P generation • Also called the parental generation. The individuals that mate in a genetic cross. Their immediate offspring are the F1 generation.
Pacemaker • That part of the heart which undergoes most rapid spontaneous contraction, thus setting the pace for the beat of the entire heart. In mammals, the sinoatrial (SA) node. Also, an artificial device, implanted in the heart, that initiates rhythmic contraction of the organ.
Pacinian corpuscle • A sensory neuron surrounded by sheaths of connective tissue. Found in the deep layers of the skin, where it senses touch and vibration.
Pair rule genes • Segmentation genes that divide the Drosophila larva into two segments each.
Paleomagnetism • The record of the changing direction of Earth’s magnetic field as stored in lava flows. Used to accurately date extremely ancient events.
Paleontology (pale´ ee on tol´ oh jee) [Gr. palaios: ancient, old + logos: discourse] • The scientific study of fossils and all aspects of extinct life.
Pancreas (pan´ cree us) • A gland, located near the stomach of vertebrates, that secretes digestive enzymes into the small intestine and releases insulin into the bloodstream.
Pangaea (pan jee´ uh) [Gk. pan: all, every] • The single land mass formed when all the continents came together in the Permian period. (Contrast with Gondwana.)
Parabronchi • Passages in the lungs of birds through which air flows.
Paradigm • A general framework within which a scientific or philosophical discipline is viewed and within which questions are asked and hypotheses are developed. Scientific revolutions usually involve major paradigm changes. (Contrast with hypothesis, theory.)
Parallel evolution • Evolutionary patterns that exist in more than one lineage. Often the result of underlying developmental processes.
Parapatric speciation [Gr. para: beside + patria: fatherland] • Development of reproductive isolation when the barrier is not geographic but is a difference in some other physical condition (such as soil nutrient content) that prevents gene flow between the subpopulations. (Contrast with allopatric speciation, sympatric speciation.)
Paraphyletic taxon • A taxon that includes some, but not all, of the descendants of a single ancestor.
Parasite • An organism that attacks and consumes parts of an organism much larger than itself. Parasites sometimes, but not always, kill the host.
Parasitoid • A parasite that is so large relative to its host that only one individual or at most a few individuals can live within a single host.
Parasympathetic nervous system • A portion of the autonomic (involuntary) nervous system. Activity in the parasympathetic nervous system produces effects such as decreased blood pressure and decelerated heart beat. (Contrast with sympathetic nervous system.)
Parathormone • Hormone secreted by the parathyroid glands. Stimulates osteoclast activity and raises blood calcium levels.
Parathyroids • Four glands on the posterior surface of the thyroid that produce and release parathormone.
Parenchyma (pair eng´ kyma) [Gr. para: beside + enchyma: infusion] • A plant tissue composed of relatively unspecialized cells without secondary walls.
Parental investment • Investment in one offspring or group of offspring that reduces the ability of the parent to assist other offspring.
Parsimony • The principle of preferring the simplest among a set of plausible explanations of a phenomenon. Commonly employed in evolutionary and biogeographic studies.
Parthenocarpy • Formation of fruit from a flower without fertilization.
Parthenogenesis (par´ then oh jen´ e sis) [Gr. parthenos: virgin + genesis: source] • The production of an organism from an unfertilized egg.
Partial pressure • The portion of the barometric pressure of a mixture of gases that is due to one component of that mixture. For example, the partial pressure of oxygen at sea level is 20.9% of barometric pressure.
Patch clamping • A technique for isolating a tiny patch of membrane to allow the study of ion movement through a particular channel.
Pathogen (path´ o jen) [Gr. pathos: suffering + gignomai: causing] • An organism that causes disease.
Pattern formation • In animal embryonic development, the organization of differentiated tissues into specific structures such as wings.
Pedigree • The pattern of transmission of a genetic trait in a family.
Pelagic zone (puh ladj´ ik) [Gr. pelagos: the sea] • The open waters of the ocean.
Penetrance • Of a genotype, the proportion of individuals with that genotype who show the expected phenotype.
