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The American HeritageŽ Student Science Dictionary

M - R

magnetic field The area around a magnet in which its magnetism can affect other objects. See Note at magnetism.

mass A measure of the amount of matter contained in a physical body. Mass is independent of gravity and is therefore different from weight. See Note at weight.
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mei·o·sis A type of cell division, occurring in two phases, that reduces the number of chromosomes in reproductive cells to half the original number. It results in the production of reproductive cells (called gametes) in animals and the formation of spores in plants, fungi, and most algae. The first phase of meiosis involves duplication and then separation of the chromosomes, followed by division into two daughter cells that each contain half the number of chromosomes as the original cell. In the second phase, each daughter cell divides to form an additional reproductive cell. See Note at mitosis.

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Men·del's law Any of the principles first proposed by Gregor Mendel to describe the inheritance of traits passed from one generation to the next. The first (also called the law of segregation) states that during the formation of reproductive cells (gametes), pairs of hereditary factors (genes) for a specific trait separate so that offspring receive one factor from each parent. The second (also called the law of independent assortment) states that chance determines which factor for a particular trait is inherited. The third (also called the law of dominance) states that one of the factors for a pair of inherited traits will be dominant and the other recessive, unless both factors are recessive. See more at inheritance.
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me·tab·o·lism The chemical processes by which cells produce the substances and energy needed to sustain life. In metabolism, organic compounds are broken down to provide heat and energy, while simpler molecules are used to build complex compounds like proteins for growth and repair of tissues. Many metabolic processes are brought about by the action of enzymes. —metabolic adjective
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met·a·mor·phism The process by which rocks are changed in composition, texture, or structure by extreme heat and pressure.
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met·a·mor·pho·sis Dramatic change in the form and often the habits of an animal during its development after birth or hatching. The transformation of a maggot into an adult fly, and of a tadpole into an adult frog, are examples of metamorphosis. The young of such animals are called larvae.
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mi·to·chon·dri·on Plural mitochondria. A structure in the cytoplasm of all cells except bacteria in which food molecules are broken down in the presence of oxygen and converted to energy in the form of ATP. Mitochondria contain their own DNA. See more at cell.

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mi·to·sis The process in cell division in which the nucleus divides to produce two new nuclei, each having the same number and type of chromosomes as the original. Early in mitosis, each chromosome duplicates itself to form two identical strands (called chromatids), which then line up along the center of the cell by attaching to the fibers of the cell spindle. The pairs of chromatids then separate, each strand of a pair moving to an opposite end of the cell. When a new membrane forms around each of the two groups of chromosomes, division of the nucleus is complete. The four main phases of mitosis are prophase, metaphase, anaphase, and telophase.


Usage
Mitosis and meiosis are easily confused, since both words refer to processes of cell division. Most cells have two full sets of chromosomes and are technically called diploid cells. When such a cell divides, it must first duplicate its chromosomes so as to produce two daughter cells that are also diploid. This type of cell division is called mitosis, and all somatic cells—that is, cells used for the maintenance, functioning, and growth of an organism—reproduce in this way. By contrast, reproductive cells, or gametes, are created by another kind of cell division, called meiosis. Meiosis also starts out by duplicating the chromosomes, but there are two divisions instead of one, with the result that four daughter cells are produced rather than two. Since the number of chromosomes is halved with each division, each daughter cell has just a single set of chromosomes and is called a haploid cell. During reproduction, the union of a female gamete with a male gamete restores the two full sets of chromosomes in a new organism.

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mole3 The amount of an element or substance that has a mass in grams numerically equal to the atomic or molecular weight of the substance. For example, carbon dioxide, CO2, has a molecular weight of 44; therefore, one mole of it weighs 44 grams. The number of atoms or molecules making up a mole is Avogadro's number.
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neu·ro·trans·mit·ter A chemical substance that helps transmit nerve impulses from one nerve cell to another.
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nit·ri·fy·ing bacteria Any of various soil bacteria that change ammonium compounds into nitrites or change nitrites into nitrates as part of the nitrogen cycle.
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no·ble gas Any of the six gases helium, neon, argon, krypton, xenon, and radon. Because the outermost electron shell of atoms of these gases is full, they do not react chemically with other substances except under certain special conditions. Also called inert gas. See Periodic Table.

Word History
What makes a noble gas so noble? The noble gases are a group of six chemical elements, normally occurring in gaseous form, that do not react chemically with other elements or chemical compounds except very rarely. Helium and neon are two familiar examples. The fact that the noble gases do not mix with other elements became the basis for a comparison with nobility. The nobility—the class that rules many societies and includes royalty—usually keep to themselves and do not mix with common folk.


nu·cle·us 1. Chemistry. The positively charged central region of an atom, composed of protons and neutrons and containing most of the mass of the atom. See more at atom. 2. Biology. The structure in the cytoplasm of a living cell that contains the cell's DNA and controls its metabolism, growth, and reproduction. A nucleus surrounded by a membrane is found in almost all the cells of eukaryotes and thus sets them apart from the cells of prokaryotes, such as bacteria, which do not contain nuclei. See more at cell. 3. Astronomy. The solid central part of a comet, composed of ice, frozen gases, and dust.
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Ohm's law A law stating that the current in an electric circuit is equal to the voltage divided by the resistance. The current increases as the voltage increases, but decreases as the resistance increases.
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os·mo·sis The movement of a solvent through a membrane separating two solutions of different concentrations. The solvent from the side of weaker concentration usually moves to the side of the stronger concentration, diluting it, until the concentrations of the solutions are equal on both sides of the membrane. The pressure exerted by the molecules of the solvent on the membrane they pass through is called osmotic pressure. Osmotic pressure is the energy driving osmosis and is important for living organisms because it allows water and nutrients dissolved in water to pass through cell membranes.

