p
(1) Prefix denoting
the polypeptide product of a gene (see e.g. Retroviridae).
(2) Designation for a
precursor molecule (as e.g. in pTP in Adenoviridae).
(3) Prefix denoting a plasmid, usually a recombinant
plasmid.
P
Proline (see amino
acids).
P agar
An agar medium containing peptone (1%), yeast
extract (0.5%), NaCl (0.5%), glucose (0.1%) and agar (1.5%).
P antigen
(parvovirus receptor)
see erythrovirus.
A
specialized region of the right atrium of the mammalian heart that sets the
rate of contraction; also called the sinoatrial (SA) node. See Sinoatrial node.
paedogenesis
(pee-doh-jen-eh-sis)
The precocious development of sexual maturity in a larva.
The precocious development of sexual maturity in a larva.
paedomorphosis
(pee-doh-mor-foh-sis)
The retention in an adult organism of the juvenile features of its evolutionary ancestors.
The retention in an adult organism of the juvenile features of its evolutionary ancestors.
paleontology
(pay-lee-un-tol-uh-jee)
[Gk. palaios, old + onta, things that exist
+ logos, discourse]
The scientific study of fossils.
The scientific study of fossils.
palisade
cells
[L. palus, stake
+ cella, a chamber]
In plant leaves, the columnar, chloroplast-containing parenchyma cells of the mesophyll.
In plant leaves, the columnar, chloroplast-containing parenchyma cells of the mesophyll.
pancreas
(pang-kree-us)
[Gk. pan, all + kreas, meat, flesh]
In vertebrates, a small, complex gland located between the stomach and the duodenum, which produces digestive enzymes and the hormones insulin and glucagon.
In vertebrates, a small, complex gland located between the stomach and the duodenum, which produces digestive enzymes and the hormones insulin and glucagon.
Pangaea
(pan-jee-uh)
The supercontinent formed near the end of the Paleozoic era when plate movements brought all the land masses of Earth together.
The supercontinent formed near the end of the Paleozoic era when plate movements brought all the land masses of Earth together.
paraphyletic
(par-uh-fy-leh-tik)
Pertaining to a taxon that excludes some members that share a common ancestor with members included in the taxon.
Pertaining to a taxon that excludes some members that share a common ancestor with members included in the taxon.
parasite
(par-uh-site)
[Gk. para, beside, akin to + sitos, food]
An organism that absorbs nutrients from the body fluids of living hosts.
An organism that absorbs nutrients from the body fluids of living hosts.
parasitism
A
symbiotic relationship in which the symbiont (parasite) benefits at the expense
of the host by living either within the host (endoparasite) or outside the host
(ectoparasite). SeeSymbiosis.
parasympathetic
division
[Gk. para, beside,
akin to]
One of two divisions of the autonomic nervous system; generally enhances body activities that gain and conserve energy, such as digestion and reduced heart rate.
One of two divisions of the autonomic nervous system; generally enhances body activities that gain and conserve energy, such as digestion and reduced heart rate.
parathyroid
glands
[Gk. para, beside,
akin to + thyra, a door]
Four endocrine glands, embedded in the surface of the thyroid gland, that secrete parathyroid hormone and raise blood calcium levels.
Four endocrine glands, embedded in the surface of the thyroid gland, that secrete parathyroid hormone and raise blood calcium levels.
parazoa
(par-uh-zoh-uh)
Members of the subkingdom of animals consisting of the sponges.
Members of the subkingdom of animals consisting of the sponges.
parenchyma
(pur-en-kim-uh)
[Gk. para, beside, akin to + en, in + chein, to
pour]
A relatively unspecialized plant cell type that carries most of the metabolism, synthesizes and stores organic products, and develops into more differentiated cell types.
A relatively unspecialized plant cell type that carries most of the metabolism, synthesizes and stores organic products, and develops into more differentiated cell types.
parental
generation
In an
experimental genetic cross, the parents of the F1 generation; homozygous for
the trait(s) being studied.
parthenogenesis
(par-then-oh-jen-eh-sis)
[Gk. parthenon, virgin + genesis, birth]
A type of reproduction in which females produce offspring from unfertilized eggs.
A type of reproduction in which females produce offspring from unfertilized eggs.
partial
pressures
The
concentration of gases; a fraction of total pressure.
passive
transport
The
diffusion of a substance across a biological membrane.
pathogen
[Gk. pathos, suffering
+ genos, origin, descent]
An organism or a virus that causes disease.