PEP carboxylase • The enzyme that combines carbon dioxide with PEP to form a 4-carbon dicarboxylic acid at the start of C4 photosynthesis or of Crassulacean acid metabolism (CAM).
Pepsin [Gr. pepsis: digestion] • An enzyme, in gastric juice, that digests protein.
Peptide linkage • The connecting group in a protein chain, -CO-NH-, formed by removal of water during the linking of amino acids, -COOH to -NH2.
Peptidoglycan • The cell wall material of many prokaryotes, consisting of a single enormous molecule that surrounds the entire cell.
Perennial (per ren´ ee al) [L. per: through + annus: a year] • Referring to a plant that lives from year to year. (Contrast with annual, biennial.)
Perfect flower • A flower with both stamens and carpels, therefore hermaphroditic.
Pericycle [Gr. peri: around + kyklos: ring or circle] • In plant roots, tissue just within the endodermis, but outside of the root vascular tissue. Meristematic activity of pericycle cells produces lateral root primordia.
Periderm • The outer tissue of the secondary plant body, consisting primarily of cork.
Period • (1) A minor category in the geological time scale. (2) The duration of a cyclical event, such as a circadian rhythm.
Peripheral nervous system • Neurons that transmit information to and from the central nervous system and whose cell bodies reside outside the brain or spinal cord.
Peristalsis (pair´ i stall´ sis) [Gr. peri: around + stellein: place] • Wavelike muscular contractions proceeding along a tubular organ, propelling the contents along the tube.
Peritoneum • The mesodermal lining of the coelom among coelomate animals.
Permease • A membrane protein that specifically transports a compound or family of compounds across the membrane.
Peroxisome • An organelle that houses reactions in which toxic peroxides are formed. The peroxisome isolates these peroxides from the rest of the cell.
Petal • In an angiosperm flower, a sterile modified leaf, nonphotosynthetic, frequently brightly colored, and often serving to attract pollinating insects.
Petiole (pet´ ee ole) [L. petiolus: small foot] • The stalk of a leaf.
pH • The negative logarithm of the hydrogen ion concentration; a measure of the acidity of a solution. A solution with pH = 7 is said to be neutral; pH values higher than 7 characterize basic solutions, while acidic solutions have pH values less than 7.
Phage (fayj) • Short for bacteriophage.
Phagocyte • A white blood cell that ingests microorganisms by endocytosis.
Phagocytosis [Gr.: phagein to eat; cell-eating] • A form of endocytosis, the uptake of a solid particle by forming a pocket of plasma membrane around the particle and pinching off the pocket to form an intracellular particle bounded by membrane. (Contrast with pinocytosis.)
Pharynx [Gr.: throat] • The part of the gut between the mouth and the esophagus.
Phenotype (fee´ no type) [Gr. phanein: to show] • The observable properties of an individual as they have developed under the combined influences of the genetic constitution of the individual and the effects of environmental factors. (Contrast with genotype.)
Phenotypic plasticity • The fact that the phenotype of an organism is determined by a complex series of developmental processes that are affected by both its genotype and its environment.
Pheromone (feer´ o mone) [Gr. phero: carry + hormon: excite, arouse] • A chemical substance used in communication between organisms of the same species.
Phloem (flo´ um) [Gr. phloos: bark] • In vascular plants, the food-conducting tissue. It consists of sieve cells or sieve tubes, fibers, and other specialized cells.
Phosphate group • The functional group -OPO3H2; the transfer of energy from one compound to another is often accomplished by the transfer of a phosphate group.
Phosphodiester linkage • The connection in a nucleic acid strand, formed by linking two nucleotides.
Phospholipids • Cellular materials that contain phosphorus and are soluble in organic solvents. An example is lecithin (phosphatidyl choline). Phospholipids are important constituents of cellular membranes. (See lipids.)
Phosphorylation • The addition of a phosphate group.
Photoautotroph • An organism that obtains energy from light and carbon from carbon dioxide. (Contrast with chemoautotroph, chemoheterotroph, photoheterotroph.)
Photoheterotroph • An organism that obtains energy from light but must obtain its carbon from organic compounds. (Contrast with chemoautotroph, chemoheterotroph, photoautotroph.)