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oxidation-reduction A chemical reaction in which an atom or ion loses electrons to another atom or ion.
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o·zone A poisonous, blue form of oxygen that has three atoms per molecule rather than the usual two. It is produced by electricity passing through air, as in a lightning strike, and also by the sun's radiation reacting with ordinary oxygen or with the pollutants in smog. Ozone is used commercially in water purification, in air conditioning, and as a bleach.

Did You Know?
For the Earth's organisms, including people, ozone can be a lifesaver or a threat to health, depending on how high it is found in the atmosphere. The ozone that lingers in the lower atmosphere is a pollutant and contributes to respiratory diseases like asthma. But in the upper atmosphere, ozone protects us from the more severe forms of the sun's radiation. The region of the atmosphere in which ozone is most concentrated is known as the ozone layer, which lies from about 10 to 20 miles (16 to 32 kilometers) above the Earth. Because ozone absorbs certain wavelengths of harmful ultraviolet radiation, this layer acts as an important protection for life on the Earth. In recent years the ozone has thinned or disappeared in parts of the ozone layer, creating an ozone hole that lets in dangerous amounts of ultraviolet radiation. Ozone holes are created in part by the presence of certain industrial or commercial chemicals released into the atmosphere.

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pas·teur·i·za·tion A process in which an unfermented liquid, such as milk, or a partially fermented one, such as beer, is heated to a specific temperature for a certain amount of time in order to kill harmful germs or prevent further fermentation. During pasteurization, the liquid is not allowed to reach its boiling point so as to avoid changing its molecular structure.
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phlo·em A tissue in vascular plants that conducts food from the leaves to the other plant parts. Phloem consists primarily of tube-like cells that have porous openings. In mature woody plants it forms a sheath-like layer of tissue in the stem, just inside the bark. See more at cambium, photosynthesis. Compare xylem.
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pho·ton The smallest unit of light or other electromagnetic energy, having no mass and no electric charge. Photons behave both as particles and waves. See Note at electromagnetic radiation.
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pho·to·syn·the·sis The process by which green plants, algae, and certain forms of bacteria make carbohydrates from carbon dioxide and water in the presence of chlorophyll, using light as energy. Photosynthesis normally releases oxygen as a byproduct. Compare chemosynthesis. See Note at transpiration.
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plate tectonics In geology, a theory that the Earth's lithosphere (crust and upper mantle) is divided into a number of large, plate-like sections that move as distinct masses. See Notes at fault, Gondwanaland. See more at tectonic boundary.

Did You Know?
Have you ever noticed that the Earth's continents seem to fit together like pieces of a puzzle? This observation is what led the German meteorologist Alfred Wegener to propose the theory of continental drift in 1912. Since rocks and fossils were found to match up in parts of different continents, it seemed that they must have once been joined, but no one could explain how such large landmasses could move so far apart. This problem was not solved until the 1960s, when the theory of plate tectonics was proposed. According to this theory, the continents move apart by riding piggyback on plates—huge slabs of the Earth's lithosphere—that are much larger than the continents themselves. The plates move like parts of a conveyor belt powered by huge convection currents of molten rock that many geologists believe is heated by the decay of radioactive elements deep within the Earth. Although they only move a few inches per year, over hundreds of millions of years the continents are carried thousands of miles. Along their boundaries, the plates crumple, scrape, or pull apart from one another, giving rise to volcanoes and earthquakes and creating and destroying rock on the ever-changing surface of the planet.

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potential energy The energy possessed by a body as a result of its position or condition rather than its motion. A raised weight, coiled spring, or charged battery has potential energy. Compare kinetic energy.

pro·kar·y·ote Any of a wide variety of one-celled organisms that lack a distinct cell nucleus or other structures bound by a membrane and that have DNA that is not organized into chromosomes. Prokaryotes reproduce asexually, are the most primitive and ancient known forms of life, and include the bacteria and blue-green algae. Prokaryotes are grouped as a separate kingdom in taxonomy. Also called moneran. Compare eukaryote. See Table at Taxonomy.
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quantum theory A theory in physics based on the principle that matter and energy behave both as particles and as waves. According to this theory, matter and energy are composed of tiny units of electromagnetic energy called quanta. Quantum theory is used to explain such phenomena as the photoelectric effect and the uncertainty principle. Another term for quantum theory is quantum mechanics, which also refers specifically to the use of quantum theory in studying the structure and behavior of atoms and molecules.
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quark Any of a group of elementary particles supposed to be the fundamental units that combine in threes to make up protons and neutrons. See Note at subatomic particle.
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qua·sar An extremely distant, compact, star-like celestial object. The power output of a quasar is several thousand times that of the Milky Way galaxy.

