Evolution of Respiratory Systems in Animals

Evolution of Respiratory Systems in Animals Jonathan Codd Respiration in animals is a necessity as it allows the exchange of respiratory gases that are required for survival. There are huge variations in the designs of respiratory systems and each has evolved due to selective pressures in environments, such as food and territory. The evolution of species is driven in part by limited resources and the adaptations in which these can be exploited. This report will describe the evolution from aquatic environments to terrestrial environments as part of this movement, of animals onto land, created a cause for the fast development of newly designed systems in order to support air breathing as oppose to gaining oxygen from water using gills. Lungfish first developed lungs, and the ability to breathe air instead of water, whilst living in aquatic environments and the appearance of air-breathing in fish is the major foundation for terrestrialization. Each species has different requirements due to different techniques of movement and feeding, for exam ple, and respiratory systems are required to support the lifestyle of each species in order to exchange the maximum amount of gas possible. The ability to develop additional processes able to assist in respiration has ensured that the oxygen demands of each animal is met. The respiratory system of animals is crucial for the life as it allows the exchange of gases between an organism and the environment. These respiratory systems have been forced to continually develop new designs depending on new evolutionary pressures from changing environments. Many species have evolved due to the availability of new niches and unexploited resources and thus have been forced to develop supporting mechanisms of respiration. This report examines the evolution of respiration from aquatic environments to the terrestrialization of land and the rapid expansion of respiratory methods that soon followed. The evolution of lungs from gills in the Sarcopterygii lineage has allowed the tetrapod transition onto land and is responsible for the ability to eventually develop fully terrestrial species that are able to respire solely air. Each system must be complementary to the requirements of the species and environment in order to meet the aerobic demands and some species are able to undergo various methods of respiration in order to undergo sufficient rates of gaseous exchange. Each method has been specifically developed for the niche, and uncinate processes have formed in order to assist with ensuring respiration can be as efficient as possible. Introduction The evolution of air breathing was vital in the transition of life from aquatic to terrestrial environments and, therefore the rapid evolution of the animal kingdoms physiology and anatomy in order to exploit all available niches (Graham, 1997). The development of air breathing would not, however, have taken place if the atmospheric composition had not altered during the Phanerozoic era, around 550 million years ago, when the concentration of oxygen showed an increase, likely due to the appearance of large vascular land plants (Ra et al., 2007). The ozone layer was thought to have formed around two billion years ago (Walker, 1978) and is essential for allowing the survival of life on earth by preventing high-energy ultra-violet radiation from entering the earths atmosphere (Parson, 2003). The movement onto land has allowed for a huge expansion in the amount of available niches and therefore caused a rapid radiation in the body plans of animals and a variety of respiratory mechanisms to evolve in support (Ra et al., 2007)    The Importance of Pulmonary Surfactant Lungs differ throughout the animal kingdom as they have evolved for the specific niche of each species; nevertheless one thing they all commonly include is a gas-liquid interface which allows surface tension to arise, causing complications (Daniels and Orgeig, 2003). The pulmonary surfactant system prevents the collapse of respiratory surfaces in lungs due to unequal pressures arising from differently sized alveoli, as well as maintaining a reduced resistance to air flow and improving lung compliance (Daniels and Orgeig, 2003). There is overwhelming evidence that there was a single evolutionary origin of the surfactant system, thought to be from the epithelial cells lining the pharynx (Daniels et al., 2004), due to Surfactant Protein-A (SP-A) or like-structures being present in all the major vertebrate groups; implying that it is an essential pre-requisite for lung evolution (Sullivan et al., 1998). Surfactant has been studied in swim bladders, which have now been shown to be a homol og of the lung, with the original principal function being an anti-adhesive but also with involvement in preventing water from entering the swim bladders or lungs (Daniels et al., 2004) Pulmonary surfactant composition is primarily lipids (around 90%) most of which are phospholipids, and the remaining ten percent is comprised of proteins. (Veldhuizen et al., 1998). There were found to be four types of surfactant proteins (SPs): A, B, C and D which all have varying properties and roles within the surfactant system; SP-B and SP-C were both found to be highly involved in the surface activity due to hydrophobic properties and SP-D is hydrophilic and part of the collectin family (Wà ¼stneck et al., 2005). Dipalmitoylphosphatidylcholine (DPPC) is the most hydrophobic lipid component and therefore DPPC-rich monolayers are able to sit packed tightly together, ensuring the exclusion of water, however they are not well suited for the expansion of the lungs and so are alternated with mixed monolayers when necessary (Wà ¼stneck et al., 2005). Respiration in Fish Fish evolution has allowed both water and air breathing to arise as a means of gas exchange and as these vary greatly in properties, such as density and the oxygen concentration, the mechanistic pumps must also show great diversity to meet the requirements for effective respiration (Brainerd and Ferry-Graham, 2005). Fish that breathe in water use gills which are highly evolved organs that provide large surface areas and thin barriers between the fishs blood and the aquatic environment, thus allowing for a high rate of gaseous diffusion (Evans et al., 2005). Whilst they show properties for gaseous exchange the gills are multifunctional organs which are also responsible for the loss of ions and nitrogenous waste, therefore fish must also have regulatory mechanisms allowing them to successfully osmoregulate (Evans et al., 2005). As fish are continually moving they all require a buccal pressure pump as well as a suction pump, most likely the operculum, working in tandem to allow for expansion and compression to move water across the gills; the suction pump is more prominent in some species such as the Osteichthyes compared to the Chondrichthyes (Ap and La, 2001). A counter-current method is established due to water flowing in the opposite direction to the movement of blood, with the secondary lamell ae being the site of gaseous exchange (Shelton and Randall, 1962). The counter-current mechanism is required as the content of dissolved oxygen is less in water than it would be in the atmosphere (Ibanez et al., 2008) and thus allows high concentrations of gas to be exchanged, whereas a con-current mechanism would too quickly reach an equilibrium and efficient extraction of oxygen would cease (Brainerd and Ferry-Graham, 2005). There are two hypothesis surrounding the origin of air breathing in fish, one suggests that lungs arose only once at the base of the Osteichthyes, whereas opposing arguments have recently suggested that lungs evolved on at least two separate occasions and instead developed in both the Actinopterygii and Sarcopterygii (Brainerd, 1994). Some air breathing fish, such as the Actinopterygian, are able to modify their buccal pump to create a four-pump mechanism, using two expiration and compression cycles, in which expired air is first pumped into the lungs before being compressed out into the atmosphere (Perry et al., 2001). When empty fresh air is inhaled via the expansion of the buccal cavity before finally being compressed into the lungs, this is shown to fully expand and compress and so there is little mixing of expired and fresh air (Perry et al., 2001). Not all air breathing fish show this mechanism as some Dipnoi, lungfish for example, will still ventilate using the primitive two-stroke mechanism (Burggren and Johansen, 1986) and even though there can be mixing of expired and fresh air in the lungs, there has been no significant evidence to suggest that this is any less effective than the four-stroke method as breathing accessories allows the increase in volume of inhaled gases (Brainerd, 1994). Terrestrialisation Lungs were an obvious pre-requisite for the transition onto land but there were many other anatomical and physiological adaptations necessary for tetrapods before they were able to fully terrestrialise and survive free of an aquatic environment (Daeschler et al., 2006). In the late Devonian, terrestralisation occurred as a means of exploring previously unexploited niches and resources through the evolution of tetrapods via the Sarcopterygian lineage, whose habitats were most likely mud-flats neighbouring the waters edge (Graham and Lee, 2004). Tetrapods share common features with both modern day land vertebrates and fishes and the discovery of Tiktaalik rosaea allowed the transitional form to be studied in detail to provide evidence on the necessary adaptations required for terrestrialisation (Sarfati, 2007). The skeleton of Tiktaalik was found to be a lot stronger than that of its sarcopterygii-like ancestors and would have allowed it to support its own body weight in substrate, it also showed a longer snout and loss of bony gill covering but still maintained fish-like qualities, such as well developed gill arches and fin rays which implies that it still spent a majority of time in an aquatic environment (Ahlberg and Clack, 2006). Tetrapod digits were seen to arise from the pectoral fins of Sarcopterygii, although