Photon (foe´ tohn) [Gr. photos: light] • A quantum of visible radiation; a "packet" of light energy.
Photoperiod (foe´ tow peer´ ee ud) • The duration of a period of light, such as the length of time in a 24-hour cycle in which daylight is present. The regulation of processes such as flowering by the changing length of day (or of night) is known as photoperiodism.
Photoreceptor • (1) A protein (pigment) that triggers a physiological response when it absorbs a photon. (2) A cell that senses and responds to light energy.
Photorespiration • Light-driven uptake of oxygen and release of carbon dioxide, the carbon being derived from the early reactions of photosynthesis.
Photosynthesis (foe tow sin´ the sis) [literally, "synthesis out of light"] • Metabolic processes, carried out by green plants, by which visible light is trapped and the energy used to synthesize compounds such as ATP and glucose.
Phototropin • A yellow protein that is the photoreceptor responsible for phototropism.
Phototropism [Gr. photos: light + trope: a turning] • A directed plant growth response to light.
Phylogenetic tree • Graphic representation of lines of descent among organisms.
Phylogeny (fy loj´ e nee) [Gr. phylon: tribe, race + genesis: source] • The evolutionary history of a particular group of organisms; also, the diagram of the "family tree" that shows genetic linkages between ancestors and descendants.
Phylum (plural: phyla) [Gr. phylon: tribe, stock] • In taxonomy, a high-level category just beneath kingdom and above the class; a group of related, similar classes.
Physiology (fiz´ ee ol´ o jee) [Gr. physis: natural form + logos: discourse, study] • The scientific study of the functions of living organisms and the individual organs, tissues, and cells of which they are composed.
Phytoalexins • Substances toxic to fungi, produced by plants in response to fungal infection.
Phytochrome (fy´ tow krome) [Gr. phyton: plant + chroma: color] • A plant pigment regulating a large number of developmental and other phenomena in plants; can exist in two different forms, one of which is active and the other is not. Different wavelengths of light can drive it from one form to the other.
Phytoplankton (fy´ tow plangk´ ton) [Gr. phyton: plant + planktos: wandering] • The autotrophic portion of the plankton, consisting mostly of algae.
Pigment • A substance that absorbs visible light.
Pilus (pill´ us) [Lat. pilus: hair] • A surface appendage by which some bacteria adhere to one another during conjugation.
Pinocytosis [Gr.: drinking cell] • A form of endocytosis; the uptake of liquids by engulfing a sample of the external medium into a pocket of the plasma membrane followed by pinching off the pocket to form an intracellular vesicle. (Contrast with phagocytosis and endocytosis.)
Pistil [L. pistillum: pestle] • The female structure of an angiosperm flower, within which the ovules are borne. May consist of a single carpel, or of several carpels fused into a single structure. Usually differentiated into ovary, style, and stigma.
Pith • In plants, relatively unspecialized tissue found within a cylinder of vascular tissue.
Pituitary • A small gland attached to the base of the brain in vertebrates. Its hormones control the activities of other glands. Also known as the hypophysis.
Placenta (pla sen´ ta) [Gr. plax: flat surface] • The organ found in most mammals that provides for the nourishment of the fetus and elimination of the fetal waste products.
Placental (pla sen´ tal) • Pertaining to mammals of the subclass Eutheria, a group characterized by the presence of a placenta; contains the majority of living species of mammals.
Plankton [Gr. planktos: wandering] • The free-floating organisms of the sea and fresh water that for the most part move passively with the water currents. Consisting mostly of microorganisms and small plants and animals. (Contrast with nekton.)
Plant • A member of the kingdom Plantae. Multicellular, gaining its nutrition by photosynthesis.
Planula (plan´ yew la) [L. planum: something flat] • The free-swimming, ciliated larva of the cnidarians.
Plaque (plack) [Fr.: a metal plate or coin] • (1) A circular clearing in a turbid layer (lawn) of bacteria growing on the surface of a nutrient agar gel. Produced by successive rounds of infection initiated by a single bacteriophage. (2) An accumulation of prokaryotic organisms on tooth enamel. Acids produced by the metabolism of these microorganisms can cause tooth decay.
