sensory-somatic nervous system and the autonomic
nervous system.
periplasm The fluid occupying the space between the
inside and outside cellular membranes of bacteria.
peristalsis Involuntary rhythmic waves or move-
ments of longitudinal and circular muscles in the diges-
tive tract (stomach, intestines, esophagus) in short or
long duration.
peroxidase A heme protein (donor: hydrogen perox-
ide OXIDOREDUCTASE, EC class 1.11.1) that catalyzes
the one-electron oxidation of a SUBSTRATEby dihydro-
gen peroxide. Substrates for different peroxidases
include various organic compounds, CYTOCHROME-c,
halides, and Mn2+.
See alsoEC NOMENCLATURE FOR ENZYMES.
peroxisome A single-membrane enzyme-containing
organelle present in most eukaryotic cells. The perox-
isome is involved in metabolic processes such as the
ß-oxidation of long- and very-long-chain fatty acids,
bile acid synthesis, cholesterol synthesis, plasmalogen
synthesis, amino acid metabolism, and purine
metabolism.
petiole A leaf stalk that attaches to the stem.
Pfeiffer’s rule States that in a series of chiral com-
pounds the EUDISMIC RATIOincreases with increasing
POTENCYof the EUTOMER.
phage (bacteriophage) Atype of virus that attacks
bacteria.
See alsoBACTERIA.
phagocyte A cell that is able to ingest, and often to
digest, large particles such as bacteria and dead tissue
cells.
phagocytosis The act of ingestion and digestion of
microorganisms, insoluble particles, damaged or dead
host cells, and cell debris by specific types of cells
called phagocytes (e.g., macrophages, neutrophils); a
form of endocytosis.
See alsoENDOCYTOSIS.pharmacokinetics The study of absorption, distri-
bution, METABOLISM, and excretion (ADME) of bioac-
tive compounds in a higher organism.
See alsoDRUG DISPOSITION.pharmacophore (pharmacophoric pattern) The
ensemble of steric and electronic features that is neces-264 periplasm
Scanning electron micrograph (SEM) of a macrophage, a white
blood cell (upper right), engulfing a protozoan, Leishmania mexi-
cana(lower left). This parasitic protozoan causes the disease
leishmaniasis. Here, the macrophage has extended part of its body
to surround and capture the Leishmania.This process is called
phagocytosis. Leishmaniasis is transmitted by the bites of infected
sandflies, causing a skin ulcer at the site of the bite. The more
serious form, kala-azar, can be fatal. Macrophages are part of the
immune system that keep the body free of invading organisms.
Magnification unknown.(Courtesy © Science Photo
Library/Photo Researchers, Inc.)