there was seen to be a pattern there remained a few anomalies throughout the development (Sarfati, 2007). When discovered Tiktaalik was a hugely important addition to the fossil record and bridged the gap between fish and tetrapods after confirmation from phylogenetic studies placed it on the Sarcopterygian to tetrapod lineage (Sarfati, 2007). The further anatomical and physiological changes that continued after Tiktaalik were responsible for allowing tetrapods to adopt new mechanisms of feeding and locomotion that were required for survival on land and thus was responsible for a huge step in the necessary radiation of respiratory systems (Clack, 2006). Respiration in Amphibians and Non-Avian Reptiles Amphibians are able to breathe by utilising cutaneous methods, using their skin to exchange gases, which could also suggest that it was an important method of respiration used during the transition onto land (Gans, 1970). [JC1]Some amphibians, that have a large enough surface area to volume ratio, such as certain species of salamanders, will rely solely on cutaneous respiration for gaseous exchange due to an absence of lungs (Feder and Burggren, 1985). Cutaneous respiration is based on an infinite pool of oxygen, through air or water mediums, in what is known as a co-current or open flow and is a passive process as there is a lack of inspiratory or expiratory flow (Burggren and Moallf, 1984). Whilst a few amphibians will rely only on cutaneous respiration, most will just use it as an accessory breathing mechanisms and will have other primary methods of respiration (Brainerd and Owerkowicz, 2006). As a means of understanding the primitive breathing in early tetrapods and air breathing fish, other species of salamanders have been studied. It was found that they use a method similar to the two-stroke mechanism previously described in Dipnoi; in which during inspiration they will expand their buccal cavity in order to create a negative pressure required to draw in fresh air, therefore supporting claims that this is most likely the primite mechanism of respiration seen in Sarcopterygii and early tetrapods (Brainerd et al., 1993). Whilst most air breathers will create a negative pressure to cause air to move into the lungs, frogs and some air breathing fish, are known as positive pressure breathers as they use their buccal chamber to fill with air which they will then actively force into the lungs (Jones, 1982). In frogs this system consists of two valves; the paired nares, which remain open for the majority of the time with the function of connecting the buccal cavity to the external air and the glottal valve which spends the majority of the time closed and is only opened when air is entering or leaving the lungs from the buccal chamber (Jones, 1982). This breathing cycle is most likely to begin with expiration as breath-holding was found to most likely occur during the end of the buccal inspiration (Jones, 1982). Reptiles, are believed to be the first group of animals to involve movements of the ribs in the assisting with lung ventilation (Nielsen, 1961). Aspiration breathing is thought to have arison in amniotes, which includes reptiles and mammals, most of which have tried to eliminate their reliance on costal aspiration by evolving accessory breathing methods to aid in respiration (Brainerd and Owerkowicz, 2006). It is evident that aspiration breathing evolved after the buccal pump mechanisms, however, there have been no transitional forms intermediate between the two found which suggests that aspiration breathing developed abruptly and amniotes soon after lost the ability to utilise a buccal cavity (Brainerd, 1999). Studies in lepidosaurs, established that most have an unidivided pleural cavity, which is also present in amphibians and air breathing fish, suggesting that this is the primitive form and that seperation occurred only later on in evolutionary history (Brainerd, 1999). Crocodiles display many unique features compared to the rest of the reptile group as they use a hepatic pistol to ventilate their lungs by utlising a muscle known as the diaphragmaticus, which is not homologous with any other diaphragmatic muscle (Brainerd, 1999). The liver divides the thoracoabdominal cavity and the diaphragmaticus muscle, orinating from the pelvis and caudal gastralia, is responsible for the expansion of the thoracic cavity by retracting the liver; this creates a negative pressure inside and fresh air is forced in, with inspiration containing an intermediate pause (Brainerd and Owerkowicz, 2006). The multicameral chamber seen in crocdiles allows high aerobic demands to be met, which is vital for their survival, and is only found in few other reptile species (Perry, 1988). Respiration in Avian Reptiles Avian reptiles, more commonly known as birds, use a lung-air sac respiratory system which allows cross-current flow, where air and blood are flowing in the same direction to one another (Scheid and Piiper, 1972). The avian respiratory system is small and compact and the thin barriers are thought to be advantageous during flight but not necessary as the mammalian bat respiratory system is dissimilar but still successful for long migratory flights (Schmidt-Nielsen, 1997). Uncinate processes, which alternate depending on the niche of each bird, are fundamental in the avian respiratory system and assist with the movement of the ribs and sternum, allowing for both inspiration and expiration to take place (Codd et al., 2008). The air sacs are used only for ventilation, with gaseous exchanges taking place as air is passed through the parabronchi, which are thin tubes with openings at each end allowing the uni-directional flow of air, which was found to be unique to avian respiration (Scheid, 1979). The parabronchi are packed into a dense hexagonal array with gas exchange tissue, known as the mantle, surrounding the lumen of each; composing a networks of both blood and air capillaries (Brown et al., 1997). The cross-current system found in birds requires these blood and air capillaries to be in close proximity and arranged parallel to one another in order for diffusion to take place; with the uni-directional flow being studied and found to be of no additional advantage to this cross-current system (Scheid, 1979). There are a total of two inspiratory and expiratory cycles that must occur for the complete flow of air through the lungs (Schmidt-Nielsen, 1997). During the first inspiration air flow is split from the trachea to the caudally grouped air sacs or the dorsobronchi, where it will enter the parabronchi and the gas that remained in the lungs from the previous inspiration is forced cranially (Brown et al., 1997). When the first expiration takes place the air remaining in the caudal air sacs moves through the parabronchi, where gas exchange takes place, and another inspiration forces the air into the cranial air sacs (Brown et al., 1997). To exit the respiratory system, the second expiration forces the air to flow from the cranial air sacs through the ventrobronchi and exits using the trachea (Reece et al., 2015). The trachea involved in avian respiration is made up of complete cartilagenous rings and is found to be around 4.5 times the size of mammalian homologues which allows larger tidal volumes and increased compliance within the system (Reece et al., 2015). There have been no valves discovered in the avian respiratory system and therefore to maintain unidirectional air flow it has been suggested that aerodynamics methods, such as jet flow, must be existing in the system during inspiration, and increased resistance through the intrapulmonary bronchus is used during expiratory flow (Scheid, 1979). Respiration in Mammals The respiratory system in mammals is completely separated from the abdominal cavity and the diaphragm and ribs are both crucial in the mechanism of respiration (Keith, 1905). The muscles of the ribs, such as the intercostals, are required for the expansion of the ribcage, which allows the neccesary generating of negative pressure caused by increased the lung volume for aspiration breathing (Perry et al., 2010). The diaphragm is responsible for the control of inspiration as it is able to contract and elongate the thoracic cavity which creates a negative pressure, thus drawing air into the lungs (Loh et al., 1977). The pericardium is closely bound to the lungs and is connected to the central tendon of the diaphragm allowing the vital pairing of both (Keith, 1905). The elevation of the rib cage, which allows further increase in the available volume for external air to enter the lungs, is also under diaphragmattic control (Loh et al., 1977). The mammalian lung is highly complex and involves lots of branching in order to increase surface area from the trachea, which then splits in series into the primary bronchi, secondary bronchi, tertiary bronchi and finally the alveoli. There are around 3108 alveolar air sacs which comprise of thin membranes to increase the surface area and allow the ease of diffusion of gases between them and the capillaries (Hoppensteadt and Peskin, 2002). Inspiration and expiration cycling allows the constant renewal of air into and out of the lungs and provides mammalian species with sufficient oxygen to meet the aerobic demands (Weibel, 1984). Respiration in Insects There is a wide variety of mechanisms adopted by insects for respiration due to the huge variation in available niches. All will utilise a network of air-filled vessels, which are known as tracheae and tracheoles, and can be as small as 1ÃŽÂ ¼m in diameter, with most terminating nearby to the mitochondria of cells (Miller, 1966). The tracheal system at rest is filled with fluid which is thought to be actively absorbed by the permeable inner tracheal wall when required for breathing, using active transport or secretion from cells (Wigglesworth, 1953). Experiments have demonstrated that during tracheal compression, which is controlled by an increased pressure inside the exoskeleton, the tracheae noticeably shrink in diameter to aid in air convection and increased diffusion of oxygen into the tissues due to a high pressure build up (Westneat et