Plasma (plaz´ muh) [Gr. plassein: to mold] • The liquid portion of blood, in which blood cells and other particulates are suspended.
Plasma cell • An antibody-secreting cell that developed from a B cell. The effector cell of the humoral immune system.
Plasma membrane • The membrane that surrounds the cell, regulating the entry and exit of molecules and ions. Every cell has a plasma membrane.
Plasmid • A DNA molecule distinct from the chromosome(s); that is, an extrachromosomal element. May replicate independently of the chromosome.
Plasmodesma (plural: plasmodesmata) [Gr. plasma: formed or molded + desmos: band] • A cytoplasmic strand connecting two adjacent plant cells.
Plasmolysis (plaz mol´ i sis) • Shrinking of the cytoplasm and plasma membrane away from the cell wall, resulting from the osmotic outflow of water. Occurs only in cells with rigid cell walls.
Plastid • Organelle in plants that serves for food manufacture (by photosynthesis) or food storage; bounded by a double membrane.
Platelet • A membrane-bounded body without a nucleus, arising as a fragment of a cell in the bone marrow of mammals. Important to blood-clotting action.
Pleiotropy (plee´ a tro pee) [Gr. pleion: more] • The determination of more than one character by a single gene.
Pleural membrane [Gk. pleuras: rib, side] • The membrane lining the outside of the lungs and the walls of the thoracic cavity. Inflammation of these membranes is a condition known as pleurisy.
Podocytes • Cells of Bowman’s capsule of the nephron that cover the capillaries of the glomerulus, forming filtration slits.
Poikilotherm (poy´ kill o therm) [Gr. poikilos: varied + therme: heat] • An animal whose body temperature tends to vary with the surrounding environment. (Contrast with homeotherm, heterotherm.)
Point mutation • A mutation that results from a small, localized alteration in the chemical structure of a gene. Such mutations can give rise to wild-type revertants as a result of reverse mutation. In genetic crosses, a point mutation behaves as if it resided at a single point on the genetic map. (Contrast with deletion.)
Polar body • A nonfunctional nucleus produced by meiosis, accompanied by very little cytoplasm. The meiosis which produces the mammalian egg produces in addition three polar bodies.
Polar molecule • A molecule in which the electric charge is not distributed evenly in the covalent bonds.
Polarity • In development, the difference between one end and the other. In chemistry, the property that makes a polar molecule.
Pollen [L.: fine powder, dust] • The fertilizing element of seed plants, containing the male gametophyte and the gamete, at the stage in which it is shed.
Pollination • Process of transferring pollen from the anther to the receptive surface (stigma) of the ovary in plants.
Poly- [Gr. poly: many] • A prefix denoting multiple entities.
Polygamy [Gr. poly: many + gamos: marriage] • A breeding system in which an individual acquires more than one mate. In polyandry, a female mates with more than one male, in polygyny, a male mates with more than one female.
Polygenes • Multiple loci whose alleles increase or decrease a continuously variable phenotypic trait.
Polymer • A large molecule made up of similar or identical subunits called monomers. (Contrast with monomer, oligomer.)
Polymerase chain reaction (PCR) • A technique for the rapid production of millions of copies of a particular stretch of DNA.
Polymerization reactions • Chemical reactions that generate polymers by means of condensation reactions.
Polymorphism (pol´ lee mor´ fiz um) [Gr. poly: many + morphe: form, shape] • (1) In genetics, the coexistence in the same population of two distinct hereditary types based on different alleles. (2) In social organisms such as colonial cnidarians and social insects, the coexistence of two or more functionally different castes within the same colony.
Polyp • The sessile, asexual stage in the life cycle of most cnidarians.
Polypeptide • A large molecule made up of many amino acids joined by peptide linkages. Large polypeptides are called proteins.
Polyphyletic group • A group containing taxa, not all of which share the most recent common ancestor.
Polyploid (pol´ lee ploid) • A cell or an organism in which the number of complete sets of chromosomes is greater than two.
Polysaccharide • A macromolecule composed of many monosaccharides (simple sugars). Common examples are cellulose and starch.
