plasmid An extrachromosomal GENETIC element
consisting generally of circular double-stranded DNA,
which can replicate independently of chromosomal
DNA. R-plasmids are responsible for the mutual trans-
fer of antibiotic resistance among microbes. Plasmids
are used as vectors for CLONINGDNA in bacteria or
yeast host cells.
plastic A synthetic material made from long chains of
molecules that has the capability of being molded or
shaped, usually by the application of heat and pressure.
A plastic is made up principally of a binder together
with PLASTICIZERs, fillers, pigments, and other addi-
tives. There are two basic types of plastic: thermoset-
ting, which cannot be resoftened after being subjected
to heat and pressure, and thermoplastic, which can be
repeatedly softened and remolded by heat and pressure.
plasticize To soften a material and make it plastic or
moldable by the application of a PLASTICIZERor the
application of heat.
plasticizer Plasticizers are added to a binder to
increase flexibility and toughness. They are low-melt-
ing solids or high-boiling organic liquids. They have
varying degrees of softening action and solvating abil-
ity resulting from a reduction of intermolecular forces
in the polymer.
plastocyanin An ELECTRON-TRANSFER PROTEIN, con-
taining a TYPE 1 COPPER site, involved in plant and
cyanobacterial PHOTOSYNTHESIS, that transfers elec-
trons to PHOTOSYSTEMI.
plutonium Element number 94, which was secretly
discovered during World War II in 1940 but was publi-
cized in 1946 by Glenn Seaborg and others. It is mostly
used for nuclear weapons, as a fission energy source (a
by-product of the fission process in nuclear reactors,
due to neutron capture by uranium-238 in particular),
and in deep-space probes. Extracted from uranium ore
in 1947. Over one-third of the energy produced in
most nuclear power plants comes from plutonium.
p-njunction In a semiconductor, it is where a p-type
material and an n-type material are in direct contact.pOH The negative logarithm of the hydroxide ion
concentration. In contrast, pH is the negative logarithm
of the hydrogen ion concentration.poikilotherm An organism (e.g., fish or reptile)
whose body temperature varies or fluctuates with the
temperature of its surroundings; an ectotherm.
See alsoENDOTHERM.polar aprotic solvent See DIPOLAR APROTIC SOL-
VENT.polar covalent bond A covalent bond in which the
electrons are not shared equally between the two
atoms, but spend more time in the vicinity of the atom
with higher ELECTRONEGATIVITY. For example, in the
bond between hydrogen and oxygen, electrons shared
by atoms spend a greater percentage of time closer to
the oxygen nucleus than the hydrogen nucleus; bonds
are polar, i.e., they have a partial electric charge
(hydrogen is positive, oxygen is negative); in organ-
isms, they can form weak HYDROGEN BONDs.polar effect For a reactant molecule RY, the polar
effect of the group R comprises all the processes
whereby a substituent may modify the electrostatic
forces operating at the reaction center Y, relative to
the standard RoY. These forces can be governed by
charge separations arising from differences in the
ELECTRONEGATIVITYof atoms (leading to the presence
of dipoles, the presence of unipoles, or electron DELO-
CALIZATION). It is synonymous with ELECTRONIC
EFFECTor “electrical effect” of a substituent, as distin-
guished from other substituent effects, e.g., STERIC
EFFECTs.
Sometimes, however, the term polar effectis taken
to refer to the influence, other than steric, that noncon-
jugated substituents exert on reaction rates, i.e., effects
connected with electron delocalization between a sub-
stituent and the molecular framework to which it ispolar effect 215