al., 2003). Spiracles are required at the external and internal barrier in the tracheael system to allow external air through the skin; and previous experiments have found if these are blocked then the insect cannot survive as respiration will cease (Fraenkel and Herford, 1938). Interneurons are essential as they are responsible for the pairing of spiracle movement with ventilation by communicating with the spiracles motor neurones (Miller, 1966). The discontinuous gas exchange cycles present in insects will typically occur in three stages, beginning the closed-spiracle phase where only small amounts of external gas exchange are able to take place (Lighton, 1996). A fluttering-spiracle phase permits oxygen uptake for the diffusion of gases into the tracheael tissues and finally an open-spiracle phase concludes the cycle whilst allowing the release of accumulated carbon dioxide (Lighton, 1996). Ventilation is under endogenously controlled rhythms produced by the central nervous system which allows aerobic respiration rates in flight muscles to be so successful that they can be challenged only by certain species of bacteria (Miller, 1966). During insect respiration air is sucked into the tracheal system by creating negative alterations in internal pressure using the pumping of hemolymph by the heart or the contracting of abdominal muscles, others can include passive diffusion or autoventilation (Westneat et al., 2003). Respiration in Cetaceans Cetaceans have evolved a much more unusual respiratory system to any terrestrial mammal, as the nasal passageway has moved to a more dorsal position to allow the exclusion of water from the system and ease of breathing as they surface (Thomas and Kastelein, 1991). A nasal plug, made up of nasal plug muscle, connective tissue and adipose tissue, is responsible for the seperation of the internal and external environment and is retracted anteriolaterally for respiration by bilaterally paired nasal plug muscles (Thomas and Kastelein, 1991). The lung size of cetaceans varies depending on the depth of the dives undertaken, due to the variety of pressures causing differing extents of thoracic collapse (Piscitelli et al., 2010). It was found that the lung size will be reduced in cetaceans that undergo deeper dives and there will be an increase in the thoracic mobility. The lungs of larger whales were found to possess extremely heavy myoelastic bundles in the air sacs and alvolar membranes that were not found in much smaller cetacea (Wislocki, 1942). Conclusion The evolution of respiratory systems has been an extremely specific process that has showed both gradual, and rapid changes throughout the many lineages of the animal kingdom in order to encorporate universal requirements, as well as accessory breathing mechanisms (Weibel, 1984). Respiration is a vital life process required for survival and it is essential that gas exchange is as efficient as possible in order to allow high oxygen-demanding aerobic activities to take place when necessary (Perry, 1988). Each respiratory system may have a variety of additional mechanisms, uncinate processes, that assist in the breathing mechanics to enable the ease of transporting larger volumes of air within each system (Codd et al., 2008). The pulmonary surfactant system is of great importance, as it allows the successful existance of such mechanisms by preventing collapse of respiratory surfaces, as well as aiding them by increasing lung compliance and reducing the resistance to air flow (Daniels and Orgeig, 2003). The evolution of efficient respiratory systems, when paired with other necessary adaptations, has provided a foundation for more complex body systems to develop to allow the utilisation of previously unexploited resources and niches, thus providing organisms advantages in the animal kingdom (Graham, 1997). Bibliography Ahlberg, P.E., Clack, J.A., 2006. Palaeontology: A firm step from water to land. Nature 440, 747-749. doi:10.1038/440747a Ap, S., La, F.-G., 2001. Ventilatory modes and mechanics of the hedgehog skate (Leucoraja erinacea): testing the continuous flow model. J. Exp. Biol. 204, 1577-1587. Brainerd, E.L., 1999. New perspectives on the evolution of lung ventilation mechanisms in vertebrates. Exp. Biol. Online 4, 1-28. doi:10.1007/s00898-999-0002-1 Brainerd, E.L., 1994. The Evolution of Lung-Gill Bimodal Breathing and the Homology of Vertebrate Respiratory Pumps. Integr. Comp. Biol. 34, 289-299. doi:10.1093/icb/34.2.289 Brainerd, E.L., Ditelberg, J.S., Bramble, D.M., 1993. Lung ventilation in salamanders and the evolution of vertebrate air-breathing mechanisms. Biol. J. Linn. Soc. 49, 163-183. doi:10.1111/j.1095-8312.1993.tb00896.x Brainerd, E.L., Ferry-Graham, L.A., 2005. Mechanics of Respiratory Pumps, in: Physiology, B.-F. (Ed.), Fish Biomechanics. Academic Press, pp. 1-28. doi:10.1016/S1546-5098(05)23001-7 Brainerd, E.L., Owerkowicz, T., 2006. Functional morphology and evolution of aspiration breathing in tetrapods. Respir. Physiol. Neurobiol., Frontiers in Comparative Physiology II: Respiratory Rhythm, Pattern and Responses to Environmental Change 154, 73-88. doi:10.1016/j.resp.2006.06.003 Brown, R.E., Brain, J.D., Wang, N., 1997. The avian respiratory system: a unique model for studies of respiratory toxicosis and for monitoring air quality. Environ. Health Perspect. 105, 188-200. Burggren, W., Moallf, R., 1984. Active regulation of cutaneous exchange by capillary recruitment in amphibians: Experimental evidence and a revised model for skin respiration. Respir. Physiol. 55, 379-392. doi:10.1016/0034-5687(84)90059-8 Burggren, W.W., Johansen, K., 1986. Circulation and respiration in lungfishes (dipnoi). J. Morphol. 190, 217-236. doi:10.1002/jmor.1051900415 Clack, J.A., 2006. The emergence of early tetrapods. Palaeogeogr. Palaeoclimatol. Palaeoecol. 232, 167-189. doi:10.1016/j.palaeo.2005.07.019 Codd, J.R., Manning, P.L., Norell, M.A., Perry, S.F., 2008. Avian-like breathing mechanics in maniraptoran dinosaurs. Proc. R. Soc. Lond. B Biol. Sci. 275, 157-161. doi:10.1098/rspb.2007.1233 Daeschler, E.B., Shubin, N.H., Jenkins, F.A., 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature 440, 757-763. doi:10.1038/nature04639 Daniels, C.B., Orgeig, S., 2003. Pulmonary Surfactant: The Key to the Evolution of Air Breathing. News Phsiology Sci. 18, 151-157. Daniels, C.B., Orgeig, S., Sullivan, L.C., Ling, N., Bennett, M.B., Schà ¼rch, S., Val, A.L., Brauner, C.J., 2004. The Origin and Evolution of the Surfactant System in Fish: Insights into the Evolution of Lungs and Swim Bladders. Physiol. Biochem. Zool. Ecol. Evol. Approaches 77, 732-749. doi:10.1086/422058 Evans, D.H., Piermarini, P.M., Choe, K.P., 2005. The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste. Physiol. Rev. 85, 97-177. doi:10.1152/physrev.00050.2003 Feder, M.E., Burggren, W.W., 1985. Cutaneous Gas Exchange in Vertebrates: Design, Patterns, Control and Implications. Biol. Rev. 60, 1-45. doi:10.1111/j.1469-185X.1985.tb00416.x Fraenkel, G., Herford, G.V.B., 1938. The Respiration of Insects Through the Skin. J. Exp. Biol. 15, 266-280. Gans, C., 1970. Respiration in Early Tetrapods-The Frog is a Red Herring. Evolution 24, 723-734. doi:10.2307/2406552 Graham, J.B., 1997. Air-Breathing Fishes: Evolution, Diversity, and Adaptation. Academic Press. Graham, J.B., Lee, H.J., 2004. Breathing Air in Air: In What Ways Might Extant Amphibious Fish Biology Relate to Prevailing Concepts about Early Tetrapods, the Evolution of Vertebrate Air Breathing, and the Vertebrate Land Transition? Physiol. Biochem. Zool. 77, 720-731. doi:10.1086/425184 Hoppensteadt, F.C., Peskin, C.S., 2002. Gas Exchange in the Lungs, in: Modeling and Simulation in Medicine and the Life Sciences, Texts in Applied Mathematics. Springer New York, pp. 75-108. doi:10.1007/978-0-387-21571-6_3 Ibanez, J.G., Hernandez-Esparza, M., Doria-Serrano, C., Fregoso-Infante, A., Singh, M.M., 2008. Dissolved Oxygen in Water, in: Environmental Chemistry. Springer New York, pp. 16-27. doi:10.1007/978-0-387-49493-7_2 Jones, R.M., 1982. How toads breathe: Control of air flow to and from the lungs by the nares in Bufo marinus. Respir. Physiol. 49, 251-265. doi:10.1016/0034-5687(82)90077-9 Keith, A., 1905. The Nature of the Mammalian Diaphragm and Pleural Cavities. J. Anat. Physiol. 39, 243-284. Lighton, J.R., 1996. Discontinuous gas exchange in insects. Annu. Rev. Entomol. 41, 309-324. doi:10.1146/annurev.en.41.010196.001521 Loh, L., Goldman, M., Davis, J.N., 1977. The assessment of diaphragm function. Medicine (Baltimore) 56, 165-169. Miller, P.L., 1966. The Regulation of Breathing in Insects, in: J.W.L. Beament, J.E.T. and V.B.W. (Ed.), Advances in Insect Physiology. Academic Press, pp. 279-354. doi:10.1016/S0065-2806(08)60189-7 Nielsen, B., 1961. On the Regulation of the Respiration in Reptiles. J. Exp. Biol. 38, 301-314. Parson, E.A., 2003. Protecting the Ozone Layer: Science and Strategy. Oxford University Press. Perry, S.F., 1988. 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Polyisobutylene Applications