An organism or a virus that causes disease.
pattern
formation
The
ordering of cells into specific three-dimensional structures, an essential part
of shaping an organism and its individual parts during development.
pedigree
A family
tree describing the occurrence of heritable characters in parents and offspring
across as many generations as possible.
pelagic
zone
(pel-ay-jik)
The area of the ocean past the continental shelf, with areas of open water often reaching to very great depths.
The area of the ocean past the continental shelf, with areas of open water often reaching to very great depths.
penetrance
In
genetics, the proportion of individuals with a particular genotype that show
the phenotype ascribed to that genotype.
peptide
bond
[Gk. pepto, to
soften, digest]
The covalent bond between two amino acid units, formed by condensation synthesis.
The covalent bond between two amino acid units, formed by condensation synthesis.
peptidoglycan
(pep-tid-oh-gly-kan)
A type of polymer in bacterial cell walls consisting of modified sugars cross-linked by short polypeptides.
A type of polymer in bacterial cell walls consisting of modified sugars cross-linked by short polypeptides.
perception
The
interpretation of sensations by the brain.
perennial
(pur-en-ee-ul)
[L. per, through + annus, year]
A plant that lives for many years.
A plant that lives for many years.
pericycle
(pair-eh-sy-kul)
[Gk. peri, around + kyklos, circle]
A layer of cells just inside the endodermis of a root that may become meristematic and begin dividing again.
A layer of cells just inside the endodermis of a root that may become meristematic and begin dividing again.
periderm
(pair-eh-durm)
The protective coat that replaces the epidermis in plants during secondary growth, formed of the cork and cork cambium.
The protective coat that replaces the epidermis in plants during secondary growth, formed of the cork and cork cambium.
peripheral
nervous system
[Gk. peripherein, to
carry around]
The sensory and motor neurons that connect to the central nervous system.
The sensory and motor neurons that connect to the central nervous system.
peristalsis
[Gk. peristellein, to
wrap around]
Rhythmic waves of contraction of smooth muscle that push food along the digestive tract.
Rhythmic waves of contraction of smooth muscle that push food along the digestive tract.
peritoneum
[Gk. peritonos, stretched
over]
A membrane that lines the body cavity and forms the external covering of the visceral organs.
A membrane that lines the body cavity and forms the external covering of the visceral organs.
peritubular
capillaries
[Gk. peri, around
+ L. tubus, tube]
In the vertebrate kidney, the capillaries that surround the renal tubule; water and solutes are reabsorbed into the bloodstream through the peritubular capillaries and some substances are secreted from them into the renal tubule.
In the vertebrate kidney, the capillaries that surround the renal tubule; water and solutes are reabsorbed into the bloodstream through the peritubular capillaries and some substances are secreted from them into the renal tubule.
permeable
[L. permeare, to
pass through]
Penetrable by molecules, ions, or atoms; usually applied to membranes that let given solutes pass through.
Penetrable by molecules, ions, or atoms; usually applied to membranes that let given solutes pass through.
peroxisome
(per-oks-eh-some)
A microbody containing enzymes that transfer hydrogen from various substrates to oxygen, producing and then degrading hydrogen peroxide.
A microbody containing enzymes that transfer hydrogen from various substrates to oxygen, producing and then degrading hydrogen peroxide.
petiole
(pet-ee-ole)
[Fr. from L. petiolus, dim. of pes, pedis, a
foot]
The stalk of a leaf, which joins the leaf to a node of the stem.
The stalk of a leaf, which joins the leaf to a node of the stem.
pH scale
A measure
of hydrogen ion concentration equal to –log [H+] and ranging in
value from 0 to 14.
phage
(fage)
A virus that infects bacteria; also called a bacteriophage.
A virus that infects bacteria; also called a bacteriophage.
phagocytosis
(fay-goh-sy-toh-sis)
[Gk. phagein, to eat + kytos, vessel]
A type of endocytosis involving large, particulate substances.
A type of endocytosis involving large, particulate substances.
pharynx
(fah-rinks)
An area in the vertebrate throat where air and food passages cross; in flatworms, the muscular tube that protrudes from the ventral side of the worm and ends in the mouth.
An area in the vertebrate throat where air and food passages cross; in flatworms, the muscular tube that protrudes from the ventral side of the worm and ends in the mouth.
phenetics
(feh-neh-tiks)
An approach to taxonomy based entirely on measurable similarities and differences in phenotypic characters, without consideration of homology, analogy, or phylogeny.
An approach to taxonomy based entirely on measurable similarities and differences in phenotypic characters, without consideration of homology, analogy, or phylogeny.
phenotype
[Gk. phainein, to
show + typos, stamp, print]
The physical and physiological traits of an organism.
The physical and physiological traits of an organism.
pheromone
(fair-uh-mone)
[Gk. phero, to bear, carry]
A small, volatile chemical signal that functions in communication between animals and acts much like a hormone in influencing physiology and behavior.