Did You Know?
"The universe is not only stranger than we imagine," Albert Einstein said. "It is stranger than we can imagine." In the 1960s, astronomers found some very strange objects that we now call quasars in the far reaches of the universe. A quasar is like a far-off floodlight. It appears to be an extremely distant star putting out huge amounts of energy. In fact, just one of these objects can be a trillion times brighter than the sun. All of the radiation that a quasar gives off comes from a small area at its center, and many astronomers believe that the source of the energy is an enormous black hole rotating at the center of a young galaxy. Quasars are among the most distant celestial objects known. Some are more than ten billion light-years away, meaning their radiation has taken ten billion years to reach us. So when we look at quasars, we're observing these objects as they were billions of years ago, and we're able to see part of the early history of the universe.

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ra·di·a·tion 1a. Energy in the form of electromagnetic waves or streams of particles, such as photons or electrons. Radiation is given off by nuclear reactions (as in fission) and by radioactive decay. b. The emission or movement of such energy through space or a medium, such as air. See Notes at conduction, electromagnetic radiation. 2. The use of such energy, especially x-rays, in medical diagnosis and treatment.

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ra·di·o·ac·tive decay The spontaneous breakdown of a radioactive nucleus into a lighter nucleus. Radioactive decay causes the release of radiation in the form of alpha particles, beta particles, or gamma rays. The end result of radioactive decay is the creation of a stable atomic nucleus.

red shift An increase in the wavelength of radiation emitted by a receding celestial body as a result of the Doppler effect. Objects appear reddish because the longer wavelengths of light are at the red end of the visible spectrum. Compare blue shift. See Note at Doppler effect.

re·frac·tion 1. The bending or turning of a wave, such as a light or sound wave, when it passes from one medium to another medium of different density. See more at wave. Compare reflection. 2. The apparent change in position of a celestial body caused by the bending of light as it enters the Earth's atmosphere.


Usage
The words refraction and reflection describe two different ways that a light wave, sound wave, or other wave can move when it encounters a boundary between two media. The media can be two different substances, such as glass and air, or they can be regions of a single substance that are in different states, such as regions of air that are at different temperatures. Reflection occurs when a wave hits the boundary and returns immediately to its original medium. Refraction occurs when a wave passes from one medium to another and is bent; that is, the wave deviates from the straight-line path it would have otherwise followed. For example, light passing through a prism is bent when it enters the prism and again when it leaves the prism. The light is therefore refracted. Light striking a mirror bounces off the silver backing without entering it. The light is therefore reflected. The boundary between the media does not have to be abrupt for reflection or refraction to occur. On a hot day, the air over the surface of an asphalt road is warmer than the air above it. Because light travels at different speeds in these two regions, we see an image that shimmers because its light waves are refracted.

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rel·a·tiv·i·ty The two-part theory of physical laws developed by Albert Einstein. The first part, called the theory of special relativity, states that the laws of physics apply equally to any body or system of bodies having unchanging motion, and that the speed of light is always constant. The second part, the theory of general relativity, extends the first part to bodies in accelerated motion, such as bodies in gravitational fields. Among the many consequences of the theory are that measurements of speed and time depend on the motion of the observer, that mass and energy are equivalent, and that time and space form a continuum called space-time. See Notes at acceleration, Einstein, gravity, space-time.

Did You Know?
Developed as part of the theory of special relativity, Einstein's formula E=mc2 expresses the equivalence of energy and mass. Energy (E) equals mass (m) multiplied by the square of the speed of light (c). Since the speed of light is a large number (186,000 miles per second), the formula shows that even small amounts of mass contain enormous amounts of energy. A mass weighing one-thirtieth of a milligram, if converted into energy, would equal the heat and light put out by a 100-watt light bulb over an entire year! This energy is stored in the mass itself and in the energy that holds it together, such as the energy that keeps the protons and neutrons together in the atomic nucleus. Einstein's formula opened the way to the discovery of nuclear energy, the energy that is released when atomic nuclei break apart or fuse together.

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Rh factor An antigen present in red blood cells, used in the classification of human blood. The blood cells of most people contain an Rh factor. For a blood transfusion to be successful, the blood of the donor must match that of the recipient—both must have or must be missing the Rh factor.
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RNA Short for ribonucleic acid. The nucleic acid that determines protein synthesis in all living cells and the genetic makeup of many viruses. RNA consists of a single strand of nucleotides in a variety of lengths and shapes and is mainly produced in the cell nucleus. Messenger RNA is RNA that carries genetic information from the cell nucleus to the structures in the cytoplasm (known as ribosomes) where protein synthesis takes place. Transfer RNA is RNA that delivers the amino acids necessary for protein synthesis to the ribosomes. Compare DNA.

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Some of the words on this list include cross-references to entries in the dictionary that are not included in the list itself.




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