sary to ensure the optimal supramolecular interactions
with a specific biological target structure and to trigger
(or to block) its biological response.
A pharmacophore does not represent a real
molecule or a real association of functional groups
but, rather, is a purely abstract concept that accounts
for the common molecular interaction capacities of a
group of compounds toward their target structure.
The pharmacophore can be considered as the largest
common denominator shared by a set of active
molecules. This definition discards a misuse often
found in the MEDICINAL CHEMISTRYliterature, which
consists of naming as pharmacophores simple chemi-
cal functionalities such as guanidines, sulfonamides,
or dihydroimidazoles (formerly imidazolines), or typi-
cal structural skeletons such as flavones, phenoth-
iazines, prostaglandins, or steroids.
pharmacophoric descriptors Used to define a
PHARMACOPHORE, including H-bonding and hydropho-
bic and electrostatic interaction sites, defined by atoms,
ring centers, and virtual points.
pharynx A shortsection of fibromuscular tube that
is the common opening for the digestive and respira-
tory systems. Located in the throat at the convergence
of the nasal passage and oral cavity it opens to the
larynx (respiratory system) and the esophagus (diges-
tive system). Composed of three parts: the nasophar-
ynx (behind the nose and above the soft palate;
tonsils), the oropharynx (back of the mouth and soft
palate, tonsils, and posterior third of the tongue wall
of the throat), and the hypopharynx or laryngophar-
ynx (lower part of the throat behind the larynx and
above the esophagus). The pharynx receives air from
the nasal cavity, while air, food, and water enter from
the mouth.
phenetics (numerical taxonomy) A former school of
taxonomy that classified organisms on the basis of
overall morphological or genetic similarity that
involved observable similarities and differences with-
out considering whether or not the organisms were
related. It involved clustering groups into types and
represented a nonphylogenetic approach to biological
classification.phenotype The outward observable features, func-
tions, or behaviors of an organism, based on the coding
of the genotype.
See alsoGENOTYPE.pheromone A volatile chemical secreted and sent
externally by an organism to send information to mem-
bers of the same species via olfactory senses that induce
a physiological or behavioral response, such as sexual
attraction.phloem The principal food-conducting living tissue
of vascular plants; one of the vascular tissues in plants,
the other being the xylem, that make up the vascular
bundle. Composed of food-conducting sieve elements:
sieve cells and sieve-tube members. Sieve cells are found
in gymnosperms, while angiosperms have sieve-tube
members.
See alsoXYLEM.phosphatase An ENZYMEthat catalyzes the hydrolysis
of orthophosphoric monoesters. Alkaline phosphatases
(EC 3.1.3.1) have an optimum pH above 7 and are zinc-
containing proteins. Acid phosphatases (EC 3.1.3.2)
have an optimum pH below 7, and some of these con-
tain a dinuclear center of iron, or iron and zinc.
See also EC NOMENCLATURE FOR ENZYMES;
NUCLEARITY.phosphate group Oxygenated phosphorus (–PO 4 )
that is attached to a carbon chain; important in energy
transfer from ATP in cell signal transduction, the bio-
chemical communication from one part of the cell to
another; also part of a DNA nucleotide.phospholipase A(phosphatide acylhydrolase) Cat-
alyzes the hydrolysis of one of the acyl groups of phos-
phoglycerides or glycerophosphatidates. Phospholipasephospholipase A 265