G-18 glossary
peroxisome Enzyme-filled vesicle in which fatty acids and
amino acids are digested first into hydrogen peroxide (which
is toxic), then converted to harmless products.
PGAL Phosphoglyceraldehyde. A key intermediate in
glycolysis.
phagocyte (fayg-uh-sight) [Gk. phagein, to eat, and -kytos,
hollow vessel] A macrophage or other white blood cell that
engulfs and destroys foreign agents.
phagocytosis (fayg-uh-sigh-toe-sis) [Gk. phagein, to eat, and
-kytos, hollow vessel] Engulfment of foreign cells or substances
by specialized white blood cells by means of endocytosis.
pharynx (fare-inks) A muscular tube by which food enters
the gastrointestinal tract; the dual entrance for the tubular
part of the digestive tract and windpipe (trachea).
phenotype (fee-no-type) [Gk. phainein, to show, and -typos,
image] Observable trait or traits of an individual; arises
from interactions between genes, and between genes and the
environment.
pheromone (fare-oh-moan) [Gk. phero, to carry, and -mone,
as in hormone] A type of signaling molecule secreted by exo-
crine glands that serves as a communication signal between
individuals of the same species.
phospholipid A type of lipid that is the main structural
component of cell membranes. Each has a hydrophobic tail
(of two fatty acids) and a hydrophilic head that incorporates
glycerol and a phosphate group.
phosphorylation (foss-for-ih-lay-shun) The attachment of
unbound (inorganic) phosphate to a molecule; also the trans-
fer of a phosphate group from one molecule to another, as
when ATP phosphorylates glucose.
photoreceptor A light-sensitive sensory cell.
pH scale A scale used to measure the concentration of free
hydrogen ions in blood, water, and other solutions; pH 0 is
the most acidic, 14 the most basic, and 7, neutral.
phytochemicals Plant molecules that may reduce the risk of
some disorders, but are not essential.
pilomotor response Contraction of smooth muscle control-
ling the erection of body hair when outside temperature
drops. This creates a layer of still air that reduces heat losses
from the body. (It is most effective in mammals that have
more body hair than humans do.)
pineal gland (py-neel) A light-sensitive endocrine gland that
secretes melatonin, a hormone that influences reproductive
cycles and the development of reproductive organs.
pituitary gland An endocrine gland that interacts with the
hypothalamus to coordinate and control many physiologi-
cal functions, including the activity of many other endocrine
glands. Its posterior lobe stores and secretes hypothalamic
hormones; the anterior lobe produces and secretes its own
hormones.
placenta (pluh-sen-tuh) Of the uterus, an organ composed
of maternal tissues and extraembryonic membranes (the cho-
rion especially); it delivers nutrients to the fetus and acceptsparathyroid glands (pare-uh-thy-royd) Endocrine glands
embedded in the thyroid gland that secrete parathyroid hor-
mone, which helps restore blood calcium levels.
passive immunity Temporary immunity conferred by delib-
erately introducing antibodies into the body.
passive transport Diffusion of a solute through a channel or
carrier protein that spans the lipid bilayer of a cell membrane.
Its passage does not require an energy input; the protein pas-
sively allows the solute to follow its concentration gradient.
pathogen (path-oh-jen) [Gk. pathos, suffering] An infectious,
disease-causing agent, such as a virus or bacterium.
PCR See polymerase chain reaction.
pectoral girdle Set of bones, including the scapula (shoulder
blade) and clavicle (collarbone), to which the long bone of
each arm attaches. The pectoral girdles form the upper part of
the appendicular skeleton and are only loosely attached to the
rest of the body by muscles.
pedigree Family history of a genetic trait.
pelvic cavity Body cavity in which the reproductive organs,
bladder, and rectum are located.
pelvic girdle Set of bones including coxal bones that form
the pelvis; the lower part of the appendicular skeleton. The
upper portions of the two coxal bones are the hipbones; the
thighbones (femurs) join the coxal bones at hip joints. The
pelvic girdle bears the body’s weight when a person stands.
penetrance In a given population, the percentage of individ-
uals in which a particular genotype is expressed (that is, the
percentage of individuals who have the genotype and also
exhibit the corresponding phenotype).
penis Male organ that deposits sperm into the female repro-
ductive tract; also houses the urethra.
pepsin Any of several digestive enzymes that are part of gas-
tric fluid in the stomach.
peptide bond Covalent bond that joins the amino group of
one amino acid to the carboxyl group of a second amino acid.
peptide hormone A hormone that consists of a short chain
of amino acids.
perception The conscious interpretation of some aspect of
the external world created by the brain from nerve impulses
generated by sensory receptors.
peripheral nervous system (per-if-ur-uhl) [Gk. peripherein,
to carry around] The nerves leading into and out from the
spinal cord and brain and the ganglia along those communi-
cation lines.
peristalsis (pare-ih-stal-sis) Rhythmic contraction of
muscles that moves food forward through the gastrointestinal
tract.
peritubular capillaries The set of blood capillaries that
threads around the tubular parts of a nephron; they func-
tion in reabsorption of water and solutes and in secretion of
hydrogen ions and some other substances as urine is formed.
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