G8 fiffGlossary
intermediate fossil A fossil that displays physical characteristics
in between those of two known fossils in a family tree.interphase The longest stage of the cell cycle. Most cells spend
90 percent or more of their life span in interphase.intron A stretch of DNA that does not code for anything. Compare
exon.invariant trait A trait that is the same in all individuals of a
species. Compare variable trait.ion An atom that has lost or gained electrons and therefore is either
negatively or positively charged.ionic bond The chemical attraction between a negatively charged
ion and a positively charged ion. Compare covalent bond and
hydrogen bond.irregular fluctuation An unpredictable pattern of change in
population size. Compare cyclical fluctuation.isotonic Describing a fluid that has a solute concentration equal to
that of the cell it surrounds. Compare hypertonic and hypotonic.isotopes Two or more forms of an element that have the same
number of protons but different numbers of neutrons.IVF See in vitro fertilization.
J
J-shaped growth curve The type of graphical curve that rep re-
sents exponential growth. Compare S-shaped growth curve.K
karyotype A depiction showing all the chromosomes of a particu-
lar individual or species arranged in homologous pairs.keystone species A species that has a disproportionately large
effect on a community, relative to the species’ abundance.kingdom The second-highest hierarchical level in the organization
of life; the unit of classification in the Linnaean hierarchy above
phylum and below domain. The six kingdoms of life are Bacteria,
Archaea, Protista, Plantae, Fungi, and Animalia.Krebs cycle Also called citric acid cycle. The second of three stages
of cellular respiration. In this sequence of enzyme-driven reac-
tions, the pyruvate made in glycolysis is broken down, releasing
CO 2 and producing large amounts of energy carriers, including
ATP, NADH, and FADH 2. Compare glycolysis and oxidative
phosphorylation.L
law of independent assortment The law, proposed by Gregor
Mendel, stating (in modern terms) that when gametes form,
the two alleles of any given gene segregate during meiosis inde-
pendently of any two alleles of other genes. Compare law of segre-
gation.law of segregation The law, proposed by Gregor Mendel, stating
(in modern terms) that the two alleles of a gene are separated
during meiosis and end up in different gametes. Compare law of
independent assortment.lichen A mutualistic association between a photosynthetic microbe
(usually a green alga or cyanobacterium) and a fungus.light-independent reactions Also called the Calvin cycle or
carbon fixation. The second of two principal stages of photosyn-
thesis, in which a series of enzyme-catalyzed chemical reactions
converts carbon dioxide (CO 2 ) into sugar, using energy delivered
by ATP and electrons and hydrogen ions donated by NADPH.
Compare light reactions.light reactions The first of two principal stages of photosynthesis,
in which chlorophyll molecules absorb energy from sunlight and
use that energy for the splitting of water, which in turn produces
oxygen gas (O 2 ) as a by-product that is released into the atmo-
sphere. Compare light-independent reactions.lignin A strengthening substance that links together cellulose fibers
in plant cells to create a rigid network.lineage A single line of descent.
Linnaean hierarchy A system of biological classification devised
by the Swedish naturalist Carolus Linnaeus in the eighteenth
century.lipid Any of a major class of biomolecules built of fatty acids and
insoluble in water.liposome A sphere formed by a phospholipid bilayer.
locus (pl. loci) The physical location of a gene on a chromosome.
logistic growth A pattern of population growth in which the
population grows nearly exponentially at first but then stabilizes
at the maximum population size that can be supported indefi-
nitely by the environment. Logistic growth is represented by an
S-shaped curve. Compare exponential growth.lysosome An organelle in animal cells that acts as a garbage or
recycling center. Compare vacuole.M
macromolecule See biomolecule.malignant cell See cancer cell.
mammals A large class of animals that have body hair, sweat
glands, and milk produced by mammary glands.marine biome An aquatic biome characterized by salt water and
encompassing both the coastal regions of all continents and the
open ocean.marsupials One of three main groups of mammals, whose
members have a simple placenta and produce offspring that
complete development in their mother’s pouch. Compare
eutherians and monotremes.mass extinction A period of time during which a great
number of Earth’s species goes extinct. The fossil record shows
that there have been five mass extinctions in the history of
Earth.matter Anything that has mass and occupies a volume of space.
meiosis A specialized type of cell division that kicks off sexual
reproduction. It occurs in two stages: meiosis I and meiosis II,
each involving one round of nuclear division followed by cytoki-
nesis. Compare mitosis.meiosis I The first stage of meiosis, in which the chromosome set
is reduced by the separation of each homologous pair into two