A Dictionary of Science

ample, green plants (which obtain their energy directly from sunlight) are the primary *producers; herbivores are primary *consumers (and secondary producers). A carnivore that eats only herbivores is a secondary consumer and a tertiary producer. Many animals feed at several different trophic levels.

tropical cyclone A *cyclone that develops over tropical or subtropical waters, in which sea temperatures are above 27°C and at least 5° of latitude away from the equator. The term encompasses tropical depression, with wind speeds of 33 knots (62 km/h) or less, and tropical storm, with wind speeds of 34–63 knots (63–117 km/h). A tropical cyclone with wind speeds of over 64 knots (117 km/h) is known as a *hurricane where it occurs in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and the east and central North PaciÜc Ocean (east of the dateline); as a typhoon where it occurs in the North PaciÜc Ocean; and as a severe tropical cyclone where it occurs in the southwest PaciÜc Ocean (west of 160°E) and southeast Indian Ocean (east of 90°E).

tropical year See year.

tropism The directional growth of a plant organ in response to an external stimulus, such as light, touch, or gravity. Growth towards the stimulus is a positive tropism; growth away from the stimulus is a negative tropism. See also geotropism;

hydrotropism; orthotropism; phototropism; plagiotropism; thigmotropism. Compare nastic movements; taxis.

tropomyosin A protein found in the

t*actin Ülaments in muscles. The molecule consists of two elongated strands that run along the length of the Ülament. When the muscle is at rest, the tropomyosin molecule covers the region of the actin molecule where interaction with myosin occurs. On contraction of the muscle, the tropomyosin is displaced by another protein, *troponin, allowing the interaction of actin with myosin.

troponin A complex of three polypeptide chains that are found at regular

intervals along the length of an *actin Ülament. During muscle contraction, troponin binds to calcium ions, displacing

*tropomyosin and exposing the active site on the actin Ülament. This allows the interaction of actin and myosin to occur.

troposphere See earth’s atmosphere.

tropylium ion The positive ion C7H7+, having a ring of seven carbon atoms. The ion is symmetrical and has characteristic properties of *aromatic compounds.

truth table A table that summarizes all possible outcomes of a logical operation. For example, for an AND *gate with inputs A and B and output C, the truth table is

This indicates that the output will be 0 unless both inputs are 1.

trypsin An enzyme that digests proteins

(see endopeptidase; protease). It is secreted in an inactive form (trypsinogen) by the pancreas into the duodenum. There, trypsinogen is acted on by an enzyme (enterokinase) produced in the duodenum to yield trypsin. The active enzyme plays an important role in the digestion of proteins in the anterior portion of the small intestine. It also activates other proteases in the pancreatic juice (see car-

boxypeptidase; chymotrypsin).

trypsinogen See trypsin.

tryptophan See amino acid.

TSH See thyroid-stimulating hormone.

tsunami A large sea wave usually generated by a submarine earthquake or volcanic eruption. It may aso be caused by a mass underwater mudslide. The waves, which can be over 10 m high, spread in concentric circles from the focus of the earthquake, often travelling hundreds of kilometres and reaching speeds of 700 km/h. A tsunami can be extremely destructive when it breaks on the shore. A devasting tsunami followed a powerful earthquake of magnitude 9.0 (see richter scale) off the west coast of N Sumatra, Indonesia, on 26 December 2004. It swept over land causing widespread destruction in Indonesia, Sri Lanka, Thailand, Malaysia, Myanmar, S India, and the Maldives,

with lesser effects as faraway as E Africa, and resulted in over 283 100 deaths.

T Tauri star An unstable young variable star in its pre-main sequence phase (see hertzsprung–russell diagram). The instability, brought about by the beginning of nuclear fusion in the core of the star, causes pulsations and stellar winds, possibly with *bipolar outÛows. Groups of such stars, often associated with *Herbig–Haro objects, are called T Tauri associations.

tuber A swollen underground stem or root in certain plants. It enables the plant to survive the winter or dry season and is also a means of propagation. A stem tuber, such as the potato, forms at the end of an underground stem. Each tuber represents several nodes and internodes. The following season several new plants develop from the terminal and axillary buds (eyes). Root tubers, such as those of the dahlia, are modiÜed food-storing adventitious roots and may also give rise to new plants.

tubulin A protein of which the *microtubules of cells are formed.

