuticles. Arthropods' methods of reproduction and development are diverse; the evolutionary ancestry of arthropods dates back to the Cambrian period. The group is regarded as monophyletic, many analyses support the placement of arthropods with cycloneuralians in a superphylum Ecdysozoa. Overall, the basal relationships of Metazoa are not yet well resolved; the relationships between various arthropod groups are still debated. Aquatic species use either external fertilization. All arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and undergo a total metamorphosis to produce the adult form; the level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by scorpions. Arthropods contribute to the human food supply both directly as food, more indirectly as pollinators of crops.
Some species are known to spread severe disease to humans and crops. The word arthropod comes from the Greek ἄρθρον árthron, "joint", πούς pous, i.e. "foot" or "leg", which together mean "jointed leg". Arthropods are invertebrates with jointed limbs; the exoskeleton or cuticles consists of a polymer of glucosamine. The cuticle of many crustaceans, beetle mites, millipedes is biomineralized with calcium carbonate. Calcification of the endosternite, an internal structure used for muscle attachments occur in some opiliones. Estimates of the number of arthropod species vary between 1,170,000 and 5 to 10 million and account for over 80 per cent of all known living animal species; the number of species remains difficult to determine. This is due to the census modeling assumptions projected onto other regions in order to scale up from counts at specific locations applied to the whole world. A study in 1992 estimated that there were 500,000 species of animals and plants in Costa Rica alone, of which 365,000 were arthropods.
They are important members of marine, freshwater and air ecosystems, are one of only two major animal groups that have adapted to life in dry environments. One arthropod sub-group, insects, is the most species-rich member of all ecological guilds in land and freshwater environments; the lightest insects weigh less than 25 micrograms. Some living crustaceans are much larger; the embryos of all arthropods are segmented, built from a series of repeated modules. The last common ancestor of living arthropods consisted of a series of undifferentiated segments, each with a pair of appendages that functioned as limbs. However, all known living and fossil arthropods have grouped segments into tagmata in which segments and their limbs are specialized in various ways; the three-
Mollusca is the second largest phylum of invertebrate animals. The members are known as mollusks. Around 85,000 extant species of molluscs are recognized; the number of fossil species is estimated between 100,000 additional species. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs live in freshwater and terrestrial habitats, they are diverse, not just in size and in anatomical structure, but in behaviour and in habitat. The phylum is divided into 8 or 9 taxonomic classes, of which two are extinct. Cephalopod molluscs, such as squid and octopus, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species; the gastropods are by far the most numerous molluscs and account for 80% of the total classified species. The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula, the structure of the nervous system.
Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc". This has a single, "limpet-like" shell on top, made of proteins and chitin reinforced with calcium carbonate, is secreted by a mantle covering the whole upper surface; the underside of the animal consists of a single muscular "foot". Although molluscs are coelomates, the coelom tends to be small; the main body cavity is a hemocoel. The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles; the generalized mollusc has three in bivalves. The brain, in species that have one, encircles the esophagus. Most molluscs have eyes, all have sensors to detect chemicals and touch; the simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur.
All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. The coelomic cavity is reduced, they have kidney-like organs for excretion. Good evidence exists for the appearance of gastropods and bivalves in the Cambrian period, 541 to 485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists. Molluscs still are an important food source for anatomically modern humans. There is a risk of food poisoning from toxins which can accumulate in certain molluscs under specific conditions and because of this, many countries have regulations to reduce this risk. Molluscs have, for centuries been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, sea silk, their shells have been used as money in some preindustrial societies. Mollusc species can represent hazards or pests for human activities.
The bite of the blue-ringed octopus is fatal, that of Octopus apollyon causes inflammation that can last for over a month. Stings from a few species of large tropical cone shells can kill, but their sophisticated, though produced, venoms have become important tools in neurological research. Schistosomiasis is transmitted to humans via water snail hosts, affects about 200 million people. Snails and slugs can be serious agricultural pests, accidental or deliberate introduction of some snail species into new environments has damaged some ecosystems; the words mollusc and mollusk are both derived from the French mollusque, which originated from the Latin molluscus, from mollis, soft. Molluscus was itself an adaptation of Aristotle's τὰ μαλάκια ta malákia, which he applied inter alia to cuttlefish; the scientific study of molluscs is accordingly called malacology. The name Molluscoida was used to denote a division of the animal kingdom containing the brachiopods and tunicates, the members of the three groups having been supposed to somewhat resemble the molluscs.