A small, volatile chemical signal that functions in communication between animals and acts much like a hormone in influencing physiology and behavior.
phloem
(floh-um)
[Gk. phloos, bark]
The portion of the vascular system in plants consisting of living cells arranged into elongated tubes that transport sugar and other organic nutrients throughout the plant.
The portion of the vascular system in plants consisting of living cells arranged into elongated tubes that transport sugar and other organic nutrients throughout the plant.
phosphate
group
A
functional group important in energy transfer.
phospholipids
(fos-fo-lip-ids)
Molecules that constitute the inner bilayer of biological membranes, having a polar, hydrophilic head and a nonpolar, hydrophobic tail.
Molecules that constitute the inner bilayer of biological membranes, having a polar, hydrophilic head and a nonpolar, hydrophobic tail.
phosphorylation
Addition
of a phosphate group or groups to a molecule.
photic
zone
(foh-tik)
The narrow top slice of the ocean, where light permeates sufficiently for photosynthesis to occur.
The narrow top slice of the ocean, where light permeates sufficiently for photosynthesis to occur.
photoautotroph
(foh-toh-aw-toh-trohf)
An organism that harnesses light energy to drive the synthesis of organic compounds from carbon dioxide.
An organism that harnesses light energy to drive the synthesis of organic compounds from carbon dioxide.
photoheterotroph
(foh-toh-het-ur-oh-trohf)
An organism that uses light to generate ATP but that must obtain carbon in organic form.
An organism that uses light to generate ATP but that must obtain carbon in organic form.
photon
(foh-tahn)
A quantum, or discrete amount, of light energy.
A quantum, or discrete amount, of light energy.
photoperiodism
[Gk. photos, light]
A physiological response to day length, such as flowering in plants.
A physiological response to day length, such as flowering in plants.
photophosphorylation
(foh-toh-fos-for-uh-lay-shun)
[Gk. photos, light + phosphoros, bringing
light]
The process of generating ATP from ADP and phosphate by means of a proton-motive force generated by the thylakoid membrane of the chloroplast during the light reactions of photosynthesis.
The process of generating ATP from ADP and phosphate by means of a proton-motive force generated by the thylakoid membrane of the chloroplast during the light reactions of photosynthesis.
photoreceptor
[Gk. photos, light]
A cell or organ capable of detecting light.
A cell or organ capable of detecting light.
photorespiration
[Gk. photos, light
+ L. respirare, to breathe]
A metabolic pathway that consumes oxygen, releases carbon dioxide, generates no ATP, and decreases photosynthetic output; generally occurs on hot, dry, bright days, when stomata close and the oxygen concentration in the leaf exceeds that of carbon dioxide.
A metabolic pathway that consumes oxygen, releases carbon dioxide, generates no ATP, and decreases photosynthetic output; generally occurs on hot, dry, bright days, when stomata close and the oxygen concentration in the leaf exceeds that of carbon dioxide.
photosynthesis
[Gk. photos, light
+ syn, together + tithenai, to place]
The conversion of light energy to chemical energy that is stored in glucose or other organic compounds; occurs in plants, algae, and certain prokaryotes.
The conversion of light energy to chemical energy that is stored in glucose or other organic compounds; occurs in plants, algae, and certain prokaryotes.
photosystem
[Gk. photos, light
+ systema, that which is put together]
The light-harvesting unit in photosynthesis, located on the thylakoid membrane of the chloroplast and consisting of the antenna complex, the reaction-center chlorophyll a, and the primary electron acceptor. There are two types of photosystems, I and II; they absorb light best at different wavelengths.
The light-harvesting unit in photosynthesis, located on the thylakoid membrane of the chloroplast and consisting of the antenna complex, the reaction-center chlorophyll a, and the primary electron acceptor. There are two types of photosystems, I and II; they absorb light best at different wavelengths.
phototropism
[Gk. photos, light
+ trope, turning]
Growth of a plant shoot toward or away from light.
Growth of a plant shoot toward or away from light.
phyletic
change
[Gk. phylon, race,
tribe]
The changes taking place in a single lineage of organisms over a long period of time; one of the principal patterns of evolutionary change.
The changes taking place in a single lineage of organisms over a long period of time; one of the principal patterns of evolutionary change.
phylogeny
(fy-loj-en-ee)
[Gk. phylon, race, tribe]
The evolutionary history of a species or group of related species.
The evolutionary history of a species or group of related species.
phylum pl. phyla
[Gk. phylon, race,
tribe]
A taxonomic category; phyla are divided into classes.
