For systems of more than one chemical component, the
concentrations of each are additional variables. A typi-
cal two-component phase diagram displays the varia-
tion of vapor pressure or melting point with
composition at a fixed pressure, for example.
phase rule (Gibbs phase rule) Describes the possible
number of degrees of freedom in a (closed) system at
equilibrium.
phase-transfer catalysis The phenomenon of rate
enhancement of a reaction between chemical species
located in different phases (immiscible liquids or solid
and liquid) by addition of a small quantity of an agent
(called the “phase-transfer CATALYST”) that extracts
one of the reactants, most commonly an anion, across
the interface into the other phase so that reaction can
proceed. These catalysts are salts of ONIUM IONs (e.g.,
tetraalkylammonium salts) or agents that complex
inorganic cations (e.g., CROWN ethers). The catalyst
cation is not consumed in the reaction, although an
anion exchange does occur.
phenol Specifically, C 6 H 5 OH; generically, a hydro-
carbon derivative containing an [OH] group bound to
an aromatic ring. Found naturally in decaying dead
organic matter like rotting vegetables and in coal.
phenonium ion SeeBRIDGED CARBOCATION.
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.
phosphatase An ENZYMEthat catalyzes the hydroly-
sis of orthophosphoric monoesters. Alkaline phos-
phatases (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 contain a DINUCLEARcenter of iron, or iron
and zinc.
See alsoECNOMENCLATURE FOR ENZYMES.phosphate group Oxygenated phosphorus (– PO 4 )
that is attached to a carbon chain; important in energy
transfer from ATPin cell signal transduction, the bio-
chemical communication from one part of the cell to
another; also part of a DNA NUCLEOTIDE.phospholipase A(phosphatide acylhydrolases) Cat-
alyzes the hydrolysis of one of the acyl groups of phos-
phoglycerides or glycerophosphatidates. Phospholipase
A1 hydrolyzes the acyl group attached to the 1-posi-
tion, while phospholipase A2 hydrolyzes the acyl group
attached to the 2-position.
See alsoPHOSPHOLIPASES.phospholipases A class of enzymes that catalyze the
hydrolysis of phosphoglycerides or glycerophosphati-
dates.
See alsoPHOSPHOLIPASEA.phospholipids The main component of cell walls; an
amphiphilic molecule (lipid). A glycerol skeleton is
attached to two fatty acids and a phosphate group, and
onto the phosphate one of three nitrogen groups, so
both phosphate and nitrogen groups make the “polar
head” larger and more polar. The phosphate part of the
molecule is water soluble, while the fatty-acid chains
are fat soluble. The phospholipids have a polar
hydrophilic head (phosphate) and nonpolar hydropho-
bic tail (fatty acids). When in water, phospholipids sort
into spherical bilayers; the phosphate groups point to
the cell exterior and interior, while the fatty acid
groups point to the interior of the membrane.
Examples include lecithin, cephalin, sphin-
gomyelin, phosphatidic acid, and plasmalogen. Two
types of phospholipids exist: glycerophospholipid and
sphingosyl phosphatide. A synthetic phospholipid,
alkylphosphocholine, has been used in biological and
therapeutic areas.phospholipids 211