in PHOTOPERIODISM and photomorphogenesis. Plant
responses regulated by phytochrome include photope-
riodic induction of flowering, chloroplast develop-
ment (minus chlorophyll synthesis), leaf senescence
and leaf abscission, seed germination, and flower
induction.
pi (π) adduct An ADDUCT formed by electron-pair
donation from a PI(π) ORBITALinto a SIGMA(σ) ORBITAL,
or from a sigma orbital into a pi orbital, or from a pi
orbital into a pi orbital. For example:
Such an adduct has commonly been known as a
“pi complex” but, as the bonding is not necessarily
weak, it is better to avoid the term COMPLEX, in accor-
dance with the recommendations in this encyclopedia.
See alsoCOORDINATION.
pi (π) bond A COVALENT BONDformed by the over-
lap between two porbitals on different atoms. Pi bonds
are superimposed on SIGMA(σ) BONDs, forming double
or triple bonds.
See alsoSIGMA, PI.
picket-fence porphyrin A PORPHYRINwith a protec-
tive enclosure for binding oxygen at one side of the
ring that is used to mimic the dioxygen-carrying prop-
erties of the HEMEgroup.
See alsoBIOMIMETIC.
+pi (π) complex SeePI(π) ADDUCT.
pi (π) electron acceptor, π-electron donor group
A SUBSTITUENTcapable of a +R (e.g., NO 2 ) or –R (e.g.,
OCH 3 ) effect, respectively.
See alsoELECTRONIC EFFECT; POLAR EFFECT; SIGMA
(σ) CONSTANT.piezoelectric effect Crystals that acquire a charge
when compressed, twisted, or distorted. The word
piezois Greek for “push.” The effect was discovered
by brothers Pierre and Jacques Curie in 1880.pi (π) orbital Bonding orbitals with two lobes lying
parallel to the bond axis.
See alsoSIGMA, PI.placebo An inert substance or dosage form identical
in appearance, flavor, and odor to the active substance
or dosage form. It is used as a negative control in a
BIOASSAYor in a clinical study.Planck’s constant(h) A physical constant used to
describe the sizes of quanta (any quantity that can only
take on integer multiples of some base value). Named
after Max Planck, it plays a central role in the theory
of quantum mechanics, in which Planck is one of the
founders of quantum theory. Its value is expressed as:
h= 6.626 × 10 –34J splasma In physics, a plasma is an ionized gas. In
biology, this term has the following three meanings:
(1) Fluid component of blood in which the blood
cells and platelets are suspended (blood plasma). Note
the distinction between plasma, which describes a part
of the blood (the fluid part of blood, outside the blood
cells), and serum, which describes a fraction derived
from blood by a manipulation (the fluid that separates
when blood coagulates).
(2) Fluid component of semen produced by the
accessory glands, the seminal vesicles, the prostate, and
the bulbourethral glands.
(3) Cell substance outside the nucleus (CYTO-
PLASM).214 pi (π) adduct