potency A comparative rather than an absolute
expression of drug activity. Drug potency depends on
bothAFFINITYand EFFICACY. Thus, two AGONISTs can
be equipotent but have different intrinsic efficacies,
with compensating differences in affinity. Potency is the
dose of DRUGrequired to produce a specific effect of
given intensity as compared with a standard reference.
potential energy Stored energy that can be released
or harnessed to do work.
power saturation A phenomenon used in ELECTRON
PARAMAGNETIC RESONANCE SPECTROSCOPYto estimate
the electron-spin relaxation times, thus providing infor-
mation about distances between PARAMAGNETICcenters.
predator An animal that preys on or eats other
animals.
predisposition, genetic A latent susceptibility to
disease, at the genetic level, that can be activated under
certain conditions.
prevalence The number of all new and old cases of a
disease in a defined population at a particular point in
time.
prezygotic barrier There aretwo types of reproduc-
tive isolation mechanisms: prezygotic (before gamete
union) and postzygotic (after gamete union). A form of
prezygotic barrier is mechanical isolation, which deals
with the mechanics of the reproductive organs and
physically prevents sexual intercourse between two dif-
ferent species. Temporal (two species reproduce at dif-
ferent times of day) and habitat isolation (two
overlapping species in same range live in different habi-
tats) are also prezygotic barriers.
See alsoPOSTZYGOTIC BARRIER.
primary consumer Any animal that eats grass, algae,
and other green plants in a food chain; a herbivore.
primary germ layers After gastrulation, develop-
ment of the three germ layers—ectoderm, mesoderm,
endoderm—occurs, and these eventually develop into
all parts of the animal.primary growth Growth initiated by the apical
meristem that takes place relatively close to the tips of
roots and stem and that involves extending the length
of the plant.primary immune response The immune response
(cellular or humoral) to a first encounter with an anti-
gen. The primary response is generally small, has a long
induction phase or lag period, consists primarily of IgM
antibodies, and generates immunologic memory.primary producer An autotroph that acts as a food
source for the next level up in the food chain. Green
plants are primary producers.primary productivity The rate at which new plant
biomass is formed by photosynthesis. Gross primary
productivity is the total rate of photosynthetic produc-
tion of biomass; net primary productivity is gross pri-
maryproductivity minus the respiration rate.primary structure The amino acid SEQUENCEof a
protein or the NUCLEOTIDEsequence of DNA or RNA.primary succession Occurs when communities
develop in a newly exposed habitat that had no former
life (e.g., bare rock, newly deposited sand).primate Mammals that include humans and other
species that are closely related. There are two main
groups: the anthropoids (humans, apes, monkeys) and
prosimians (aye-ayes, galagos, lemurs, lorises, pottos,
and tarsiers).primer A short preexisting polynucleotide chain to
which new deoxyribonucleotides can be added by
DNA polymerase.274 potency