Tullgren funnel A device used to remove and collect small animals, such as insects, from a sample of soil or leaf litter. The sample is placed on a coarse sieve Üxed across the wide end of a funnel and a 100-watt light bulb, in a metal reÛector, is placed about 25 cm above the funnel. The heat from the bulb dries and warms the sample, causing the animals to move downwards and fall through the sieve into the funnel, which directs them into a collecting dish or tube below. The dish can contain water or alcohol to prevent the animals from escaping.

tumour See neoplasm.

tundra A terrestrial *biome characterized by a lack of trees and a permanently frozen subsoil. Tundra lies to the north of the *taiga in North America and Eurasia; the vegetation is dominated by grasses, sedges, lichens, mosses, heathers, and low shrubs. The growing season, which occurs during the warmest part of the year when the average daily mean temperature is about 10°C, lasts only 2–4 months, during which the topsoil thaws to a depth of

30 cm, allowing roots to penetrate it.

However, below this level the soil is permanently frozen (permafrost); water cannot Ülter through the soil and may lie in surface depressions during the growing season. Compare taiga.

tuneable laser See dye laser.

tungsten Symbol W. A white or grey metallic *transition element (formerly called wolfram); a.n. 74; r.a.m. 183.85; r.d. 19.3; m.p. 3410°C; b.p. 5660°C. It is found in a number of ores, including the oxides wolframite, (Fe,Mn)WO4, and scheelite, CaWO4. The ore is heated with concentrated sodium hydroxide solution to form a soluble tungstate. The oxide WO3 is precipitated from this by adding acid, and is reduced to the metal using hydrogen. It is used in various alloys, especially highspeed steels (for cutting tools) and in lamp Ülaments. Tungsten forms a protective oxide in air and can be oxidized at high temperature. It does not dissolve in dilute acids. It forms compounds in which the oxidation state ranges from +2 to +6. The metal was Ürst isolated by Juan and Fausto d’Elhuyer in 1783.

tungsten carbide A black powder, WC, made by heating powdered tungsten metal with lamp black at 1600°C. It is extremely hard (9.5 on Mohs’ scale) and is used in dies and cutting tools. A ditungsten carbide, W2C, also exists.

tuning fork A metal two-pronged fork that when struck produces an almost pure tone of a predetermined frequency. It is used for tuning musical instruments and in experiments in acoustics.

nelling from one classically allowed region to another arises as a consequence of *quantum mechanics. The effect is made use of in the *tunnel diode. Alpha decay (see alpha particle) is an example of a tunnelling process.

Turbellaria A class of free-living Ûatworms (see platyhelminthes) comprising the planarians, which occur in wet soils, fresh water, and marine environments. Their undersurface is covered with cilia, used for gliding over stones and weeds. Planarians can also swim by means of undulations of the body.

turbine A machine in which a Ûuid is used to produce rotational motion. The most widely used turbines are the steam turbines and water turbines that provide some 95% of the world’s electric power (in the form of *turbogenerators) and the gas turbines that power all the world’s jetpropelled aircraft. In the impulse turbine a high-pressure low-velocity Ûuid is expanded through stationary nozzles, producing low-pressure high-velocity jets, which are directed onto the blades of a rotor. The rotor blades reduce the speed of the jets and thus convert some of the Ûuid’s kinetic energy into rotational kinetic energy of the rotor shaft. In the reaction turbine the discharge nozzles are themselves attached to the rotor. The acceleration of the Ûuid leaving the nozzles produces a force of reaction on the pipes, causing the rotor to move in the opposite direction to that of the Ûuid. (See illustrations.) Many turbines work on a combination of the impulse and reaction principles.