Polysome • A complex consisting of a threadlike molecule of messenger RNA and several (or many) ribosomes. The ribosomes move along the mRNA, synthesizing polypeptide chains as they proceed.
Polytene (pol´ lee teen) [Gr. poly: many + taenia: ribbon] • An adjective describing giant interphase chromosomes, such as those found in the salivary glands of fly larvae. The characteristic, reproducible pattern of bands and bulges seen on these chromosomes has provided a method for preparing detailed chromosome maps of several organisms.
Pons [L. pons: bridge] • Region of the brain stem anterior to the medulla.
Population • Any group of organisms coexisting at the same time and in the same place and capable of interbreeding with one another.
Population density • The number of individuals (or modules) of a population in a unit of area or volume.
Population genetics • The study of genetic variation and its causes within populations.
Population structure • The proportions of individuals in a population belonging to different age classes (age structure). Also, the distribution of the population in space.
Portal vein • A vein connecting two capillary beds, as in the hepatic portal system.
Positive control • The situation in which a regulatory macromolecule is needed to turn transcription of structural genes on. In its absence, transcription will not occur.
Positive cooperativity • Occurs when a molecule can bind several ligands and each one that binds alters the conformation of the molecule so that it can bind the next ligand more easily. The binding of four molecules of O2 by hemoglobin is an example of positive cooperativity.
Postabsorptive period • When there is no food in the gut and no nutrients are being absorbed.
Postsynaptic cell • The cell whose membranes receive the neurotransmitter released at a synapse.
Predator • An organism that kills and eats other organisms. Predation is usually thought of as involving the consumption of animals by animals, but it can also mean the eating of plants.
Presynaptic excitation/inhibition • Occurs when a neuron modifies activity at a synapse by releasing a neurotransmitter onto the presynaptic nerve terminal.
Prey [L. praeda: booty] • An organism consumed as an energy source.
Primary active transport • Form of active transport in which ATP is hydrolyzed, yielding the energy required to transport ions against their concentration gradients. (Contrast with secondary active transport.)
Primary growth • In plants, growth produced by the apical meristems. (Contrast with secondary growth.)
Primary producer • A photosynthetic or chemosynthetic organism that synthesizes complex organic molecules from simple inorganic ones.
Primary succession • Succession that begins in an areas initially devoid of life, such as on recently exposed glacial till or lava flows.
Primary structure • The specific sequence of amino acids in a protein.
Primary wall • Cellulose-rich cell wall layers laid down by a growing plant cell.
Primate (pry´ mate) • A member of the order Primates, such as a lemur, monkey, ape, or human.
Primer • A short, single-stranded segment of DNA serving as the necessary starting material for the synthesis of a new DNA strand, which is synthesized from the 3´ end of the primer.
Primitive streak • A line running axially along the blastodisc, the site of inward cell migration during formation of the three-layered embryo. Formed in the embryos of birds and fish.
Primordium [L. primordium: origin] • The most rudimentary stage of an organ or other part.
Principle of continuity • States that because life probably evolved from nonlife by a continuous, gradual process, all postulated stages in the evolution of life should be derivable from preexisting states. (Compare with signature principle.)
Pro- [L.: first, before, favoring] • A prefix often used in biology to denote a developmental stage that comes first or an evolutionary form that appeared earlier than another. For example, prokaryote, prophase.
Probe • A segment of single stranded nucleic acid used to identify DNA molecules containing the complementary sequence.
Procambium • Primary meristem that produces the vascular tissue.
Progesterone [L. pro: favoring + gestare: to bear] • A vertebrate female sex hormone that maintains pregnancy.
Prokaryotes (pro kar´ ry otes) [L. pro: before + Gk. karyon: kernel, nucleus] • Organisms whose genetic material is not contained within a nucleus. The bacteria. Considered an earlier stage in the evolution of life than the eukaryotes.
Prometaphase • The phase of nuclear division that begins with the disintegration of the nuclear envelope.
Promoter • The region of an operon that acts as the initial binding site for RNA polymerase.
Proofreading • The correction of an error in DNA replication just after an incorrectly paired base is added to the growing polynucleotide chain.