Polyisobutylene Applications Fuel and lubricant additive. Polyisobutylene (in the form of polyisobutylene succinimide) has interesting properties when used as an additive in lubricating oils and motor fuels. Polyisobutylene added in small amounts to the lubricating oils used in machining results in a significant reduction in the generation of oil mist and thus reduces the operators inhalation of oil mist.[2] It is also used to clean up waterborne oil spills as part of the commercial product Elastol. When added to crude oil it increases the oils viscoelasticity when pulled, causing the oil to resist breakup when it is vacuumed from the surface of the water. As a fuel additive, polyisobutylene has detergent properties. When added to diesel fuel, it resists fouling of fuel injectors, leading to reduced hydrocarbon and particulate emissions.[3] It is blended with other detergents and additives to make a detergent package that is added to gasoline and diesel fuel to resist buildup of deposits and engine knock.[4] Polyisobutylene is used in some formulations as a thickening agent. [edit]Sporting equipment Butyl rubber is used for the bladders in basketballs, footballs, soccer balls and other inflatable balls to provide a tough, airtight inner compartment. [edit]Roof Repair Butyl rubber sealant is used for rubber roof repair and for maintenance of roof membranes (especially around the edges). It is important to have the roof membrane fixed, as a lot of fixtures (i.e., air conditioner vents, plumbing and other pipes, etc.) can considerably loosen it. Rubber roofing typically refers to a specific type of roofing materials that are made of ethylene propylene diene monomers (EPDM). It is crucial to the integrity of such roofs to avoid using harsh abrasive materials and petroleum-based solvents for their maintenance. Polyester fabric laminated to butyl rubber binder provides a single-sided waterproof tape that can be used on metal, PVC, and cement joints. It is ideal for repairing and waterproofing metal roofs. [edit]Gas masks and chemical agent protection Butyl rubber is one of the most robust elastomers when subjected to chemical warfare agents and decontamination materials. It is a harder and less porous material than other elastomers, such as natural rubber or silicone, but still has enough elasticity to form an airtight seal. While butyl rubber will break down when exposed to agents such as NH3 (ammonia) or certain solvents, it breaks down more slowly than comparable elastomers. It is therefore used to create seals in gas masks and other protective clothing. [edit]Chewing gum Molecular structure: Rubber Chemical Structure Introduction to rubber chemical structure: In the organic chemistry section of chemistry ,we learn about the various polymers , monomers,elastomers etc.Monomer is a single unit and when huge number of monomers are combined or say polymerised then polymers are formed.The process of conversion of monomer to polymer is known as p-olymerisation.Elastomer is an another category of polymers having a specific properties of regaining of its structure even if it is stretched.Rubber comes under the category of elastomer.In general life rubber has variety of uses.the important property of rubber is that it regains its structure even if it is stretched.Stretching can be done up to a certain limit.If it is stretched beyond limit then it can break. Rubber can be found in two forms 1)Natural rubber 2)Synthetic Rubber Natural rubber is a kind of rubber which which is found directly from the nature.And when the natural rubber is processed under some chemical processes then a new kind of rubber is formed ,this rubber is known as synthetic rubber.We can also say that natural rubber are synthesised from the natural rubber.Both this rubber are of great use because of its specific features. Structure of rubber: Main composition of crude rubber is hydrocarbons.It also contains some proteins and materials which are soluble in acetone.The hydrocarbons which possess the properties of rubber are usually high in molecular weight and it ranges from 45000 to 3000000.Isoprene is a monomer of natural rubber.When huge number of isoprene units are polymerised then a polymer is formed . Cis and Trans Configuration of Rubber The cis configuration of the natural rubber is the reason for the rubber properties in it. Cis configuration means that extension of the chain is on the same side of the ethylene bond.If the configuration is trans, it means that the extension of chain is on the both sides of ethylene bond,then it is a hard plastic.In case of trans it does not show the properties of rubber. Synthetic rubber is of great use in the industry.Some of the widely used synthetic rubber are butyl rubber which is formed by the copolymerisation of isobutylene and a little amount isoprene.Another synthetic rubber is Styrene Butadiene Rubber also known as SBR.Buna N and buna S is also a kind of synthetic rubber often use in the industry. Vulcanisation of Rubber In the rubber molecules the cross linking between the chains are very less.This leads to the softness in the rubber .To make the rubber hard some chemicals are added to it.The process is known as vulcanisation.In this process the natural rubber is treated with some chemicals ,more often chemical used is sulphur.When sulphur reacts with the natural rubber then it increases the cross linking between the molecules in the rubber.It also forms many sulphide bonds.Due to formation of many new crosslinkings and many sulphide bonds the natural rubber becomes hard.Natural rubber is a kind of thermoplastic,it means that it becomes soft when it is subjected to heat and it becomes hard when it is subjected to cold. butyl rubber (IIR), also called isobutylene-isoprene rubber, a synthetic rubber produced by copolymerizing isobutylene with small amounts of isoprene. Valued for its chemical inertness, impermeability to gases, and weatherability, butyl rubber is employed in the inner linings of automobile tires and in other specialty applications. Both isobutylene (C[CH3]2=CH2) and isoprene (CH2=C[CH3]-CH=CH2) are usually obtained by the thermal cracking of natural gas or of the lighter fractions of crude oil. At normal temperature and pressure isobutylene is a gas and isoprene is a volatile liquid. For processing into IIR, isobutylene, refrigerated to very low temperatures (approximately à ¢Ã‹â€ Ã¢â‚¬â„¢100  °C [à ¢Ã‹â€ Ã¢â‚¬â„¢150  °F]), is diluted with methyl chloride. Low concentrations (1.5 to 4.5 percent) of isoprene are added in the presence of aluminum chloride, which initiates the reaction in which the two compounds copolymerize (i.e., their single-unit molecules link together to form giant, multiple-unit molecules). The polymer repeating units have the following structures: Because the base polymer, polyisobutylene, is stereoregular (i.e., its pendant groups are arranged in a regular order along the polymer chains) and because the chains crystallize rapidly on stretching, IIR containing only a small amount of isoprene is as strong as natural rubber. In addition, because the copolymer contains few unsaturated groups (represented by the carbon-carbon double bond located in each isoprene repeating unit), IIR is relatively resistant to oxidation-a process by which oxygen in the atmosphere reacts with the double bonds and breaks the polymer chains, thereby degrading the material. Butyl rubber also shows an unusually low rate of molecular motion well above the glass transition temperature (the temperature above which the molecules are no longer frozen in a rigid,glassy state). This lack of motion is reflected in the copolymers unusually low permeability to gases as well as in its outstanding resistance to attack by ozone. The copolymer is recovered from the solvent as a crumb, which can be compounded with fillers and other modifiers and then vulcanized into practical rubber products. Owing to its excellent air retention, butyl rubber is the preferred material for inner tubes in all but the largest sizes. It also plays an important part in the inner liners of tubeless tires. (Because of poor tread durability, all-butyl tires have not proved successful.) IIR is also used for many other automobile components, including window strips, because of its resistance to oxidation. Its resistance to heat has made it indispensable in tire manufacture, where it forms the bladders that retain the steam or hot water used to vulcanize tires. Bromine or chlorine can be added to the small isoprene fraction of IIR to make BIIR or CIIR (known as halobutyls). The properties of these polymers are similar to those of IIR, but they can be cured more rapidly and with different and smaller amounts of curative agents. As a result, BIIR and CIIR can be cocured more readily in contact with other elastomers making up a rubber product. Butyl rubber was first produced by American chemists William Sparks and Robert Thomas at the Standard Oil Company of New Jersey (nowExxon Corporation) in 1937. Earlier attempts to produce synthetic rubbers had involved the polymerization of dienes (hydrocarbon molecules containing two carbon-carbon double bonds) such as isoprene and butadiene. Sparks and Thomas defied convention by copolymerizing isobutylene, an olefin (hydrocarbon molecules containing only one carbon-carbon double bond) with small amounts-e.g., less than 2 percent-of isoprene. As a diene, isoprene provided the extra double bond required to cross-link the otherwise inert polymer chains, which were essentially polyisobutylene. Before experimental difficulties were resolved, butyl rubber was called futile butyl, but with improvements it enjoyed wide acceptance for its low permeability to gases and its excellent resistance to oxygen and ozone at normal temperatures. During World War IIthe copolymer was called GR-I, for Government Rubber-Isobutylene. LINKS Related Articles Top of Form Polymers 1. Introduction Prior to the early 1920s, chemists doubted the existence of molecules having molecular weights greater than a few thousand. This limiting view was challenged by Hermann Staudinger, a German chemist with experience in studying natural compounds such as rubber and cellulose. In contrast to the prevailing rationalization of these substances as aggregates of small molecules, Staudinger proposed they were made up of macromolecules composed of 10,000 or more atoms. He formulated a polymeric structure for rubber, based on a repeating isoprene unit (referred to as a monomer). For his contributions to chemistry, Staudinger received the 1953 Nobel Prize. The terms polymer and monomer were derived from the Greek roots poly (many), mono (one) and meros (part). Recognition that polymeric macromolecules make up many important natural materials was followed by the creation of synthetic analogs having a variety of properties. Indeed, applications of these materials as fibers, flexible films, adhesives, resistant paints and tough but light solids have transformed modern society. Some important examples of these substances are discussed in the following sections. art 2. Writing Formulas for Polymeric Macromolecules The repeating structural unit of most simple polymers not only reflects the monomer(s) from which the polymers are constructed, but also provides a concise means for drawing