Principle of the turbine

turbogenerator A steam turbine driving an electric generator. This is the normal method of generating electricity in

power stations. In a conventional power station the steam is raised by burning a fossil fuel (coal, oil, or natural gas); in a nuclear power station the steam is raised by heat transfer from a nuclear reactor.

turbojet See jet propulsion.

turbulence A form of Ûuid Ûow in which the particles of the Ûuid move in a disordered manner in irregular paths, resulting in an exchange of momentum from one portion of a Ûuid to another. Turbulent Ûow takes over from *laminar Ûow when high values of the *Reynolds number are reached.

turgor The condition in a plant cell when its *vacuole is distended with water, pushing the protoplast against the cell wall. In this condition the force causing water to enter the cell by *osmosis is balanced by the hydrostatic pressure of the protoplast against the cell wall (see also water potential). Turgidity assists in maintaining the rigidity of plants; a decrease in turgidity leads to *wilting. Compare plasmolysis.

Turing, Alan Mathison (1912–54) British mathematician, who after studying at Cambridge University went to Princeton, where in 1937 he published his most important work on computable numbers, which contained a description of the hypothetical *Turing machine. He returned to Britain at the outbreak of World War II and worked on cracking German codes. This led to his involvement in the development of computers. He committed suicide after being convicted of indecency (as a homosexual).

Turing machine A hypothetical machine that determines whether or not a problem is computable. It has an inÜnite memory represented by an inÜnitely long ribbon of paper tape passing through the machine, which can be in several discrete internal states. The tape is divided into cells that can each hold one of a given number of symbols. The machine can move left or right along the tape, acting on one cell at a time. It is programmed by a set of instructions that make it change symbols, change state, and move one cell left or right (or remain at the same cell). If an operation can be performed by using

an algorithm (i.e. if it is computable), a Turing machine can do it. It was devised by Alan Turing.

turion 1. A winter bud, covered with scale leaves and mucilage, that is produced by certain aquatic plants, such as frogbit. Turions become detached and remain dormant on the pond or lake bottom during the winter before developing into new plants the following season.

2. See sucker.

Turner’s syndrome A genetic disorder of women caused by the absence of the second *sex chromosome (such women are XO, rather than the normal XX). It is characterized by a lack of ovaries and menstrual cycle. Affected women are sterile and lack secondary sexual characteristics, although the external genitalia are present. The syndrome is named after the US endocrinologist Henry Turner (1892– 1970), who Ürst described it.

turns ratio See transformer.

turpentine An oily liquid extracted from pine resin. It contains pinene, C10H16, and other terpenes and is mainly used as a solvent.

turquoise A mineral consisting of a hydrated phosphate of aluminium and copper, CuAl6(PO4)4(OH)8.4H2O, that is prized as a semiprecious stone. It crystallizes in the triclinic system and is generally blue in colour, the ‘robin’s egg’ blue variety being the most sought after. It usually occurs in veinlets and as masses and is formed by the action of surface waters on aluminium-rich rocks. The Ünest specimens are obtained from Iran.

T violation See time reversal.

tweeter A small loudspeaker capable of reproducing sounds of relatively high frequency, i.e. 5 kilohertz upwards. In highÜdelity equipment a tweeter is used in conjunction with a *woofer.

twinning A process in which two crystals of the same material form with orientations such that the two crystals are related to each other by a symmetry operation. This may be either reÛection in a plane (the twinning plane) or rotation

about an axis (the twinning axis). The plane or axis is common to the two crystals.

twins Two individuals born to the same mother at the same time. Twins can develop from the same egg (see identical twins) or from two separately fertilized eggs (see fraternal twins).

tympanic cavity See middle ear.

tympanum (tympanic membrane; eardrum) The membrane that separates the *outer ear from the *middle ear. It vibrates in response to sound waves and transmits these vibrations via the *ear ossicles of the middle ear to the site of hearing (the *cochlea of the *inner ear). In amphibians and some reptiles there is no external ear and the tympanum is exposed at the skin surface.