A taxonomic category; phyla are divided into classes.
physiology
[Gk. physis, nature
+ logos, a discourse]
The study of function in cells, organs, or entire organisms; the processes of life.
The study of function in cells, organs, or entire organisms; the processes of life.
phytoalexin
(fy-toh-ah-lek-sin)
An antibiotic, produced by plants, that destroys microorganisms or inhibits their growth.
An antibiotic, produced by plants, that destroys microorganisms or inhibits their growth.
phytochrome
(fy-tuh-krome)
[Gk. phyton, plant + chroma, color]
A pigment involved in many responses of plants to light.
A pigment involved in many responses of plants to light.
phytoplankton
[Gk. phyton, plant
+ planktos, wandering]
Aquatic, free-floating, microscopic, photosynthetic organisms.
Aquatic, free-floating, microscopic, photosynthetic organisms.
pigment
[L. pigmentum, paint]
A colored substance that absorbs light over a narrow band of wavelengths.
A colored substance that absorbs light over a narrow band of wavelengths.
pilus pl. pili
(pill-us)
A surface appendage in certain bacteria that functions in adherence and the transfer of DNA during conjugation.
A surface appendage in certain bacteria that functions in adherence and the transfer of DNA during conjugation.
pineal
gland
(pin-ee-ul)
A small endocrine gland on the dorsal surface of the vertebrate forebrain; secretes the hormone melatonin, which regulates body functions related to seasonal day length.
A small endocrine gland on the dorsal surface of the vertebrate forebrain; secretes the hormone melatonin, which regulates body functions related to seasonal day length.
pinocytosis
(py-noh-sy-toh-sis)
[Gk. pinein, to drink + kytos, vessel]
A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes.
A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes.
pith
The core
of the central vascular cylinder of monocot roots, consisting of parenchyma cells, which are ringed by
vascular tissue; ground tissue interior to vascular bundles in dicot stems.
pituitary
gland
(pi-too-ih-tair-ee)
[L. pituita, phlegm]
An endocrine gland at the base of the hypothalamus; consists of a posterior lobe (neurohypophysis), which stores and releases two hormones produced by the hypothalamus, and an anterior lobe (adenohypophysis), which produces and secretes many hormones that regulate diverse body functions.
An endocrine gland at the base of the hypothalamus; consists of a posterior lobe (neurohypophysis), which stores and releases two hormones produced by the hypothalamus, and an anterior lobe (adenohypophysis), which produces and secretes many hormones that regulate diverse body functions.
placenta
(pluh-sen-tuh)
[Gk. plax, a flat object]
A structure in the pregnant uterus for nourishing a viviparous fetus with the mother's blood supply; formed from the uterine lining and embryonic membranes.
A structure in the pregnant uterus for nourishing a viviparous fetus with the mother's blood supply; formed from the uterine lining and embryonic membranes.
placental
mammal
A member
of a group of mammals, including humans, whose young complete their embryonic
development in the uterus, joined to the mother by a placenta. See Eutherian mammals.
placoderm
(plak-oh-durm)
A member of an extinct class of fishlike vertebrates that had jaws and were enclosed in a tough, outer armor.
A member of an extinct class of fishlike vertebrates that had jaws and were enclosed in a tough, outer armor.
plankton
[Gk. planktos, wandering]
Mostly microscopic organisms that drift passively or swim weakly near the surface of oceans, ponds, and lakes.
Mostly microscopic organisms that drift passively or swim weakly near the surface of oceans, ponds, and lakes.
planula
[L. dim.
of planus, a wanderer]
The ciliated, free-swimming type of larva formed by many cnidarians.
The ciliated, free-swimming type of larva formed by many cnidarians.
plasma
[Gk. form
or mold]
The liquid matrix of blood in which the cells are suspended.
The liquid matrix of blood in which the cells are suspended.
plasma
cell
A
derivative of B cells that secretes antibodies.
plasma
membrane
[Gk. plasma, form
or mold + L. membrana, skin, parchment]
The membrane at the boundary of every cell that acts as a selective barrier, thereby regulating the cell's chemical composition.
The membrane at the boundary of every cell that acts as a selective barrier, thereby regulating the cell's chemical composition.
plasmid
(plaz-mid)
A small ring of DNA that carries accessory genes separate from those of a bacterial chromosome. Also found in some eukaryotes, such as yeast.
A small ring of DNA that carries accessory genes separate from those of a bacterial chromosome. Also found in some eukaryotes, such as yeast.
plasmodesma pl. plasmodesmata
(plaz-moh-dez-muh)
[Gk. plassein, to mold + desmos, band, bond]
An open channel in the cell wall of plants through which strands of cytosol connect from adjacent cells.
