polyp A cnidarian body form that is the sessile
reproductive stage; the alternate is the mobile medusa
stage. In medicine, a polyp is a small stalked benign
growth or tumor protruding from a mucous mem-
brane; can be precursors of cancer.
polypeptide A polymer chain of amino acids linked
by covalent peptide bonds. One or more polypeptides
form proteins. Each polypeptide has two terminal ends;
one, called the amino terminal or N-terminal, has a free
amino group, while the other end is called the carboxyl
terminal or C-terminal with a free carboxyl group.
polyphyletic Refers to a group of organisms that
might have some similarities but that do not include the
most recent common ancestor of all the member organ-
isms due to that ancestor lacking some or all character-
istics of the group. Polyphyletic groups are not
recognized in accepted taxonomies.
polyploidy When the number of chromosomes in a
cell gets doubled; two complete chromosome sets; a
mutation. Polyploidy is very common in plants, where
30 percent to 70 percent of modern angiosperms are
believed to be polyploids. Polyploidy is rare inanimals
butis found in some insects, amphibians, reptiles, and
one mammal (a rat in Argentina).
polyribosome(polysome) A cluster of ribosomes
translating on one messenger RNA molecule.
polysaccharide Acarbohydrate (polymer) made by
polymerizing any of more than 1,000 monosaccha-
rides; a complex sugar.
population Any group of interbreeding individuals of
a particular species living in a specific geographic area.
population viability analysis (PVA) A process to
evaluate the likelihood of a population surviving and to
identify threats facing the species. Used in endangered
species recovery and management.
porins A class of proteins that create water-filled
channels across cell membranes.porphyrin A macrocyclic molecule that contains
four pyrrole rings linked together by single carbon
atom bridges between the alpha positions of the pyr-
role rings. Porphyrins usually occur in their dianionic
form coordinated to a metal ion.
See alsoCOORDINATION.positional information Cells send signals indicating
their locations relative to each other in the embryo
development process; positional information is commu-
nicated through gap junctions, specific cell-cell adhe-
sions, or diffusible signal molecules; part of the process
of determining cell fate.positive feedback When a change occurs in a vari-
able during homeostasis, the response is to reinforce
the change in the variable. Examples include nerve
impulse conduction, blood clotting, female ovarian
cycles, labor and birth, and immune responses. A
resulting NEGATIVE FEEDBACKmay need to take over to
halt the process.postsynaptic membrane The presynaptic mem-
brane is at the tip of each nerve ending. A small cleft
called the synaptic cleft separates the presynaptic
membrane from the postsynaptic membrane, a special
area on the receiving cell. When the presynaptic nerve
ending receives nerve impulses, it sends neurotransmit-
ters stored in synaptic vesicles into the synaptic cleft
(the synapse), which in turn diffuse across and trans-
mit the signal to the postsynaptic membrane receptor
molecules, which are specialized molecules that open
or close certain ion channels when activated by the
correct neurotransmitter.postzygotic barrier A species-isolating mechanism
preventing hybrids that are produced by two
different species from developing into fertile and
viable adults.
See alsoPREZYGOTIC BARRIER.postzygotic barrier 273