Tyndall effect The scattering of light as it passes through a medium containing small particles. If a polychromatic beam of light is passed through a medium containing particles with diameters less than about one-twentieth of the wavelength of the light, the scattered light appears blue. This accounts for the blue appearance of tobacco smoke. At higher particle diameters, the scattered light remains polychromatic. It is named after John Tyndall (1820–93). See also scattering of electromagnetic radiation.

type specimen The specimen used for naming and describing a *species or subspecies. If this is the original specimen collected by the author who named the species it is termed a holotype. The type

specimen is not necessarily the most char- t acteristic representative of the species.

The term type is also applied to any taxon selected as being representative of the rank to which it belongs. For example, the genus Solanum (potato) is said to be the type genus of the family Solanaceae.

typhoon A *tropical cyclone or *hurricane that occurs in the W and N PaciÜc Ocean and the South China Sea.

tyrosine See amino acid.

ubiquinone (coenzyme Q) Any of a group of related quinone-derived compounds that serve as electron carriers in the *electron transport chain reactions of cellular respiration. Ubiquinone molecules have side chains of different lengths in different types of organisms but function in similar ways.

ulna The larger of the two bones in the forearm of vertebrates (compare radius). It articulates with the outer carpals at the wrist and with the humerus at the elbow.

ultracentrifuge A high-speed centrifuge used to measure the rate of sedimentation of colloidal particles or to separate macromolecules, such as proteins or nucleic acids, from solutions. Ultracentrifuges are electrically driven and capable of speeds up to 60 000 rpm.

ultradian rhythm See biorhythm.

ultraÜltration The process in which hydrostatic pressure causes water and small dissolved molecules and ions to move across a membrane against a *concentration gradient. UltraÜltration is responsible for the formation of *tissue Ûuid and *glomerular Ültrate from blood. In both these processes the ultraÜltered Ûuid has the same composition as the plasma except that it does not contain blood cells or large protein molecules.

ultrahigh frequency (UHF) A radio frequency in the range 3 × 109– 0.3 × 109 Hz; i.e. having a wavelength in the range 10 cm to 1 m.

ultramicroscope A form of microscope that uses the *Tyndall effect to reveal the presence of particles that cannot be seen with a normal optical microscope. Colloidal particles, smoke particles, etc., are suspended in a liquid or gas in a cell with a black background and illuminated by an intense cone of light that enters the cell from the side and has its apex in the Üeld of view. The particles then produce

diffraction-ring systems, appearing as bright specks on the dark background.

ultramicrotome See microtome.

ultrasonics The study and use of pressure waves that have a frequency in excess of 20 000 Hz and are therefore inaudible to the human ear. Ultrasonic generators make use of the *piezoelectric effect, *ferroelectric materials, or *magnetostriction to act as transducers in converting electrical energy into mechanical energy. Ultrasonics are used in medicine for diagnosis, particularly in conditions such as pregnancy, in which X-rays could have a harmful effect, and for treatment, the vibratory effect being used to break up kidney stones, etc. Ultrasonic techniques are also used industrially to test for Ûaws in metals, to clean surfaces, to test the thickness of parts, and to form colloids.

ultrastructure The submicroscopic, almost molecular, structure of living cells, which is revealed by the use of an electron microscope.

ultraviolet microscope A *microscope that has quartz lenses and slides and uses *ultraviolet radiation as the illumination. The use of shorter wavelengths than the visible range enables the instrument to resolve smaller objects and to provide greater magniÜcation than the normal optical microscope. The Ünal image is either photographed or made visible by means of an *image converter.

ultraviolet radiation (UV) Electromagnetic radiation having wavelengths between that of violet light and long X-rays, i.e. between 400 nanometres and 4 nm. In the range 400–300 nm the radiation is known as the near ultraviolet. In the range 300–200 nm it is known as the far ultraviolet. Below 200 nm it is known as the extreme ultraviolet or the vacuum ultraviolet, as absorption by the oxygen in the air makes the use of evacuated ap-

paratus essential. The sun is a strong emitter of UV radiation but only the near UV reaches the surface of the earth as the *ozone layer of the atmosphere absorbs all wavelengths below 290 nm. Ultraviolet radiation is classiÜed in three ranges according to its effect on the skin. The ranges are:

UV-A (320–400 nm); UV-B (290–320 nm); UV-C (230–290 nm).