An open channel in the cell wall of plants through which strands of cytosol connect from adjacent cells.
plasmogamy
The
fusion of the cytoplasm of cells from two individuals; occurs as one stage of
syngamy.
plasmolysis
(plaz-mol-eh-sis)
A phenomenon in walled cells in which the cytoplasm shrivels and the plasma membrane pulls away from the cell wall when the cell loses water to a hypertonic environment.
A phenomenon in walled cells in which the cytoplasm shrivels and the plasma membrane pulls away from the cell wall when the cell loses water to a hypertonic environment.
plastid
[Gk. plastos, formed
or molded]
One of a family of closely related plant organelles, including chloroplasts, chromoplasts, and amyloplasts (leucoplasts).
One of a family of closely related plant organelles, including chloroplasts, chromoplasts, and amyloplasts (leucoplasts).
platelet
(plate-let)
[Gk. platus, flat]
A small enucleated blood cell important in blood clotting; derived from large cells in the bone marrow.
A small enucleated blood cell important in blood clotting; derived from large cells in the bone marrow.
pleated
sheet
One form
of the secondary structure of proteins in which the polypeptide chain folds
back and forth, or where two regions of the chain lie parallel to each other
and are held together by hydrogen bonds.
pleiotropy
(ply-o-trop-ee)
[Gk. pleios, more + trope, a turning]
The ability of a single gene to have multiple effects.
The ability of a single gene to have multiple effects.
plesiomorphic
character
(pleez-ee-oh-mor-fik)
A primitive phenotypic character possessed by a remote ancestor.
A primitive phenotypic character possessed by a remote ancestor.
pluripotent
stem cell
A cell
within bone marrow that is a progenitor for any kind of blood cell.
point
mutation
A change
in a gene at a single nucleotide pair.
polar
[L. polus, end
of axis]
Having parts or areas with opposed or contrasting properties, such as positive and negative charges, head and tail.
Having parts or areas with opposed or contrasting properties, such as positive and negative charges, head and tail.
polar
body
Minute,
nonfunctioning cell produced during those meiotic divisions that lead to egg
cells; contains a nucleus but very little cytoplasm.
polar
covalent bond
A type of
covalent bond between atoms that differ in electronegativity. The shared
electrons are pulled closer to the more electronegative atom, making it
slightly negative and the other atom slightly positive.
polar
molecule
A
molecule (such as water) with opposite charges on opposite sides.
polar
nuclei
In
angiosperms, the two nuclei of the central cell of the female gametophyte; they
fuse with a sperm nucleus to form the triploid (3n) endosperm nucleus.
pollen
[L. fine
dust]
An immature male gametophyte that develops within the anthers of stamens in a flower.
An immature male gametophyte that develops within the anthers of stamens in a flower.
pollination
(pol-eh-nay-shun)
[L. pollen, fine dust]
The placement of pollen onto the stigma of a carpel by wind or animal carriers, a prerequisite to fertilization.
The placement of pollen onto the stigma of a carpel by wind or animal carriers, a prerequisite to fertilization.
polyandry
(pol-ee-an-dree)
A polygamous mating system involving one female and many males.
A polygamous mating system involving one female and many males.
polygenic
inheritance
(pol-ee-jen-ik)
[Gk. polus, many + genos, race, descent]
An additive effect of two or more gene loci on a single phenotypic character.
An additive effect of two or more gene loci on a single phenotypic character.
polygyny
(pol-ij-en-ee)
A polygamous mating system involving one male and many females.
A polygamous mating system involving one male and many females.
polymer
(pol-eh-mur)
[Gk. polus, many + meris, part or portion]
A large molecule consisting of many identical or similar monomers linked together.
A large molecule consisting of many identical or similar monomers linked together.
polymerase
An
enzyme, such as DNA polymerase or RNA polymerase, that catalyzes the synthesis
of a polymer from its subunits.
polymerase
chain reaction (PCR)
A technique
for amplifying DNA in vitro by incubating with special primers, DNA polymerase
molecules and nucleotides.
polymorphic
(pol-ee-mor-fik)
Referring to a population in which two or more physical forms are present in readily noticeable frequencies.
Referring to a population in which two or more physical forms are present in readily noticeable frequencies.
polymorphism
(pol-ee-mor-fiz-um)
[Gk. polus, many + morphe, form]
The coexistence of two or more distinct forms of individuals (polymorphic characters) in the same population.
The coexistence of two or more distinct forms of individuals (polymorphic characters) in the same population.
polynucleotide
(pol-ee-noo-klee-oh-tide)
A polymer made up of many nucleotides covalently bonded together.
