parasitoid Anorganism that lives in or on the body
of a single host individual during its development and
eventually kills the host. Falls between a predator and a
true parasite.
parasympathetic division (craniosacral division)
One of two divisions of the autonomic nervous system.
The parasympathetic deals with conserving energy,
digestion of food, and excretory functions; encourages
sedentary functions as opposed to the sympathetic divi-
sion; increases the nutrient content in the blood, which
stimulates growth and storage of energy reserves. Other
responses include a decreased heart rate and airway
and pupil diameters.
parathyroid glands A set of four glands, two in the
left lobe of the thyroid gland and two in the right lobe.
They function to control blood calcium levels by secret-
ing parathyroid hormone (PTH). If too much calcium
is allowed in the blood, a condition known as hyper-
parathyroidism occurs; if the blood calcium is too low,
a condition known as hypoparathyroidism can exist.
The parathyroid hormone also stimulates absorption of
food by the intestines and conservation of calcium by
the kidneys.
Parazoa The animal subkingdom of sponges, the
phylum Porifera. Mostly mouthless marine multicellu-
lar sessile animals that feed by drawing in microorgan-
isms through their pores (suspension feeders). Four
classes of sponges exist: Calcarea (calcareous sponges),
Hexactinellida (glass sponges), and Demospongiae (the
largest and most diverse, 90 percent of known species),
and Sclerospongiae (mostly fossil records). There are
about 5,000 species of sponges.
parenchyma One of the three types of plant tis-
sue; unspecialized, composed of large thin-walled
cells forming the greater part of leaves, roots, the
pulp of fruit, and the pith of stems; Has an abun-
dance of plastids, chloroplast-containing cells able to
perform photosynthesis in leaves and stems (when
filled with chloroplasts, called chlorenchyma). The
cells of a plant’s “ground tissue,” composed of
parenchyma cells that function in assimilation and
storage, are called assimilates. Parenchyma tissues
are the mesophyll, an important assimilation tissue;
the palisade parenchyma, directly beneath the epider-
mis of the upper part of a leaf; and the spongy
parenchyma, which fills the spaces beneath the pal-
isade parenchyma. Parenchyma are the only cells
that can engage in mitosis and are the only type of
cell found in apical meristems. Parenchyma found in
the air-filled floating leaves of aquatic plants are
called aerenchyma.parthenogenesis A form of reproduction in many
lower animals in which the egg develops into a new
individual without fertilization. In certain social
insects, such as ants and honeybees, the unfertilized
eggs develop into male drones.partial agonist An AGONISTthat is unable to induce
maximal activation of a RECEPTORpopulation, regard-
less of the amount of DRUGapplied.
See alsoINTRINSIC ACTIVITY.partial pressure Each gas in a mixture of
gases exerts a pressure called the partial pressure. It
is the pressure exerted by one gas in a mixture of
gases.passive transport(diffusion) A molecule or ion
that crosses a biological membrane by moving down a
concentration or electrochemical gradient with no
expenditure of metabolic energy. Passive transport, in
the same direction as a concentration gradient, can
occur spontaneously, or proteins can mediate passive
transport and provide the pathway for this movement
across the lipid bilayer without supplying energy for
the action. These proteins are called channels if they
mediate ions and permeases if they mediate large
molecules. This type of transport always operates from
regions of greater concentration to regions of lesser
concentration.
See alsoACTIVE TRANSPORT.260 parasitoid