The longest-wavelength range, UV-A, is not harmful in normal doses and is used clinically in the treatment of certain skin complaints, such as psoriasis. It is also used to induce *vitamin D formation in patients that are allergic to vitamin D preparations. UV-B causes reddening of the skin followed by pigmentation (tanning). Excessive exposure can cause severe blistering. UV-C, with the shortest wavelengths, is particularly damaging. It is thought that short-wavelength ultraviolet radiation causes skin cancer and that the risk of contracting this has been increased by the depletion of the ozone layer.

Most UV radiation for practical use is produced by various types of *mercuryvapour lamps. Ordinary glass absorbs UV radiation and therefore lenses and prisms for use in the UV are made from quartz.

ultraviolet–visible spectroscopy

(UV–visible spectroscopy) A technique for chemical analysis and the determination of structure. It is based on the principle that electronic transitions in molecules occur in the visible and ultraviolet regions of the electromagnetic spectrum, and that a given transition occurs at a characteristic wavelength. The spectrometer has two sources, one of ultraviolet and the other of white visible light, which together cover the whole wavelength range of the instrument. If the whole wavelength range is used, the source is changed over at the appropriate point. The radiation from the source is split into two beams of equal intensity. One beam is passed through a dilute solution of the sample while the other is passed through the pure solvent and is used as a reference against which the Ürst is compared after transmittance. The cells used for the sam-

ple and reference solutions are usually made of silica and are matched.

umbel A type of *racemose inÛorescence in which stalked Ûowers arise from the same point on the Ûower axis, resembling the spokes of an umbrella. An involucre (cluster) of bracts may occur at the point where the stalks emerge. This arrangement is characteristic of the family Umbelliferae (Apiaceae; e.g. carrot, hogweed, parsley, parsnip), in which the inÛorescence is usually a compound umbel.

umbilical cord The cord that connects the embryo to the *placenta in mammals. It contains a vein and two arteries that carry blood between the embryo and placenta. It is severed after birth to free the newly born animal from the placenta, and shrivels to leave a scar, the navel, on the animal.

umbra See shadow.

uncertainty principle (Heisenberg uncertainty principle; principle of indeterminism) The principle that it is not possible to know with unlimited accuracy both the position and momentum of a particle. This principle, discovered in 1927 by Werner Heisenberg, is usually stated in the form: ∆x∆px ≥ h/4π, where ∆x is the uncertainty in the x-coordinate of the particle, ∆px is the uncertainty in the x- component of the particle’s momentum, and h is the *Planck constant. An explanation of the uncertainty is that in order to locate a particle exactly, an observer must be able to bounce off it a photon of radiation; this act of location itself alters the position of the particle in an unpredictable way. To locate the position accurately, photons of short wavelength

would have to be used. These would have u associated large momenta and cause a

large effect on the position. On the other hand, using long-wavelength photons would have less effect on the particle’s position, but would be less accurate because of the longer wavelength. The principle has had a profound effect on scientiÜc thought as it appears to upset the classical relationship between cause and effect at the atomic level.

underdamped See damping.

undernourishment See malnutrition.

undulipodium (pl. undulipodia) A slender Ûexible outgrowth of a eukaryote cell used for locomotion or propelling Ûuids over the surface of the cell. The term ‘undulipodium’ is used to designate a eukaryotic ‘Ûagellum’ or a *cilium (which have the same structure), to emphasize the distinction between these structures and the *Ûagellum of a bacterium. Many protoctists and sperm cells swim by means of undulipodia, and various organisms use them to establish feeding currents, or to clear debris from epithelial surfaces. All undulipodia have a shaft, about 0.25 µm in diameter, consisting of a longitudinal array of *microtubules, the axoneme, which is surrounded by an extension of the cell’s plasma membrane. The axoneme has two single microtubules running down the middle surrounded by nine pairs of microtubules. At its base the axoneme connects with a basal body (or kinetosome), which organizes assembly of the axoneme microtubules. Cilia are shorter than Ûagella and move by a whiplike power stroke followed by a recovery stroke in the opposite direction. Flagella generate successive waves that pass from the base to the tip. In both cases, Ûexing of the shaft is produced by a sliding motion of the microtubule pairs relative to each other. This involves the successive formation and breakage of molecular bridges between adjacent pairs. The bridges are composed of a protein, dynein, and their formation requires energy in the form of ATP.

