G-20 glossary
reflex [L. reflectere, to bend back] A simple, stereotyped
movement in response to a stimulus. Sensory neurons syn-
apse on motor neurons in the simplest reflex pathways.
regulatory protein A protein that enhances or suppresses
transcription of a gene.
renewable resource A natural resource that can, in theory,
be tapped indefinitely if replenished.
reproduction In biology, processes by which a new genera-
tion of cells or multicellular individuals is produced. Sexual
reproduction requires meiosis, formation of gametes, and
fertilization. Asexual reproduction refers to the produc-
tion of new individuals by any mode that does not involve
gametes.
reproductive cloning Creating a cloned embryo to produce
a pregnancy.
respiration [L. respirare, to breathe] The exchange of oxygen
from the environment for carbon dioxide wastes from cells
by way of circulating blood. Compare aerobic respiration;
cellular respiration.
respiratory cycle One inhalation, one exhalation of air into
and out of the lungs.
respiratory membrane Two-layer membrane between the
walls of lung capillaries and alveoli; blood gases diffuse
across it.
respiratory surface In alveoli of the lungs, the thin, moist
membrane across which gases diffuse.
respiratory system An organ system specialized for bring-
ing in oxygen and carrying away carbon dioxide wastes;
human lungs and airways.
resting membrane potential Of neurons and other excit-
able cells that are not being stimulated, the steady voltage
difference across the plasma membrane.
reticular formation A major network of neurons in the brain
stem that helps govern activity of the whole nervous system.
retina A thin layer of neural tissue in the eye that contains
densely packed photoreceptors.
retrovirus An RNA virus that infects animal cells and with
reverse transcriptase creates an RNA template to synthesize a
DNA molecule that integrates itself into the host’s DNA.
Rh blood typing A method of characterizing red blood cells
on the basis of a protein that serves as a self marker at their
surface; Rh+ signifies its presence and Rh–, its absence.
rib cage Portion of the axial skeleton in the upper torso,
formed by the ribs and sternum, which supports and protects
the heart, lungs, and other organs.
ribosomal RNA (rRNA) Type of RNA molecule that
combines with proteins to form ribosomes, on which the
polypeptide chains of proteins are assembled.
ribosome The cell structure at which amino acids are strung
together to form the polypeptide chains of proteins. An intact
ribosome consists of two subunits, each composed of ribo-
somal RNA and protein molecules.proto-oncogene A gene similar to an oncogene but that
codes for a protein required in normal cell function; may trig-
ger cancer, generally when mutations alter its structure or
function.
proximal tubule The region of a nephron tubule that receives
water and solutes filtered from the blood.
psychoactive drug A chemical that acts on the central
nervous system, altering the activity of brain neurons and
associated mental and physical states.
puberty Period of human development that marks the
onset of sexual maturity as the reproductive organs begin to
function.
pulmonary circuit Blood circulation route between the heart
and lungs.
pulmonary valve Valve in the heart that opens from the right
ventricle into the pulmonary artery.
pulse Rhythmic pressure surge of blood flowing in an artery,
created during each cardiac cycle when a ventricle contracts.
Punnett square A method to predict the probable outcome
of a mating or an experimental cross in a simple diagram.
pyruvate (pie-roo-vate) A compound with a backbone of
three carbon atoms that is the end product of glycolysis.
radiation therapy Cancer treatment that relies on radiation
from radioisotopes to damage or destroy cancer cells.
radioisotope An isotope with an unstable nucleus that spon-
taneously decays to a new, stable atom that is not radioactive.
radiometric dating A method of dating fossils that tracks the
radioactive decay of material in the specimen.
radius One of two long bones of the forearm that extend
from the humerus (at the elbow joint) to the wrist. The radius
runs along the thumb side of the forearm, parallel to the ulna.
recessive (allele or trait) [L. recedere, to recede] Allele whose
expression in heterozygotes is fully or partially masked by
expression of its partner; fully expressed only in the homozy-
gous recessive condition.
recombinant DNA A DNA molecule that contains genetic
material from more than one organism of the same species or
from different species.
recombinant DNA technology Procedures by which DNA
(genes) from different species may be isolated, cut, spliced
together, and the new recombinant molecules multiplied in
quantity in a population of rapidly dividing cells such as
bacteria.
rectum Final region of the gastrointestinal tract, which
receives and temporarily stores undigested food residues
(feces).
red blood cell (RBC) Erythrocyte; an oxygen-transporting
cell in blood.
red marrow A substance in the spongy tissue of many bones
that serves as a major site of blood cell formation.
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