As it is now known these groups have no relation to molluscs, little to one another, the name Molluscoida has been abandoned. The most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, the organization of the nervous system. Many have a calcareous shell. Molluscs have developed such a varied range of body structures, it is difficult to find synapomorphies to apply to all modern groups; the most general characteristic of molluscs is they are bilaterally symmetrical. The following are present in all modern molluscs: The dorsal part of the body wall is a mantle which secretes calcareous spicules, plates or shells, it overlaps the body with enough spare room to form a mantle cavity. The anus and genitals open into the mantle cavity. There are two pairs of main nerve cords. Other characteristics that appear in textbooks have significant exceptions: Estimates of accepted described living species of molluscs vary from 50,000 to a maximum of 120,000 species.
In 1969 David Nicol estimated the probable total number of living mollusc species at 107,000 of which were ab
Polysaccharides are polymeric carbohydrate molecules composed of long chains of monosaccharide units bound together by glycosidic linkages, on hydrolysis give the constituent monosaccharides or oligosaccharides. They range in structure from linear to branched. Examples include storage polysaccharides such as starch and glycogen, structural polysaccharides such as cellulose and chitin. Polysaccharides are quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure, these macromolecules can have distinct properties from their monosaccharide building blocks, they may be amorphous or insoluble in water. When all the monosaccharides in a polysaccharide are the same type, the polysaccharide is called a homopolysaccharide or homoglycan, but when more than one type of monosaccharide is present they are called heteropolysaccharides or heteroglycans. Natural saccharides are of simple carbohydrates called monosaccharides with general formula n where n is three or more.
Examples of monosaccharides are glucose and glyceraldehyde. Polysaccharides, have a general formula of Cxy where x is a large number between 200 and 2500; when the repeating units in the polymer backbone are six-carbon monosaccharides, as is the case, the general formula simplifies to n, where 40≤n≤3000. As a rule of thumb, polysaccharides contain more than ten monosaccharide units, whereas oligosaccharides contain three to ten monosaccharide units. Polysaccharides are an important class of biological polymers, their function in living organisms is either structure- or storage-related. Starch is used as a storage polysaccharide in plants, being found in the form of both amylose and the branched amylopectin. In animals, the structurally similar glucose polymer is the more densely branched glycogen, sometimes called "animal starch". Glycogen's properties allow it to be metabolized more which suits the active lives of moving animals. Cellulose and chitin are examples of structural polysaccharides.
Cellulose is used in the cell walls of plants and other organisms, is said to be the most abundant organic molecule on Earth. It has many uses such as a significant role in the paper and textile industries, is used as a feedstock for the production of rayon, cellulose acetate and nitrocellulose. Chitin has nitrogen-containing side branches, increasing its strength, it is found in the cell walls of some fungi. It has multiple uses, including surgical threads. Polysaccharides include callose or laminarin, xylan, mannan and galactomannan. Nutrition polysaccharides are common sources of energy. Many organisms can break down starches into glucose; these carbohydrate types can be metabolized by some protists. Ruminants and termites, for example, use microorganisms to process cellulose. Though these complex polysaccharides are not digestible, they provide important dietary elements for humans. Called dietary fiber, these carbohydrates enhance digestion among other benefits; the main action of dietary fiber is to change the nature of the contents of the gastrointestinal tract, to change how other nutrients and chemicals are absorbed.
Soluble fiber binds to bile acids in the small intestine, making them less to enter the body. Soluble fiber attenuates the absorption of sugar, reduces sugar response after eating, normalizes blood lipid levels and, once fermented in the colon, produces short-chain fatty acids as byproducts with wide-ranging physiological activities. Although insoluble fiber is associated with reduced diabetes risk, the mechanism by which this occurs is unknown. Not yet formally proposed as an essential macronutrient, dietary fiber is regarded as important for the diet, with regulatory authorities in many developed countries recommending increases in fiber intake. Starch is a glucose polymer, it is made up of a mixture of amylopectin. Amylose consists of a linear chain of several hundred glucose molecules and Amylopectin is a branched molecule made of several thousand glucose units. Starches are insoluble in water, they can be digested by breaking the alpha-linkages. Both humans and other animals have amylases, so they can digest starches.
Potato, rice and maize are major sources of starch in the human diet. The formations of starches are the ways. Glycogen serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue. Glycogen is made by the liver and the muscles, but can be made by glycogenesis within the brain and stomach. Glycogen is analogous to starch, a glucose polymer in plants, is sometimes referred to as animal starch, having a similar structure to amylopectin but more extensively branched and compact than starch. Glycogen is a polymer of α glycosidic bonds linked, with α-linked branches. Glycogen is found in the form of granules in the cytosol/cytoplasm in many cell types, plays an important role in the glucose cycle. Glycogen forms an energy reserve that can be mobilized to meet a sudden need for glucose, but one, less compact and more available a