A polymer made up of many nucleotides covalently bonded together.
polyp
(pol-ip)
[Gk. polus, many + pous, foot]
The sessile variant of the cniderian body plan. The alternate form is the medusa.
The sessile variant of the cniderian body plan. The alternate form is the medusa.
polypeptide
(pol-ee-pep-tide)
[Gk. polus, many + pepto, to soften, digest]
A polymer (chain) of many amino acids linked together by peptide bonds.
A polymer (chain) of many amino acids linked together by peptide bonds.
polyphyletic
Pertaining
to a taxon whose members were derived from two or more ancestral forms not
common to all members.
polyploid
[Gk. polus, many
+ ploion, vessel]
Cell with more than two complete sets of chromosomes per nucleus.
Cell with more than two complete sets of chromosomes per nucleus.
polyploidy
(pol-ee-ploid-ee)
A chromosomal alteration in which the organism possesses more than two complete chromosome sets.
A chromosomal alteration in which the organism possesses more than two complete chromosome sets.
polyribosome
(pol-ee-ry-boh-some)
An aggregation of several ribosomes attached to one messenger RNA molecule.
An aggregation of several ribosomes attached to one messenger RNA molecule.
polysaccharide
(pol-ee-sak-ur-ide)
[Gk. polus, many + sakcharon, sugar]
A polymer of up to over a thousand monosaccharides, formed by condensation synthesis.
A polymer of up to over a thousand monosaccharides, formed by condensation synthesis.
polysome
See Polyribosome.
population
A group
of individuals of one species that live in a particular geographic area.
population
bottleneck
Type
of genetic drift that occurs as the
result of a population being drastically reduced in numbers by an event having
little to do with the usual forces of natural selection.
population
density
The
number of individuals of a population per unit area or volume of living space.
population
viability analysis (PVA)
A method
of predicting whether or not a species will persist in a particular
environment.
positional
information
Signals,
to which genes regulating development respond, indicating a cell's location
relative to other cells in an embryonic structure.
positive
feedback
A
physiological control mechanism in which a change in some variable triggers
mechanisms that amplify the change.
posterior
Of or
pertaining to the rear, or tail, end.
postsynaptic
membrane
(post-sin-ap-tik)
The surface of the cell on the opposite side of the synapse from the synaptic terminal of the stimulating neuron that contains receptor proteins and degradative enzymes for the neurotransmitter.
The surface of the cell on the opposite side of the synapse from the synaptic terminal of the stimulating neuron that contains receptor proteins and degradative enzymes for the neurotransmitter.
postzygotic
barrier
(post-zy-got-ik)
Any of several species-isolating mechanisms that prevent hybrids produced by two different species from developing into viable, fertile adults.
Any of several species-isolating mechanisms that prevent hybrids produced by two different species from developing into viable, fertile adults.
potential
energy
The
energy stored by matter as a result of its location or spatial arrangement.
predation
An
interaction between species in which one species, the predator, eats the other,
the prey.
predator
[L. praedari, to
prey upon; from prehendere, to grasp, seize]
An organism that eats other living organisms.
An organism that eats other living organisms.
pressure-flow
hypothesis
A
hypothesis accounting for sap flow through the phloem system. According to this
hypothesis, the solution containing nutrient sugars moves through the sieve
tubes by bulk flow, moving into and out of the sieve tubes by active transport
and diffusion.
prey
[L. prehendere, to
grasp, seize]
An organism eaten by another organism.
An organism eaten by another organism.
prezygotic
barrier
(pree-zy-got-ik)
A reproductive barrier that impedes mating between species or hinders fertilization of ova if interspecific mating is attempted.
A reproductive barrier that impedes mating between species or hinders fertilization of ova if interspecific mating is attempted.
primary
consumer
An
herbivore; an organism in the trophic level of an ecosystem that eats plants or
algae.
primary
germ layers
The three
layers (ectoderm, mesoderm, endoderm) of the late gastrula, which develop into all parts of
an animal.
primary
growth
Growth
initiated by the apical meristems of a plant root or shoot.
primary
immune response
The
initial immune response to an antigen, which appears after a lag of several
days.
primary
producer
An
autotroph, which collectively make up the trophic level of an ecosystem that
ultimately supports all other levels; usually a photosynthetic organism.
primary
productivity
The rate
at which light energy or inorganic chemical energy is converted to the chemical
energy of organic compounds by autotrophs in an ecosystem.
primary
structure
The level
of protein structure referring to the specific sequence of amino acids.
primary
succession
A type of
ecological succession that occurs in an area where there were originally no
organisms.
primer
An
already existing short RNA chain bound to template DNA to which DNA nucleotides
are added during DNA synthesis.
primate
A member
of the order of mammals that includes anthropoids and prosimians.
primitive
[L. primus, first]
Not specialized; at an early stage of evolution or development.
