glossary G -19positive feedback mechanism Homeostatic mechanism
by which a chain of events is set in motion that intensifies a
change from an original condition.
potential energy A capacity to do work.
precapillary sphincter A ring of smooth muscle that regu-
lates the flow of blood into a capillary.
prediction A statement about what one can expect to observe
in nature if a theory or hypothesis is correct.
preimplantation diagnosis Test for birth defects in an early
embryo that was conceived by in vitro fertilization.
primary immune response Activity of white blood cells and
their products elicited by a first-time encounter with
an antigen; includes both antibody-mediated and cell-
mediated responses.
primary productivity Of ecosystems, gross primary produc-
tivity is the rate at which the producer organisms capture and
store a given amount of energy during a specified interval.
primary structure The particular sequence of amino acids
that make up a given protein.
primate A type of mammal; primates include monkeys, apes,
and humans.
principle of sustainability See sustainability.
probability With respect to any chance event, the most likely
number of times it will turn out a certain way, divided by the
total number of all possible outcomes.
producer Of ecosystems, any of the organisms that secure
energy from the physical environment, as by photosynthesis
or chemosynthesis. Green plants are Earth’s main primary
producers.
progesterone (pro-jess-tuh-rown) Female sex hormone
secreted by the ovaries.
prokaryotic cell (pro-care-ee-ah-tik) [L. pro, before, and Gk.
karyon, kernel] A single-celled organism that has no nucleus
or any of the other membrane-bound organelles characteristic
of eukaryotic cells. Bacteria are prokaryotic.
prophase Of mitosis, the stage when each duplicated chro-
mosome starts to condense, microtubules form a spindle
apparatus, and the nuclear envelope starts to break up.
prostaglandin Any of various local signaling molecules that
typically cause smooth muscle to contract or relax, as in blood
vessels, the uterus, and airways.
prostate gland Gland in males that wraps around the
urethra and ejaculatory ducts; its secretions become part of
semen.
protein A large organic compound composed of one or more
chains of amino acids held together by peptide bonds. Pro-
teins have unique sequences of different kinds of amino acids
in their polypeptide chains; such sequences are the basis of a
protein’s three-dimensional structure and chemical behavior.
protein hormone A hormone that consists of a long amino
acid chain.wastes from it, yet allows the fetal circulatory system to
develop separately from the mother’s.
plaque Cholesterol and other lipids that build up in the arte-
rial wall, leaving less room for flowing blood.
plasma (plaz-muh) Liquid portion of blood; consists of
water, various proteins, ions, sugars, dissolved gases, and
other substances.
plasma cell In adaptive immunity, an effector B cell that
quickly floods the bloodstream with antibodies.
plasma membrane The outermost cell membrane. Pro-
teins in its lipid bilayer carry out most functions, including
transport across the membrane and reception of extracellular
signals.
platelet (playt-let) A cell fragment in blood that releases
substances necessary for blood clotting.
pleiotropy (ply-ah-trow-pee) [Gk. pleon, more, and trope,
direction] A type of gene interaction in which a single gene
exerts multiple effects on seemingly unrelated aspects of an
individual’s phenotype.
pleura (plural: pleurae) Thin, double membrane surround-
ing each lung.
pollutant Any substance with which an ecosystem has
had no prior evolutionary experience in terms of kinds or
amounts, and that can accumulate to disruptive or harmful
levels. Can be naturally occurring or synthetic.
polygenic trait Trait that results from the combined expres-
sion of several genes.
polymer (pah-lih-mur) [Gk. polus, many, and meris, part] A
molecule composed of three to millions of small subunits that
may or may not be identical.
polymerase chain reaction (PCR) DNA amplification
method; DNA containing a gene of interest is split into single
strands, which enzymes (polymerases) copy; the enzymes
also act on the accumulating copies, multiplying the gene
sequence by the millions.
polypeptide chain Three or more amino acids joined by pep-
tide bonds.
polysaccharide [Gk. polus, many, and sakharon, sugar] A
straight or branched chain of covalently bonded monomers
of the same or different kinds of sugars. The most common
polysaccharides are cellulose, starch, and glycogen.
pons (pahnz) Hindbrain traffic center for signals between
centers of the cerebellum and forebrain.
population A group of individuals of the same species occu-
pying a given area.
population density The number of individuals of a popula-
tion that are living in a specified area or volume.
population size The number of individuals that make up the
gene pool of a population.
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