ungulate A herbivorous mammal with hoofed feet (see unguligrade). Ungulates are grouped into two orders: *Artiodac-

utyla and *Perissodactyla.

unguligrade Describing the gait of ungulates (e.g. horses and cows), in which only the tips of the digits (i.e. the hooves) are on the ground and the rest of the foot is off the ground. Compare digitigrade; plantigrade.

uniaxial crystal A double-refracting crystal (see double refraction) having only one *optic axis.

unicellular Describing tissues, organs, or organisms consisting of a single cell.

For example, the reproductive organs of some algae and fungi are unicellular. Unicellular organisms include bacteria, protozoans, and certain algae. Compare acellular; multicellular.

uniÜed-Üeld theory A comprehensive theory that would relate the electromagnetic, gravitational, strong, and weak interactions (see fundamental interactions) in one set of equations. In its original context the expression referred only to the uniÜcation of general *relativity and classical electromagnetic theory. No such theory has yet been found but some progress has been made in the uniÜcation of the electromagnetic and weak interactions (see electroweak theory).

Einstein attempted to derive *quantum mechanics from uniÜed-Üeld theory, but it is now thought that any uniÜed-Üeld theory has to start with quantum mechanics. Attempts to construct uniÜed-Üeld theories, such as *supergravity and *Kaluza–Klein theory, have run into great difÜculties. At the present time it is not clear whether the framework of relativistic *quantum Üeld theory is adequate to give a uniÜed theory for all the known fundamental interactions and elementary particles, or whether one has to go to extended objects, such as superstrings or supermembranes. UniÜed-Üeld theories and other fundamental theories, such as *superstring theory and *supermembrane theory, are of great importance in understanding cosmology, particularly the *early universe. In turn cosmology puts constraints on uniÜed-Üeld theories. See also grand unified theory.

unimolecular reaction A chemical reaction or step involving only one molecule. An example is the decomposition of dinitrogen tetroxide:

N2O4 → 2NO2

Molecules colliding with other molecules acquire sufÜcient activation energy to react, and the activated complex only involves the atoms of a single molecule.

union See sets.

unisexual Describing animals or plants with either male or female reproductive organs but not both. Most of the more ad-

vanced animals are unisexual but plants are often *hermaphrodite. Flowers that contain either stamens or carpels but not both are also described as unisexual. See

also monoecious; dioecious.

unit A speciÜed measure of a physical quantity, such as length, mass, time, etc., speciÜed multiples of which are used to express magnitudes of that physical quantity. For many scientiÜc purposes previous systems of units have now been replaced by *SI units.

unit cell The group of particles (atoms, ions, or molecules) in a crystal that is repeated in three dimensions in the *crystal lattice. See also crystal system.

unit magnetic pole See magnetic poles.

unit vector A vector that has the magnitude 1. If a is any non-zero vector the unit vector in the direction of a is given by a/|a| and is denoted â.

univalent (monovalent) Having a valency of one.

universal constants See fundamental constants.

universal indicator A mixture of acid–base *indicators that changes colour (e.g. red-yellow-orange-green-blue) over a range of pH.

universality See phase transition.

universal motor See electric motor.

universe All the matter, energy, and space that exists. See cosmology; early

universe; heat death of the universe.

UNIX A general-purpose *computer operating system that allows several users, at different terminals, to use the machine at the same time. It was developed in 1969 and became generally available in 1971.

unnil- See transactinide elements.