structures to represent these macromolecules. For polyethylene, arguably the simplest polymer, this is demonstrated by the following equation. Here ethylene (ethene) is the monomer, and the corresponding linear polymer is called high-density polyethylene (HDPE). HDPE is composed of macromolecules in which n ranges from 10,000 to 100,000 (molecular weight 2*105 to 3 *106 ). If Y and Z represent moles of monomer and polymer respectively, Z is approximately 10-5 Y. This polymer is called polyethylene rather than polymethylene, (-CH2-)n, because ethylene is a stable compound (methylene is not), and it also serves as the synthetic precursor of the polymer. The two open bonds remaining at the ends of the long chain of carbons (colored magenta) are normally not specified, because the atoms or groups found there depend on the chemical process used for polymerization. The synthetic methods used to prepare this and other polymers will be described later in this chapter. Unlike simpler pure compounds, most polymers are not composed of identical molecules. The HDPE molecules, for example, are all long carbon chains, but the lengths may vary by thousands of monomer units. Because of this, polymer molecular weights are usually given as averages. Two experimentally determined values are common: Mn , the number average molecular weight, is calculated from the mole fraction distribution of different sized molecules in a sample, and Mw , the weight average molecular weight, is calculated from the weight fraction distribution of different sized molecules. These are defined below. Since larger molecules in a sample weigh more than smaller molecules, the weight average Mw is necessarily skewed to higher values, and is always greater than Mn. As the weight dispersion of molecules in a sample narrows, Mw approaches Mn, and in the unlikely case that all the polymer molecules have identical weights (a pure mono-disperse sample), the ratio Mw / Mn becomes unity. The influence of different mass distributions on Mn and Mw may be examined with the aid of a simple mass calculator. To use this device Click Here. Many polymeric materials having chain-like structures similar to polyethylene are known. Polymers formed by a straightforward linking together of monomer units, with no loss or gain of material, are called addition polymers or chain-growth polymers. A listing of some important addition polymers and their monomer precursors is presented in the following table. Some Common Addition Polymers Name(s) Formula Monomer Properties Uses Polyethylene low density (LDPE) -(CH2-CH2)n- ethylene CH2=CH2 soft, waxy solid film wrap, plastic bags Polyethylene high density (HDPE) -(CH2-CH2)n- ethylene CH2=CH2 rigid, translucent solid electrical insulation bottles, toys Polypropylene (PP) different grades -[CH2-CH(CH3)]n- propylene CH2=CHCH3 atactic: soft, elastic solid isotactic: hard, strong solid similar to LDPE carpet, upholstery Poly(vinyl chloride) (PVC) -(CH2-CHCl)n- vinyl chloride CH2=CHCl strong rigid solid pipes, siding, flooring Poly(vinylidene chloride) (Saran A) -(CH2-CCl2)n- vinylidene chloride CH2=CCl2 dense, high-melting solid seat covers, films Polystyrene (PS) -[CH2-CH(C6H5)]n- styrene CH2=CHC6H5 hard, rigid, clear solid soluble in organic solvents toys, cabinets packaging (foamed) Polyacrylonitrile (PAN, Orlon, Acrilan) -(CH2-CHCN)n- acrylonitrile CH2=CHCN high-melting solid soluble in organic solvents rugs, blankets clothing Polytetrafluoroethylene (PTFE, Teflon) -(CF2-CF2)n- tetrafluoroethylene CF2=CF2 resistant, smooth solid non-stick surfaces electrical insulation Poly(methyl methacrylate) (PMMA, Lucite, Plexiglas) -[CH2-C(CH3)CO2CH3]n- methyl methacrylate CH2=C(CH3)CO2CH3 hard, transparent solid lighting covers, signs skylights Poly(vinyl acetate) (PVAc) -(CH2-CHOCOCH3)n- vinyl acetate CH2=CHOCOCH3 soft, sticky solid latex paints, adhesives cis-Polyisoprene natural rubber -[CH2-CH=C(CH3)-CH2]n- isoprene CH2=CH-C(CH3)=CH2 soft, sticky solid requires vulcanization for practical use Polychloroprene (cis + trans) (Neoprene) -[CH2-CH=CCl-CH2]n- chloroprene CH2=CH-CCl=CH2 tough, rubbery solid synthetic rubber oil resistant 3. Properties of Macromolecules A comparison of the properties of polyethylene (both LDPE HDPE) with the natural polymers rubber and cellulose is instructive. As noted above, synthetic HDPE macromolecules have masses ranging from 105 to 106 amu (LDPE molecules are more than a hundred times smaller). Rubber and cellulose molecules have similar mass ranges, but fewer monomer units because of the monomers larger size. The physical properties of these three polymeric substances differ from each other, and of course from their monomers. à ¢Ã¢â€š ¬Ã‚ ¢ HDPE is a rigid translucent solid which softens on heating above 100 ° C, and can be fashioned into various forms including films. It is not as easily stretched and deformed as is LDPE. HDPE is insoluble in water and most organic solvents, although some swelling may occur on immersion in the latter. HDPE is an excellent electrical insulator. à ¢Ã¢â€š ¬Ã‚ ¢ LDPE is a soft translucent solid which deforms badly above 75 ° C. Films made from LDPE stretch easily and are commonly used for wrapping. LDPE is insoluble in water, but softens and swells on exposure to hydrocarbon solvents. Both LDPE and HDPE become brittle at very low temperatures (below -80 ° C). Ethylene, the common monomer for these polymers, is a low boiling (-104 ° C) gas. à ¢Ã¢â€š ¬Ã‚ ¢ Natural (latex) rubber is an opaque, soft, easily deformable solid that becomes sticky when heated (above. 60 ° C), and brittle when cooled below -50 ° C. It swells to more than double its size in nonpolar organic solvents like toluene, eventually dissolving, but is impermeable to water. The C5H8 monomer isoprene is a volatile liquid (b.p. 34 ° C). à ¢Ã¢â€š ¬Ã‚ ¢ Pure cellulose, in the form of cotton, is a soft flexible fiber, essentially unchanged by variations in temperature ranging from -70 to 80 ° C. Cotton absorbs water readily, but is unaffected by immersion in toluene or most other organic solvents. Cellulose fibers may be bent and twisted, but do not stretch much before breaking. The monomer of cellulose is the C6H12O6aldohexose D-glucose. Glucose is a water soluble solid melting below 150 ° C. To account for the differences noted here we need to consider the nature of the aggregate macromolecular structure, or morphology, of each substance. Because polymer molecules are so large, they generally pack together in a non-uniform fashion, with ordered or crystalline-like regions mixed together with disordered or amorphous domains. In some cases the entire solid may be amorphous, composed entirely of coiled and tangled macromolecular chains. Crystallinity occurs when linear polymer chains are structurally oriented in a uniform three-dimensional matrix. In the diagram on the right, crystalline domains are colored blue. Increased crystallinity is associated with an increase in rigidity, tensile strength and opacity (due to light scattering). Amorphous polymers are usually less rigid, weaker and more easily deformed. They are often transparent. Three factors that influence the degree of crystallinity are: i) Chain length ii) Chain branching iii) Interchain bonding The importance of the first two factors is nicely illustrated by the differences between LDPE and HDPE. As noted earlier, HDPE is composed of very long unbranched hydrocarbon chains. These pack together easily in crystalline domains that alternate with amorphous segments, and the resulting material, while relatively strong and stiff, retains a degree of flexibility. In contrast, LDPE is composed of smaller and more highly branched chains which do not easily adopt crystalline structures. This material is therefore softer, weaker, less dense and more easily deformed than HDPE. As a rule, mechanical properties such as ductility, tensile strength, and hardness rise and eventually level off with increasing chain length. The nature of cellulose supports the above analysis and demonstrates the importance of the third factor (iii). To begin with, cellulose chains easily adopt a stable rod-like conformation. These molecules align themselves side by side into fibers that are stabilized by inter-chain hydrogen bonding between the three hydroxyl groups on each monomer unit. Consequently, crystallinity is high and the cellulose molecules do not move or slip relative to each other. The high concentration of hydroxyl groups also accounts for the facile absorption of water that is characteristic of cotton. Natural rubber is a completely amorphous polymer. Unfortunately, the potentially useful properties of raw latex rubber are limited by temperature dependence; however, these properties can be modified by chemical change. The cis-double bonds in the hydrocarbon chain provide planar segments that stiffen, but do not straighten the chain. If these rigid segments are completely removed by hydrogenation (H2 Pt catalyst), the chains lose all constrainment, and the product is a low melting paraffin-like semisolid of little value. If instead, the chains of rubber molecules are slightly cross-linked by sulfur atoms, a process called vulcanization which was discovered by Charles Goodyear in 1839, the desirable elastomeric properties of rubber are substantially improved. At 2 to 3% crosslinking a useful soft rubber, that no longer suffers stickiness and brittleness problems on heating and cooling, is obtained. At 25 to 35% crosslinking a rigid hard rubber product is formed. The following illust ration shows a cross-linked section of amorphous rubber. By clicking on the diagram it will change to a display of the corresponding stretched section. The more highly-ordered chains in the stretched conformation are entropically unstable and return to their original coiled state when allowed to relax (click a second time). On heating or cooling most polymers undergo thermal transitions that provide insight into their morphology. These are defined as the melt transition, Tm , and the glass transition, Tg . Tm is the temperature at which crystalline domains lose their structure, or melt. As crystallinity increases, so does