Not specialized; at an early stage of evolution or development.
primordium pl. primordia
[L. primus, first
+ ordiri, to begin to weave]
A cell or organ in its earliest stage of differentiation.
A cell or organ in its earliest stage of differentiation.
principle
of allocation
The
concept that each organism has an energy budget, or a limited amount of total
energy available for all of its maintenance and reproductive needs.
prion
An
infectious form of protein that may increase in number by converting related
proteins to more prions.
probe
See Nucleic acid probe.
procambium
(pro-kam-bee-um)
[L. pro, before + cambium, exchange]
A primary meristem of roots and shoots that forms the vascular tissue.
A primary meristem of roots and shoots that forms the vascular tissue.
producer,
in ecological systems
An
autotrophic organism, usually a photosynthesizer, that contributes to the net
primary productivity of a community.
progesterone
(pro-jes-teh-roan)
[L. progerere, to carry forth or out + steiras, barren]
A steroid hormone secreted by the corpus luteum of the ovary; maintains the uterine lining during pregnancy.
A steroid hormone secreted by the corpus luteum of the ovary; maintains the uterine lining during pregnancy.
prokaryotic
cell
[L. pro, before
+ Gk. karyon, nut, kernel]
A type of cell lacking a membrane-enclosed nucleus and membrane-enclosed organelles; found only in the domains Bacteria and Archaea.
A type of cell lacking a membrane-enclosed nucleus and membrane-enclosed organelles; found only in the domains Bacteria and Archaea.
prometaphase
The phase
of mitosis in which the nuclear envelope breaks into fragments. Some of the
spindle fibers reach the chromosomes and attach to protein structures at the
centromeres, called kinetochores, while others make contact with microtubules
coming from the opposite pole. The opposing spindle fibers move the chromosomes
toward the metaphase plate, an imaginary plane equidistant from the poles.
promoter
A
specific nucleotide sequence in DNA that binds RNA polymerase and indicates
where to start transcribing RNA.
prophage
A phage
genome that has been inserted into a specific site on the bacterial chromosome.
prophase
[Gk. pro, before
+ phasis, form]
The first stage of mitosis, during which duplicated chromosomes condense from chromatin, and the mitotic spindle forms and begins moving the chromosomes toward the center of the cell.
The first stage of mitosis, during which duplicated chromosomes condense from chromatin, and the mitotic spindle forms and begins moving the chromosomes toward the center of the cell.
prosimian
[L. pro, before
+ simia, ape]
A lower primate; includes lemurs, lorises, tarsiers, and bush babies, as well as many fossil forms.
A lower primate; includes lemurs, lorises, tarsiers, and bush babies, as well as many fossil forms.
prostaglandin
(PG)
(pros-tuh-glan-din)
[Gk. prostas, a porch or vestibule + L. glans, acorn]
One of a group of modified fatty acids secreted by virtually all tissues and performing a wide variety of functions as messengers.
One of a group of modified fatty acids secreted by virtually all tissues and performing a wide variety of functions as messengers.
prostate
gland
[Gk. prostas, a
porch or vestibule + L. glans, acorn]
A gland in human males that secretes an acid-neutralizing component of semen.
A gland in human males that secretes an acid-neutralizing component of semen.
protein
(pro-teen)
[Gk. proteios, primary]
A three-dimensional biological polymer constructed from a set of 20 different monomers called amino acids.
A three-dimensional biological polymer constructed from a set of 20 different monomers called amino acids.
protein
kinase
An enzyme
that transfers phosphate groups from ATP to a protein.
protein
phosphatase
An enzyme
that removes phosphate groups from proteins, often functioning to reverse the
effect of a protein kinase.
proteoglycans
(pro-tee-oh-gly-kanz)
A glycoprotein in the extracellular matrix of animal cells, rich in carbohydrate.
A glycoprotein in the extracellular matrix of animal cells, rich in carbohydrate.
proteasome
A giant
protein complex that recognizes and destroys proteins tagged for elimination by
the small protein ubiquitin.
protoderm
(pro-toh-durm)
[Gk. protos, first + derma, skin]
The outermost primary meristem, which gives rise to the epidermis of roots and shoots.
The outermost primary meristem, which gives rise to the epidermis of roots and shoots.
proton
A
subatomic particle with a single positive electrical charge, found in the
nucleus of the atom.
proton-motive
force
The
potential energy stored in the form of an electrochemical gradient, generated
by the pumping of hydrogen ions across biological membranes during
chemiosmosis.
proton
pump
An active
transport mechanism in cell membranes that consumes ATP to force hydrogen ions
out of a cell and, in the process, generates a membrane potential.
protonephridium
(pro-toh-nef-rid-ee-um)
An excretory system, such as the flame-cell system of flatworms, consisting of a network of closed tubules having external openings called nephridiopores and lacking internal openings.