Prophage (pro´ fayj) • The noninfectious units that are linked with the chromosomes of the host bacteria and multiply with them but do not cause dissolution of the cell. Prophage can later enter into the lytic phase to complete the virus life cycle.
Prophase (pro´ phase) • The first stage of nuclear division, during which chromosomes condense from diffuse, threadlike material to discrete, compact bodies.
Prostaglandin • Any one of a group of specialized lipids with hormone-like functions. It is not clear that they act at any considerable distance from the site of their production.
Prosthetic group • Any nonprotein portion of an enzyme.
Protease (pro´ tee ase) • See proteolytic enzyme.
Protein (pro´ teen) [Gr. protos: first] • One of the most fundamental building substances of living organisms. A long-chain polymer of amino acids with twenty different common side chains. Occurs with its polymer chain extended in fibrous proteins, or coiled into a compact macromolecule in enzymes and other globular proteins.
Proteolytic enzyme • An enzyme whose main catalytic function is the digestion of a protein or polypeptide chain. The digestive enzymes trypsin, pepsin, and carboxypeptidase are all proteolytic enzymes (proteases).
Protist • Those eukaryotes not included in the kingdoms Animalia, Fungi, or Plantae.
Protobiont • Aggregates of abiotically produced molecules that cannot reproduce but do maintain internal chemical environments that differ from their surroundings.
Protoderm • Primary meristem that gives rise to epidermis.
Proton (pro´ ton) [Gr. protos: first] • One of the three most fundamental particles of matter, with mass approximately 1 amu and an electrical charge of +1.
Proto-oncogenes • The normal alleles of genes possessing oncogenes (cancer-causing genes) as mutant alleles. Proto-oncogenes encode growth factors and receptor proteins.
Protostome • One of the major lineages of animal evolution. Characterized by spiral, determinate cleavage of the egg, and by schizocoelous development. (Compare with deuterostome.)
Prototroph (pro´ tow trofe´) [Gr. protos: first + trophein: to nourish] • The nutritional wild type, or reference form, of an organism. Any deviant form that requires growth nutrients not required by the prototrophic form is said to be a nutritional mutant, or auxotroph.
Protozoa • A group of single-celled organisms classified by some biologists as a single phylum; includes the flagellates, amoebas, and ciliates. This textbook follows most modern classifications in elevating the protozoans to a distinct kingdom (Protista) and each of their major subgroups to the rank of phylum.
Proximal • Near the point of attachment or other reference point. (Contrast with distal.)
Pseudocoelom • A body cavity not surrounded by a peritoneum. Characteristic of nematodes and rotifers.
Pseudogene • A DNA segment that is homologous to a functional gene but contains a nucleotide change that prevents its expression.
Pseudoplasmodium [Gr. pseudes: false + plasma: mold or form] • In the cellular slime molds such as Dictyostelium, an aggregation of single amoeboid cells. Occurs prior to formation of a fruiting structure.
Pseudopod (soo´ do pod) [Gr. pseudes: false + podos: foot] • A temporary, soft extension of the cell body that is used in location, attachment to surfaces, or engulfing particles.
Pulmonary • Pertaining to the lungs.
Punctuated equiilibrium • An evolutionary pattern in which periods of rapid change are separated by longer periods of little or no change.
Pupa (pew´ pa) [L.: doll, puppet] • In certain insects (the Holometabola), the encased developmental stage that intervenes between the larva and the adult.
Pupil • The opening in the vertebrate eye through which light passes.
Purine (pure´ een) • A type of nitrogenous base. The purines adenine and guanine are found in nucleic acids.
Purkinje fibers • Specialized heart muscle cells that conduct excitation throughout the ventricular muscle.
Pyramid of biomass • Graphical representation of the total body masses at different trophic levels in an ecosystem.
Pyramid of energy • Graphical representation of the total energy contents at different trophic levels in an ecosystem.
Pyrimidine (peer im´ a deen) • A type of nitrogenous base. The pyrimidines cytosine, thymine, and uracil are found in nucleic acids.
Pyruvate • A three-carbon acid; the end product of glycolysis and the raw material for the citric acid cycle.