Unruh effect The phenomenon, predicted in 1976 by William Unruh, that an accelerating body would seem to be surrounded by particles at a non-zero temperature, which is proportional to the acceleration. The vacuum state of a nonaccelerating observer is different to that of an accelerating observer because of distortion of the zero-point Ûuctuations.

There is Unruh radiation associated with this effect. The effect itself is very small and has not been veriÜed experimentally.

unsaturated 1. (of a compound) Having double or triple bonds in its molecules. Unsaturated compounds can undergo addition reactions as well as substitution. Compare saturated. 2. (of a solution) See saturated.

unstable equilibrium See equilibrium.

upper atmosphere The upper part of the *earth’s atmosphere above about

30 km. This is the part of the atmosphere that cannot be reached by balloons.

noids; a.n. 92; r.a.m. 238.03; r.d. 19.05 (20°C); m.p. 1132±1°C; b.p. 3818°C. It occurs as *uraninite, from which the metal is extracted by an ion-exchange process. Three isotopes are found in nature: ura- nium–238 (99.28%), uranium–235 (0.71%), and uranium–234 (0.006%). As uranium– 235 undergoes *nuclear Üssion with slow neutrons it is the fuel used in *nuclear reactors and *nuclear weapons; uranium has therefore assumed enormous technical and political importance since their invention. It was discovered by Martin Klaproth (1743–1817) in 1789.

uranium(VI) Ûuoride (uranium hexa-

Ûuoride) A volatile white solid, UF6; r.d. 4.68; m.p. 64.5°C. It is used in the separation of uranium isotopes by gas diffusion.

uranium hexaÛuoride See uranium(vi) fluoride.

uranium–lead dating A group of *dating techniques for certain rocks that depends on the decay of the radioisotopes uranium–238 to lead–206 (half-life 4.5 × 109 years) or the decay of uranium–235 to lead–207 (half-life 7.1 × 108 years). One form of uranium–lead dating depends on measuring the ratio of the amount of helium trapped in the rock to the amount of uranium present (since the decay 238U → 206Pb releases eight alpha particles). Another method of calculating the age of the rocks is to measure the ratio of radiogenic lead (206Pb, 207Pb, and 208Pb) present to nonradiogenic lead (204Pb). These methods give reliable results for ages of the order 107–109 years.

uranium(IV) oxide A black solid, UO2; r.d. 10.96; m.p. 2500°C. It occurs naturally as *uraninite and is used in nuclear reactors.

uuranium series See radioactive series.

Uranus A planet having its orbit be-

tween Saturn and Neptune. Its mean distance from the sun is 2870.97 × 106 km and its mean diameter is 51 118 km; it has

a *sidereal period of 83.75 years. The equator of Uranus is tilted at 98° with respect to its orbit, so that each pole is almost facing the sun during part of each orbit. The temperature of Uranus is very low, about 35 K, and its atmosphere is thought to contain methane and hydro-

gen. The planet itself is believed to have an ice mantle some 8000 km thick surrounding a rocky core. The planet has 27 known satellites and a system of about 20 rings, nine of which were discovered in 1977 with the rest being photographed in 1986 by the US Voyager 2 probe.

urea (carbamide) A white crystalline solid, CO(NH2)2; r.d. 1.3; m.p. 135°C. It is soluble in water but insoluble in certain organic solvents. Urea is the major end product of nitrogen excretion in mammals, being synthesized by the *urea cycle. Urea is synthesized industrially from ammonia and carbon dioxide for use in *urea–formaldehyde resins and pharmaceuticals, as a source of nonprotein nitrogen for ruminant livestock, and as a nitrogen fertilizer.

urea cycle (ornithine cycle) The series of biochemical reactions that converts ammonia, which is highly toxic, and carbon dioxide to the much less toxic *urea during the excretion of metabolic nitrogen. These reactions take place in the liver in mammals and, to a lesser extent, in some other animals. The urea is ultimately excreted in solution in *urine.

urea–formaldehyde resins Synthetic resins made by copolymerizing urea with formaldehyde (methanal). They are used as adhesives or thermosetting plastics.

ureter The duct in vertebrates that conveys urine from the *kidney to the *bladder.

urethane resins (polyurethanes) Synthetic resins containing the repeating group –NH–CO–O–. There are numerous types made by copolymerizing isocyanate esters with polyhydric alcohols. They have a variety of uses in plastics, paints, and solid foams.

urethra The duct in mammals that conveys urine from the *bladder to be discharged to the outside of the body. In males the urethra passes through the penis and is joined by the *vas deferens; it therefore also serves as a channel for sperm.

