polymerization 'ypo-.li-mo-ro-'za-shonN Any process in which mono¬ mers combine chemically to produce a polymer. The monomer molecules— which in the polymer usually number from at least 100 to many thousands—may or may not all be the same. In nature, enzymes carry out polymerization under ordinary conditions to form proteins, nucleic acids, and carbohydrate polymers; in industry, the reaction is usually done with a catalyst, often under high pressure or heat. In addition polymerization, monomers are added successively to the reactive ends of a growing poly¬ mer molecule, similar to adding links to a chain; during the reactions, no by-products are formed. In condensation polymerization, growth of the polymer advances stepwise—monomers having reactive functional groups combine into larger molecules with their own functional groups; each reaction splits off a small molecule, often water, as a by-product.
polymorphism Discontinuous genetic variation that results in the occurrence of several different forms or types of individuals among the members of a single species. The most obvious example of polymorphism is the separation of most higher organisms into male and female sexes. Another example is the different blood types in humans. A polymorphism that persists over many generations is usually maintained because no one form has an overall advantage or disadvantage over the others in terms of natural selection. Some polymorphisms have no visible manifestations. The castes that occur in social insects are a special form of polymorphism that results from differences in nutrition rather than from genetic varia¬ tion.
Polynesia ^pa-lo-'ne-zhsN Island group, scattered across a huge trian¬ gular area of the east-central Pacific Ocean. A subdivision of Oceania, Polynesia includes New Zealand, Hawaii, Samoa, the Line Islands, French Polynesia, the Cook Islands, the Phoenix Islands, Tuvalu, Tonga, and Easter Island. Fiji is sometimes included because of its Polynesian population. The islands are mostly small coral atolls; some are of volcanic origin. Most of the inhabitants are Polynesians, some of whom might be related to the Malay. Their languages belong to a subfamily of the Austronesian languages. Contact with European culture began in the late 1700s with the arrival of Spanish explorers and radically altered life in Polynesia. Colonizers, imposing Western belief systems and cultural ways, effec¬ tively wiped out local traditions and customs. Samoa and Tonga retain more of the traditional culture than the other islands.
polynomial In algebra, an expression consisting of numbers and vari¬ ables grouped according to certain patterns. Specifically, polynomials are sums of monomials of the form ay! 1 , where a (the coefficient) can be any real number and n (the degree) must be a whole number. A polynomial’s degree is that of its monomial of highest degree. Like whole numbers, polynomials may be prime or factorable into products of primes. They may contain any number of variables, provided that the power of each variable is a nonnegative integer. They are the basis of algebraic equation solving. Setting a polynomial equal to zero results in a polynomial equa¬ tion; equating it to a variable results in a polynomial function, a particu¬ larly useful tool in modeling physical situations. Polynomial equations and functions can be analyzed completely by methods of algebra and cal¬ culus. See also orthoganal polynomial.
polynomial, orthogonal See orthoganal polynomial
polyoma V.pa-le-'o-moX virus Minute infectious agent normally present in extremely small amounts in wild mice without causing obvi¬ ous ill effects. It may induce cancerous tumours if grown in tissue cul¬ ture and injected in large quantities into newborn mice or young hamsters, guinea pigs, and rabbits. It belongs to the Papovaviridae family of viRUSes.
polyp \'pa-bp\ Growth projecting from the wall of a cavity lined with a mucous membrane. Shape varies widely; it may have a stalk or many lobes. Polyps most often occur in the nose, urinary bladder, and digestive tract, especially in the rectum and colon. Symptoms, if any, depend on location and size; they may result from pressure or from blockage of a passage. Polyps occasionally bleed. Because a small percentage are pre¬ cursors to cancers or actually contain cancers, it is advisable to have them removed and examined microscopically and to undergo routine colonos¬ copy after age 50.
polyp In zoology, one of two principal cnidarian body forms and, some¬ times, an individual in a bryozoan colony. The cnidarian polyp body is a hollow cylindrical structure. The lower end attaches to another body or surface. The upper, or free, end is directed upward and has a mouth sur¬ rounded by extensible tentacles that bear stinging structures called nem- atocysts. The tentacles capture prey, which is then drawn into the mouth. The polyp may be solitary (see sea anemone) or colonial (see coral). The body wall consists of three dermal layers. The other cnidarian body form is the MEDUSA.
Polyphemus V.pa-lo-'fe-mssV In Greek mythology, a Cyclops. He was the son of Poseidon and the nymph Thoosa. When Odysseus and his com¬ panions were cast ashore on the coast of Sicily, Polyphemus imprisoned them in his cave with the intention of eating them. Odysseus got the giant drunk and then blinded him by plunging a burning stake into his single eye. When Polyphemus opened his cave in the morning, Odysseus and the six men the giant had not yet devoured made their escape by clinging to the bellies of sheep let out to pasture.
polyphony See counterpoint
polyploidy See ploidy
polysaccharide V.pa-le-'sa-ko-.ridX Any of a large class of long-chain sugars composed of monosaccharides. Because the chains may be unbranched or branched and the monosaccharides may be of one, two, or occasionally more kinds, polysaccharides can be categorized in various ways. Cellulose, starch, glycogen, and dextran are all polysaccharides of glucose, with different configurations. Pectins are composed of a galac¬ tose derivative, chitin of a glucose derivative. Connective tissues, joint fluid, and cartilage contain two-component polysaccharides, including heparin. See also oligosaccharide.
polysiloxane See silicone
polytheism Belief in many gods. Though Judaism, Christianity, and Islam are monotheistic (see monotheism), most other religions throughout history have been polytheistic. The numerous gods may be dominated by a supreme god or by a small group of powerful gods. The gods originated as abstractions of the forces of nature such as the sky or the sea and of human and social functions such as love, war, marriage, or the arts. In many religions the sky god is powerful and all-knowing (e.g., Dievs), and the earth goddess is maternal and associated with fertility. Gods of death and the underworld (e.g., Osiris and Hel) are also important. In addition to many gods, polytheistic religions generally also include malevolent or benevolent spiritual forces or powers. See also god and goddess.
polyurethane Any of a class of very versatile polymers that are made into flexible and rigid foams, fibres, elastomers (elastic polymers), sur¬ face coatings, and adhesives. They are produced by reacting a diisocyan¬ ate (a compound with two functional groups of the type —NCO) with a diol (a compound with two hydroxyl, or —OH, groups). Foamed poly¬ urethanes, used for insulation and mattress and upholstery filler, are made with organic compounds containing carboxyl groups, causing a reaction that liberates carbon dioxide bubbles throughout the product. Spandex fibres are highly elastic and have replaced natural and synthetic rubber fibres for many textile purposes. Polyurethane elastomers are made into auto parts, rollers, flexible molds, medical equipment, and shoe soles. Polyurethane surface coatings are applied as sealants to wood, concrete, and machine parts and as linings for tanks and pipes; moisture-curing polyurethane resin is used as a general-purpose waterproof glue.