Tm. Tg is the temperature below which amorphous domains lose the structural mobility of the polymer chains and become rigid glasses. Tg often depends on the history of the sample, particularly previous heat treatment, mechanical manipulation and annealing. It is sometimes interpreted as the temperature above which significant portions of polymer chains are able to slide past each other in response to an applied force. The introduction of relatively large and stiff substituents (such as benzene rings) will interfere with this chain movement, thus increasing Tg (note polystyrene below). The introduction of small molecular compounds called plasticizers into the polymer matrix increases the interchain spacing, allowing chain movement at lower temperatures. with a resulting decrease in Tg. The outgassing of plasticizers used to modify interior plastic components of automobiles produces the new-car smell to which we are accustomed. Tm and Tg values for some common addition polymers are listed below. Note that cellulose has neither a Tm nor a Tg. Polymer LDPE HDPE PP PVC PS PAN PTFE PMMA Rubber Tm ( °C) 110 130 175 180 175 >200 330 180 30 Tg ( °C) _110 _100 _10 80 90 95 _110 105 _70 Rubber is a member of an important group of polymers called elastomers. Elastomers are amorphous polymers that have the ability to stretch and then return to their original shape at temperatures above Tg. This property is important in applications such as gaskets and O-rings, so the development of synthetic elastomers that can function under harsh or demanding conditions remains a practical goal. At temperatures below Tg elastomers become rigid glassy solids and lose all elasticity. A tragic example of this caused the space shuttle Challenger disaster. The heat and chemical resistant O-rings used to seal sections of the solid booster rockets had an unfortunately high Tg near 0  °C. The unexpectedly low temperatures on the morning of the launch were below this Tg, allowing hot rocket gases to escape the seals. Copolymers The synthesis of macromolecules composed of more than one monomeric repeating unit has been explored as a means of controlling the properties of the resulting material. In this respect, it is useful to distinguish several ways in which different monomeric units might be incorporated in a polymeric molecule. The following examples refer to a two component system, in which one monomer is designated A and the other B. Statistical Copolymers Also called random copolymers. Here the monomeric units are distributed randomly, and sometimes unevenly, in the polymer chain: ~ABBAAABAABBBABAABA~. Alternating Copolymers Here the monomeric units are distributed in a regular alternating fashion, with nearly equimolar amounts of each in the chain: ~ABABABABABABABAB~. Block Copolymers Instead of a mixed distribution of monomeric units, a long sequence or block of one monomer is joined to a block of the second monomer: ~AAAAA-BBBBBBB~AAAAAAA~BBB~. Graft Copolymers As the name suggests, side chains of a given monomer are attached to the main chain of the second monomer: ~AAAAAAA(BBBBBBB~)AAAAAAA(BBBB~)AAA~. 1. Addition Copolymerization Most direct copolymerizations of equimolar mixtures of different monomers give statistical copolymers, or if one monomer is much more reactive a nearly homopolymer of that monomer. The copolymerization of styrene with methyl methacrylate, for example, proceeds differently depending on the mechanism. Radical polymerization gives a statistical copolymer. However, the product of cationic polymerization is largely polystyrene, and anionic polymerization favors formation of poly(methyl methacrylate). In cases where the relative reactivities are different, the copolymer composition can sometimes be controlled by continuous introduction of a biased mixture of monomers into the reaction. Formation of alternating copolymers is favored when the monomers have different polar substituents (e.g. one electron withdrawing and the other electron donating), and both have similar reactivities toward radicals. For example, styrene and acrylonitrile copolymerize in a largely alternating fashion. Some Useful Copolymers Monomer A Monomer B Copolymer Uses H2C=CHCl H2C=CCl2 Saran films fibers H2C=CHC6H5 H2C=C-CH=CH2 SBR styrene butadiene rubber tires H2C=CHCN H2C=C-CH=CH2 Nitrile Rubber adhesives hoses H2C=C(CH3)2 H2C=C-CH=CH2 Butyl Rubber inner tubes F2C=CF(CF3) H2C=CHF Viton gaskets A terpolymer of acrylonitrile, butadiene and styrene, called ABS rubber, is used for high-impact containers, pipes and gaskets. For polyisobutylene at a glance, click here! Polyisobutylene is a synthetic rubber, or elastomer. Its special because its the only rubber thats gas impermeable, that is, its the only rubber that can hold air for long periods of time. You may have noticed that balloons will go flat after a few days. This is because they are made of polyisoprene, which is not gas impermeable. Because polyisobutylene will hold air, it is used to make things like the inner liner of tires, and the inner liners of basketballs. Polyisobutylene, sometimes called butyl rubber, and other times PIB, is a vinyl polymer. Its very similar to polyethylene and polypropylene in structure, except that every other carbon is substituted with two methyl groups. It is made from the monomer isobutylene, by cationic vinyl polymerization. And this is that monomer isobutylene: Usually, a small amount of isoprene is added to the isobutylene. The polymerization is carried out at a right frosty -100 oC, or -148 oF for you Americans out there. This is because the reaction is so fast we cant control it unless we freeze it colder than a brass toilet seat in the Yukon. Polyisobutylene was first developed during the early 1940s. At that time, the most widely used rubber was natural rubber, polyisoprene. Polyisoprene was an excellent elastomer, and easy to isolate from the sap of the hevea tree. Huge plantations thrived in Malaysia and grew hevea trees to supply the worlds rubber needs. There was only one slight problem, and that was that Malaysia had just been conquered by the Imperial Japanese Army, and wouldnt you know we just so happened to be fighting the Second World War against them right at that moment. Before the war was over more than sixty million people would be dead. Deprived of natural rubber, the Allied nations did some quick thinking and came up with PIB. It obviously worked, because the Allies won the war. Ok, we didnt actually invent polyisobutylene during the war. It had been invented long before the war by chemists in Germany. Theres irony! But it wasnt very useful until American chemists came up with a way to crosslink it. What they did was to copolymerize isobutylene with a little bit, say, around one percent, isoprene. This is isoprene: When isoprene is polymerized with the isobutylene we get a polymer that looks like this: About one or two out of every hundred repeat units is an isoprene unit, shown in blue. These have double bonds, which means the polymer can be crosslinked byvulcanization just like natural rubber. What is this vulcanization? To find out, click here. Stealing Vulcans Fire There was a time long past when the only rubber we had was natural rubber latex, polyisoprene. Straight out of the tree, natural rubber latex isnt good for much. It gets runny and sticky when it gets warm, and it gets hard and brittle when its cold. Tires made out of it wouldnt be much good unless one lived in some happy land where the temperature was seventy degrees year round. A long time agohow long, you ask? It was about a hundred and sixty years ago, 1839 to be exact. This was before there were any cars to need tires, but the idea of a useable rubber was still attractive. One person trying to make rubber more useful was named Charles Goodyear, a tinkerer and inventor, and by no means a successful one at this point. While goofing around in his kitchen with a piece of fabric coated with a m

The Ethics of Stem Cell Research Essay example -- Genetic Engineering

According to the National Institutes of Health (NIH), stem cells are 1 "Cells with the ability to divide for indefinite periods in culture and to give rise to specialized cells." Stem cells are basically unspecialized cells that can, with proper physiologic or experimental conditions, become specialized cells. Specialized cells are usually called differentiated cells. These differentiated cells can then be used to repair damaged cells and eventually cure many diseases and disorders in humans. This could revolutionize the way society treats health issues. Instead of trying to destroy and sure a disease or disorder, doctors could simply grow new cells that would be able to repair or replace the damaged cells and tissue. Adult Stem Cells There are two major types of stem cells, embryonic and adult stem cells. Adult stem cells are the undifferentiated cells that are found in tissue and organs in adults. These cells are usually found mixed in with differentiated cells. These cells are used to repair and maintain the tissue in which they are found. Research of adult stem cells began in the 1960?s when scientists discovered them in bone marrow. Stem cells from bone marrow have been used in transplants for the past 30 years. Currently, scientists are unsure as to how adult stem cells develop. A major advantage to using adult stem cells is that each patient?s stem cells can be extracted, grown, and then reinserted into their body. This would ensure that the immune system would not reject the new cells. One of the problems with adult stem cells is that there are a very few number of stem cells in a sample of tissue. Another problem is that scientists are unsure about the flexibility of adult stem cells. Since they are found only i... ...ws-item94.htm >. Stem Cell Basics. 2002. National Institutes of Health (NIH). 16 Nov. 2003. ?Stem Cell Debate Revives an Old Ideological Battle,? New York Times [NY] 6 Jul. 2001. first ed.: A17 ?Stem Cell Research; Global Differences: As the U.S. Hesitates, Other Countries Move Ahead With Studies on Embryos,? San Jose Mercury News [CA] 7 May 2002, morning final: 1E. ?Tangled Issues In Congress: Cloning and Stem Cell Study,? New York Times [NY] 31 Jul 2001, first ed.: A17 ?Use of Cloning to Tailor Treatment Has Big Hurdles, Including Cost,? New York Times [NY] 18 Dec. 2001 first ed.: F2 What Are Stem Cells and What are They Used For? 2002. How Stuff Works. 15 Nov. 2003. . P1.http://www4.od.nih.gov/stemcell/figure1_primer0902big.jpg