Urey, Harold Clayton (1894–1981) US physical chemist, who became a professor at the University of California in 1958. His

best-known work was the discovery of *deuterium (heavy hydrogen) in 1932, for which he was awarded the 1939 Nobel Prize for physics.

uric acid The end product of purine breakdown in most mammals, birds, terrestrial reptiles, and insects and also (except in mammals; see urea) the major form in which metabolic nitrogen is excreted. Being fairly insoluble, uric acid can be expelled in solid form, which conserves valuable water in arid environments. The accumulation of uric acid in the synovial Ûuid of joints causes gout.

uridine A nucleoside consisting of one uracil molecule linked to a d-ribose sugar molecule. The derived nucleotide uridine diphosphate (UDP) is important in carbohydrate metabolism.

urinary system The collection of organs and tissues that perform *osmoregulation and *excretion. The mammalian urinary system consists of two *kidneys each linked to the bladder by a ureter.

urine The aqueous Ûuid formed by the excretory organs of animals for the removal of metabolic waste products. In higher animals, urine is produced by the *kidneys, stored in the *bladder, and excreted through the *urethra or *cloaca. Apart from water, the major constituents of urine are one or more of the end products of nitrogen metabolism – ammonia, urea, uric acid, and creatinine. It may also contain various inorganic ions, the pigments urochrome and urobilin, amino acids, and purines. Precise composition depends on many factors, especially the habitat of a particular species: aquatic animals produce copious volumes; terrestrial animals need to conserve water and produce much less (about 1.0–1.5 litres per day in humans).

uriniferous tubule See nephron.

URL See world wide web.

USB drive In general, any storage device that can be attached to a computer through a special type of connection (universal serial bus connection). The term is particularly used for small portable storage devices typically sealed in plastic. Their physical size is reÛected in the vari-

ous names for this type of device – thumb drive, pen drive, keyring drive. They have capacities as high as 1 gigabyte and on most modern personal computers they can be recognized as an additional drive without the need for a special driver. Storage devices of this type are increasingly used for backup, data transfer, storage of photographs or MP3 Üles, etc.

uterus (womb) The organ of female mammals in which the embryo develops. Paired in most mammals but single in humans, it is situated between the bladder and rectum and is connected to the *fallopian tubes and to the *vagina. The lining (see endometrium) shows cyclical changes (see menstrual cycle; oestrous cycle) associated with egg production and provides a thick spongy layer in which the fertilized egg becomes embedded. The outer wall of the uterus is thick and muscular; by contracting, it forces the fully grown fetus through the vagina to the outside.

utriculus (utricle) A chamber of the *inner ear from which the *semicircular canals arise. It bears patches of sensory epithelium concerned with detecting changes in the direction and speed of movement (see macula).

UV See ultraviolet radiation.

UV–visible spectroscopy See ultravi- olet–visible spectroscopy.

vacancy See crystal defect.

vaccination See immunization.

vaccine A liquid preparation of treated disease-producing microorganisms or their products used to stimulate an *immune response in the body and so confer resistance to the disease (see immunization). Vaccines are administered orally or by injection (inoculation). They take the

form of dead viruses or bacteria that can v still act as antigens, live but weakened microorganisms (see attenuation), spe-

cially treated *toxins, or antigenic extracts of the microorganism.

vacuole A space within the cytoplasm of a living *cell that is Ülled with air, water or other liquid, sap, or food particles. In plant cells there is usually one large vacuole bounded by a single-layered mem-