Biology Now, 2e

Glossary ■ (^) G7
glycolysis The first of three stages of cellular respiration.
During glycolysis, sugars (mainly glucose) are split to make
the three-carbon compound pyruvate. For each glucose mole-
cule that is split, two molecules of ATP and two molecules of
NADH are released. Compare Krebs cycle and oxidative
phosphorylation.
GM See genetic modification.
Golgi apparatus A collection of flattened membranes that pack-
ages and directs proteins and lipids produced by the ER to their
final destinations either inside or outside the cell.
grassland A terrestrial biome characterized by low moisture levels
(but not as low as in deserts), and dominated by grasses and
herbaceous plants.
greenhouse effect The process by which greenhouse gases let in
sunlight and trap heat.
greenhouse gas A gas in Earth’s atmosphere that absorbs heat
that radiates away from Earth’s surface. Examples include carbon
dioxide (CO 2 ), water vapor (H 2 O), methane (CH 4 ), and nitrous
oxide (N 2 O).
guanine (G) One of the four nucleotides that make up DNA. The
other three are adenine (A), thymine (T), and cytosine (C).
gymnosperms One of two main groups of seed-bearing plants,
characterized by naked seeds. Compare angiosperms.
H
halophile A prokaryote, usually archaean, that can live in extr-
emely salty environments.
haploid Possessing a single set of genetic information, represented
by 2n. Gametes are haploid. Compare diploid.
herbivore An animal that eats plants. Compare carnivore and
omnivore.
herd immunity Protection against disease that is brought about
by vaccination of a critical portion of a population.
heterotroph A metabolic consumer, an organism that obtains
energy by taking it from other sources. Chemoheterotrophs
consume organic molecules as a source of energy and carbon.
Photo heterotrophs absorb the energy of sunlight but require an
organic source of carbon. Compare autotroph.
heterozygous Carrying two different alleles for a given phenotype
(Bb). Compare homozygous.
histone protein One of a class of specific proteins that, together
with DNA, form nucleosomes.
homeostasis The process of maintaining constant internal
conditions.
hominids The ape family, which includes humans and chimpan-
zees. All hominids are capable of tool use, symbolic language, and
deliberate acts of deception. Compare hominins.
hominins The “human” branch of the hominids, including modern
humans and extinct relatives such as Neanderthals.
homologous pair A pair of chromosomes consisting of one chro-
mosome received from the father and one from the mother.
homologous trait A feature that is similar across species because
of common descent. Homologous traits may begin to look differ-
ent from one another over time. Compare analogous trait.
homozygous Carrying two copies of the same allele (such as BB or
bb) for a particular gene. Compare heterozygous.
horizontal gene transfer The transfer of genes on plasmids
from one bacterium to another.
host An organism in which a parasite lives.
human microbiome The complete collection of microbes that
live in and on our cells and bodies.
hydrogen bond The weak electrical attraction between a hydrogen
atom with a partial positive charge and a neighboring atom with
a partial negative charge. Compare covalent bond and ionic bond.
hydrologic cycle The movement of water as it circulates from the
land to the sky and back again.
hydrophilic Literally, “water-loving.” Soluble in water. Compare
hydrophobic.
hydrophobic Literally, “water-fearing.” Excluded from water.
Compare hydrophilic.
hypertonic Describing a fluid that has a solute concentration
higher than that of the cell it surrounds. Compare hypotonic and
isotonic.
hyphae (sing. hypha) The fine, branching threads of fungi that
absorb nutrients from the environment.
hypothesis (pl. hypotheses) An informed, logical, and plausible
explanation for observations of the natural world.
hypotonic Describing a fluid that has a solute concentration
lower than that of the cell it surrounds. Compare hypertonic and
isotonic.
I
in vitro fertilization (IVF) Fertilization of an egg by a sperm in
a petri dish, followed by implantation of one or more embryos
into a woman’s uterus.
incomplete dominance An interaction between two alleles of
a gene in which neither one can exert its full effect, causing a
heterozygote to display an intermediate phenotype. Compare
codominance.
independent assortment The random distribution of the
homologous chromosomes into daughter cells during meiosis I.
independent variable The variable that is manipulated by
the researcher in a scientific experiment. Compare dependent
variable.
induced fit The way an enzyme changes shape when molecules
bind to its active site.
insertion A mutation in which a base is inserted into the DNA
sequence of a gene. Compare deletion and substitution.
interference competition Competition between species in
which one organism directly excludes another from the use of a
resource. Compare exploitative competition.