The Storm: An Inner Reflection Essays -- Emotions Psychology Essays

The Storm: An Inner Reflection Memories are all we have sometimes, but what if memories bring out unwelcome feelings? In Romesh Gunesekera's short story "Ranvali," a young lady goes back to her father's old holiday bungalow and begins to discover new feelings toward her beloved Communist father. The story is set in an idyllic bungalow in Ranvali, by the coast of India. Theorists such as Roland Barthes would argue that setting in modern narratives "no longer need meaning: they simply are: that is their meaning." (qtd in Chatman 145). However, in "Ranvali," the storm that besieges the bungalow while the young lady is there clearly mimics her thoughts and gives the reader a greater sense of the inner turmoil that she must be going through. It can thus be shown that the storm is an essential part of the setting that Gunesekera uses to evoke certain feelings in the readers of "Ranvali." In the story, the storm mimics the narrator's inner turmoil at discovering new feelings about her father. But is the storm part of setting? Chatman makes a distinction between existents - characters and setting. For Chatman, "setting 'sets the character off'; it is the place and collection of objects 'against which' his actions and passions appropriately emerge" (Chatman 134). The storm is part of the description of the 'place' where the story unfolds. The memories of the narrator's father, which may be considered the 'actions and passions' within the story, emerge before and after the occurrence of the storm. The storm is thus part of the background to which the events in "Ranvali" occur. Chatman also gives three criteria for being a character - presence, being named and importance (Chatman 139). The storm in "Ranvali" is clearly not explicitly... ...der is given a definite analogy to how she might be feeling. Gunesekera's use of this narrative device as opposed to using the narrator to describe her emotions makes the reader sympathetic to the narrator's plight in an almost unconscious way. Although the storm is part of the setting, it subconsciously draws a connection to the narrator's inner thoughts. The reader thus can imagine that a storm rages within her mind, with thoughts about her father's idealism conflicting with her love for him. Without this narrative device of the storm, the story would have been much impoverished, as the final effect of "Ranvali" would have been much reduced. Works Cited Chatman, Seymour, "Existents" Story and Discourse: Narrative Structure in Fiction and Film. Ithaca: Cornell UP, 1978. 131-145. Gunesekera, Romesh. "Ranvali." Monkfish Moon. London: Granta, 1992: 89-102.

War from Myceneans to Rome Essay -- essays research papers

WAR FROM MYCENEANS TO ROME The modern day soldier did not arrive at the current level of training methods overnight. Throughout history warfare techniques and strategies have evolved from the earliest primitive battles to the latest technologies. The only way to learn about war is to study the past engagements and lessons learned. There are nine principles of war as follows: Objective, Offensive, Mass, Economy of force, Maneuver, Unity of command, Security, Surprise, and Simplicity. These are the areas of study in order to gain a better understanding of what to do and what to avoid during any engagement.   Ã‚  Ã‚  Ã‚  Ã‚  The battles from yesterday differ from those in recent years and today, because the more primitive cultures fought under their leader for food, territory, or the domination of another group. Today’s motives are based more on economic, political, or social reasons regarded as appropriate by a group of individuals instead of the thoughts or intentions of one man.   Ã‚  Ã‚  Ã‚  Ã‚  Mainland Greece is the first study of warfare in the selected readings and by 1600 B.C. a civilization emerged from the Hellas culture and the Minoan culture. This group, known as the Myceneans, fought using chariots and armor made of bronze. By the eighth century B.C., the Myceneans art of war consisted of the phalanx. The phalanx was a solid rectangle of infantrymen carrying armor and spears eight deep. When an army approached another army the phalanxes of both sides would come head to head. The soldiers, who were normally citizens not professional soldiers, would find themselves in the midst of blood and sweat pouring out of the bodies surrounding them from the hand to hand combat. The only way of victory was to hold the lines strong and fight until the other side fled. The problems with this type of formation was that there was no overall leadership within the phalanx, no reserve was established to outflank the opposing army, and there was no way to pursue the fleeing enemy, left them capable to heal and fight another day.   Ã‚  Ã‚  Ã‚  Ã‚  The technique of phalanx had not changed for some time and the Greek warfare stayed the same due to no major opposition force that used different techniques against Greek system. The phalanx was also used because it was a proven technique that had been tested and used successfully. O... ...diterranean, Hannibal moved to engage the Romans and almost conquered them at Cannae (216 B.C.), where the largest Roman army was surrounded, enveloped, and destroyed. The Romans needed military leadership to outwit Hannibal and found it in Scipio.   Ã‚  Ã‚  Ã‚  Ã‚  Scipio made the maniples stronger than ever and increased the amount of horsemen in the cavalry to solve the problem that the Romans had against Hannibal. By using adapting techniques to envelop and control sea power, Scipio was able to defeat Carthage at the battle of Zama (202 B.C.), and thus the Romans were ready to expand their empire from Spain to Asia Minor and from Britain to Northern Africa.   Ã‚  Ã‚  Ã‚  Ã‚   SOURCES USED Preston, Richard A., Alex Roland, and Sydney F. Wise. Men In Arms: A History of Warfare and its interrelationships With Western Society. (Belmont, California:Wadsworth/ Thomson Learning, 2001). Chap 1-3 Warry, John. Warfare in the Classical World: An Illustrated Encyclopedia of Weapons, Warriors, and Warfare in the Ancient Civilisations of Greece and Rome. (Norman, Oklahoma:University of Oklahoma Press, 1995). Chap 1-13

Contrastive Analysis

Advanced writing April 24, 2009 Contrastive analysis: Prosperity, Decline and new Hopes of revival It should be mentioned that the history of foreign language teaching is so complicated. The complexities are the outcome of the rise of the assumptions of so many theories, approaches, methods and hypotheses that dominated this field , especially beginning from1940s and up till now. Today there are innumerable assumptions for approaches and methods that relate to language learning and teaching. All of them claim to be the right approach for learning and teaching a language. In the midst of these situations, foreign language teachers find it extremely difficult to decide upon an approach, a method or a hypothesis to adopt, so as the process of teaching becomes easier to them and this, of course, would make the process of learning easier to the students in turn. The purpose of this short paper is to explain the assumptions behind what is called ‘ Contrastive Analysis Hypothesis' (CAH) to language teaching and learning. Examples from English and Arabic; English and German will be cited, and then demonstrate why contrastive analysis was rejected after decades of prosperity in which it dominated the area of foreign language teaching for almost 20 years. In this effect, According to Larsen-Freeman & Long (1991) in (Yoon,2002): this was a time when structural linguistics and behavioral psychology were rather dominant in the study of language learning. CA proponents came to advocate that L2 instructional aterials could be prepared more efficiently by comparing two languages and, in the process, predict learners' behaviors and difficulties(qtd. in Dina 2). Contrastive Analysis(CA) was developed by Charles Fries, and was more explained and clarified by Robert Lado. CA is based on the assumptions that the majority of the errors done by non-native learners, throughout their course of studying a language, are related to the interference of the students' native language. That's why there is difficulty in learn ing a language. The learners native language's habits do not easily allow for the development of any new habits for another different language to take place. Lado and Fries believe that: Individuals tend to transfer the forms and meanings and the distribution of forms and meanings of their native language and culture to the foreign language and culture- both productively when attempting to speak the language and act in the culture and receptively when attempting to grasp and understand the language and the culture as practiced by natives. (qtd. in Abbas 2) The process of learning is more difficult when there are differences between the systems of these two languages and more easy when there are similarities. If it is true that most of the difficulties encountered by the students, in their path of learning a foreign language, result from the differences rather than the similarities between the L1 and L2, then the students errors could be predicted, and hence can be encountered by teaching materials that focuses mainly on the differences rather than the similarities between the L1 and L2. CA is classified into two forms. A strong form and A weak form. The strong form predicts that most of L2 errors are due to negative transfer resulting from the differences between the L1 and L2. The weak form explains errors once they are made with out making prediction. To give an example about the possible difficulties that learners may encounter in learning a foreign language, lets consider some language features of two completely different languages such as English and Arabic. English