An excretory system, such as the flame-cell system of flatworms, consisting of a network of closed tubules having external openings called nephridiopores and lacking internal openings.
proto-oncogene
(pro-toh-onk-oh-jeen)
A normal cellular gene corresponding to an oncogene; a gene with a potential to cause cancer, but that requires some alteration to become an oncogene.
A normal cellular gene corresponding to an oncogene; a gene with a potential to cause cancer, but that requires some alteration to become an oncogene.
protoplasm
[Gk. protos, first
+ plasma, anything molded]
Living matter.
Living matter.
protoplast
The
contents of a plant cell exclusive of the cell wall.
protostome
(pro-toh-stome)
[Gk. protos, first + stoma, mouth]
A member of one of two distinct evolutionary lines of coelomates, consisting of the annelids, mollusks, and arthropods, and characterized by spiral, determinate cleavage, schizocoelous formation of the coelom, and development of the mouth from the blastopore.
A member of one of two distinct evolutionary lines of coelomates, consisting of the annelids, mollusks, and arthropods, and characterized by spiral, determinate cleavage, schizocoelous formation of the coelom, and development of the mouth from the blastopore.
protozoan pl. protozoa
A protist
that lives primarily by ingesting food, an animal-like mode of nutrition.
provirus
Viral DNA
that inserts into a host genome.
proximate
causation
The
hypothesis about why natural selection favored a particular animal behavior.
pseudocoelom
[Gk. pseudes, false
+ koilos, a hollow]
A body cavity consisting of a fluid-filled space between the endoderm and the mesoderm; characteristic of the nematodes.
A body cavity consisting of a fluid-filled space between the endoderm and the mesoderm; characteristic of the nematodes.
pseudocoelomate
(soo-doh-seel-oh-mate)
An animal, such as a rotifer or roundworm, whose body cavity is not completely lined by mesoderm.
An animal, such as a rotifer or roundworm, whose body cavity is not completely lined by mesoderm.
pseudopodium
(soo-doh-poh-dee-um)
[Gk. pseudes, false + pous, pod-, foot]
A cellular extension of amoeboid cells used in moving and feeding.
A cellular extension of amoeboid cells used in moving and feeding.
pulmonary
[L. pulmonis, lung]
Pertaining to the lungs.
Pertaining to the lungs.
pulmonary
artery
[L. pulmonis, lung]
In birds and mammals, an artery that carries deoxygenated blood from the right ventricle of the heart to the lungs, where it is oxygenated.
In birds and mammals, an artery that carries deoxygenated blood from the right ventricle of the heart to the lungs, where it is oxygenated.
pulmonary
vein
[L. pulmonis, lung]
In birds and mammals, a vein that carries oxygenated blood from the lungs to the left atrium of the heart, from which blood is pumped into the left ventricle and from there to the body tissues.
In birds and mammals, a vein that carries oxygenated blood from the lungs to the left atrium of the heart, from which blood is pumped into the left ventricle and from there to the body tissues.
pulse
A
measurement of heart rate; distention of an artery that can be felt each time
the heart contracts.
punctuated
equilibrium
A theory
of evolution advocating spurts of relatively rapid change followed by long
periods of stasis.
Punnett
square
The
checkerboard diagram used for analysis of allele segregation.
pupa
[L. girl,
doll]
A developmental stage of some insects, in which the organism is nonfeeding, immotile, and sometimes encapsulated or in a cocoon; the pupal stage occurs between the larval and adult phases.
A developmental stage of some insects, in which the organism is nonfeeding, immotile, and sometimes encapsulated or in a cocoon; the pupal stage occurs between the larval and adult phases.
purine
[Gk. purinos, fiery,
sparkling]
A nitrogenous base, such as adenine or guanine, with a characteristic two-ring structure; one of the components of nucleic acids.
A nitrogenous base, such as adenine or guanine, with a characteristic two-ring structure; one of the components of nucleic acids.
pyramid,
ecological
See Ecological pyramid.
pyramid
of energy
A diagram
of the energy flow between the trophic levels of an ecosystem; plants or other
autotrophs (at the base of the pyramid) represent the greatest amount of
energy, herbivores next, then primary carnivores, secondary carnivores, etc.
pyrimidine
A
nitrogenous base, such as cytosine, thymine, or uracil, with a characteristic single-ring
structure; one of the components of nucleic acids.