100 Words Toward Science Literacy
The American Heritage®
Student Science Dictionary
by Editors of the American Heritage Dictionaries
M-R
magnetic field The area around a magnet in which its magnetism can affect other objects. See
Note at magnetism.
A measure of the amount of matter contained in a physical body. Mass is
mass
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.
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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
impulses from one nerve cell to another.
A chemical substance that helps transmit nerve
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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.
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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
The pressure exerted by the molecules of
solutions are equal on both sides of the membrane.
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.
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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.
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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.
Any of a wide variety of one-celled organisms that lack a distinct
pro·kar·y·ote
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
Another term for
phenomena as the photoelectric effect and the uncertainty principle.
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.
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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.
1. The bending or turning of a wave, such as a light or sound wave,
re·frac·tion
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.
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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
Transfer RNA is
in the cytoplasm (known as ribosomes) where protein synthesis takes place.
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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