belongs to the Indo-European family of languages, and Arabic belongs to the Semitic languages, therefore, they are almost quite different from one another in many ways. If a comparison is drawn between English and Arabic in terms of phonology, contrastive analysis predicts that, because Arabic language lacks speech sounds such as / p /, /v / , / n / and / /, Arabic learners of English will encounter problems in the correct pronunciation of words containing the above mentioned speech sounds. For example, the voiceless / p / will be replaced by the voiced / b / , / v / by / f / , / n / by / g / and / / by / d /. Another difficulty Arab learners of English encounter lies in the area of prepositions. There are more prepositions in English than in Arabic On top of that, they are used differently in the two languages, and therefore a lot of confusion occur when an Arab learner uses the prepositions inappropriately. For example, in an answer to the question, How long have you been living in Colorado, it is more likely that an Arab learner of English answers saying ‘ I have been living in Colorado since 10 years. Hence, Instead of using ‘for', s/he uses ‘since'. It was argued that: English prepositions can be used with different parts of speech of the same root word. We use one preposition with the verb form, another with the adjective and still another with the noun form of the word. For example, we are fond of something, but we have fondness for it. In English, prepositions are either simple, single words, or complex consisting of more than one word(Rami and Hanna 184). Similarly, English learners trying to learn Arabic find it extremely difficult to correctly pronounce some Arabic speech sounds such as / / , / /, / / and / /, simply because these speech sounds do not exist in English at all. On the other hand, the areas of similarities, between the two languages, lead to ease in learning. Schuster argues that: English learners of German or German learners of English are destined to have a positive transfer because the two languages do have many similarities. On the other hand, theory stipulates that learning will be quite difficult, or even unsuccessful, when the two languages are different(qtd. in Dena 3) Decline of Contrastive analysis Contrastive analysis started losing interest and was gradually abandoned and replaced by other new assumptions and views about language learning and teaching. Many scholars started to direct sound criticisms to contrastive analysis, beginning from the mid of 1960s. For example, it is argued that most of the language learners do not result from interference, but rather they could be developmental. In this effect, Dulay and Burt mention that: report on a number of studies of the errors made by children learning English as a second language and consider that they are similar to those made by children learning English natively . The greatest number(87 percent) they considered to be developmental, that is, like those that a native language learner makes(qtd. in Spolsky 253). Other researchers arrived to the same conclusion. For example, it was pointed out that: Similarly, Baily, Madden, and Krashen(1974)report results similar to Dulay and Burt's showing some similarity in order of acquisition between adults and children learning English as a second language that is still different from the order in first language acquisition(Spolsky 254). Contrastive analysis was also challenged by the views of the prominent linguist, Noam Chomsky believes of â€Å"the existence of language acquisition device(LAD) in order to construct a generative grammar of linguistic competence out of language samples one encounters†(Neda 3). Furthermore, Slinker believes that Corder contributions were very important. He argues that, Corder points out that,†The errors of a learner, whether adult or child are (a) not ‘negative' or ‘interfering' in any way with learning a TL but are, on the contrary, a necessary positive factor, indicative of testing hypothesis†(qtd. in Abbas 2). Similarly, Dulay, Burt and Krashen argue that, â€Å"learners first language are no longer believed to interfere with their attempts to acquire a second language grammar, and language teachers no longer need to create special grammar lessons for students from each language background†(qtd. in Abbas 2). New hopes of revival Many of proponents to contrastive analysis believe that the criticisms directed to it, are not reliable. For example, it was noted that: The Dulay et al postion is suspect for two reasons. First, it ignores other findings with appreciably higher estimates. Hocking(1973), Cornu(1973)†¦ James(1981) †¦ arrived at findings which estimate the magnitude of such errors(interfernce) in the 60% range†¦. Second, a substantial number of studies involved in the body of method comparison research of the 60% and 70's had demonstrated the effectiveness of teaching methods using CA input(Abbas2 ). Although contrastive analysis was gradually abandoned in favor of new views and assumptions, there are still many scholars who consider it of a great importance. For example, it was pointed out that: it(CA)will be useful to teachers, students, and linguists. Even though contrastive analysis has lived up to its promise of explaining the nature of the language learning process and of making it possible to develop error-free learning, it has played a useful role in encouraging the kind of language descriptions that are needed by language teachers and learners(Abbas 253). It is also argued that contrastive analysis was disfavored not due to sound critisisms that relate to its validity only, but also for financial and economic factors. It is believed that: The text books for FSLT†¦ are all solely in English, making no refrence to the L1 of the learners. This, of course, suits publishers for it enables them to sell the same text books all over the world thus increasing thier sales many fold. It also suits the many anglophone teachers of English as a second or foreign language for it enables them to teach anywhere in the world without knowing the L1's of the students they teach(Abbas 2). Consequently, many supporters to contrastive analysis still have hopes that one day contrastive analysis will be able to find its way back to the foreign language text books. Works Cited Abbas, Elbadri. â€Å"The Relevance of Error Analysis and Contrastive Analysis. † Http://www. teaching. org. uk /blogs/badri/relevance-error-analsis. 28 Feb. 2009. Teachingenglish. org. 22 Apr. 2009 . Al-Sibia, Dina M. â€Å"The decline of contrastive analysis – Search. † Google. 26 Oct. 2004. 03 May 2009 . Al-Sibai M, Dina. The Decline of Contrastive Analysis Pedagogy. English 523.

Art: in the beginning Essay

Art, can you imagine a time when art wasn’t part of your culture, it has been around for an extremely long time even the crave man did it. Imagine how much it has evolved since then. In order to understand the technical aspect of art, how the art was created and understanding it’s meaning, you have to travel back in time to Egyptian art. Egypt started it all which influenced the Greeks’ art. Both Egyptian and Greek art has played a heavy influence on today’s Western traditional art. However, they both went about the creation of Art differently. They had different focal points, technics and humanistic views. Embark through the journey of learning about Ancient Art and maybe your view point on art will change. Egyptian imagery was governed by sociality ideals of what was suitable at the time which is why their art seem to appear extremely stagnant. This was done on purpose because they believe that it made their art seem stable with a deific balance. Egyptian art had a common theme, its focused on gods/goddess, royalty and the afterlife. Their art conveyed an idealized version of a person. However, Greeks art may have been influenced by Egyptian art but they took a more of a realistic focal point of art. When creating a human they created a realistic idea of a person by emphasizing facial expressions, stance and their location upon the canvas. When the Egyptian may have focus more on the afterlife then the living the Greeks was the opposite, they value life. Their differences doesn’t stop there it is evident that they both used different approaches to create art. Every artist has a median they used to create the picture they have in their mind. For instance the Egyptian used stone, wood and metal to create sculptures. This limited the amount of movement and actions of the depicted image. Egyptian artists embraced the two-dimensional surfaces. This is where registers where created, separate the scene in the picture as well as a floor for the figure to appear to be standing on. Without the registers the scene would appear chaotic; battles and hunting scenes. The Greeks enjoy creating art on pottery, the Black-and-Red (later there was also Red-and-Black) techniques were used to decorate fine pottery. The formation and creation of the pot in its self is art with the added effect of the  picture that was place on the surface. This form of art had a practical used to it because the specific shapes signified its daily usage, storing and transportation of wine and amphora (food things). Both Egypt and Greece biggest influence of art was based on the human world. They saw the beauty of the naked body but they still have different ideals. The Egypt highlighted the physic of a person rather the humanistic characteristic. While the Greek figure is in the nude, athletic built, youthful, and has a perfect proportion. Egyptians mainly focus on the complete picture. If you standing in an ancient art museum will you be able to identify the Egyptian art and the Greek art by now looking at the technical aspect? They may have a big influence on today’s world of art. The creation of pottery is relatively the same just more efficient with modern technology. The center point of art may have shifted but they all universally go back to the same theme, human life. The technical aspect of art varies now a days and humans remain the focal point of art. Both Egyptians and Greeks approach of art was great, both embodies today’s art.