P generation • Parental generation. The individuals that mate in a genetic cross. Their immediate offspring are the F1 generation.
P generation • 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 modified nerve ending that senses touch and vibration.
Pair rule genes • Segmentation genes that divide the Drosophila larva into two segments each.
Paleontology (pale´ ee on tol´ oh jee) [Gr. palaios: ancient + 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.
Para- [Gk. para: akin to, beside] • Prefix indicating association in being along side or accessory to.
Parabronchi • Passages in the lungs of birds through which air flows.
Paracrine • A hormone that acts locally, near the site of its secretion. (Compare with endocrine gland.)
Parallel evolution • Evolutionary patterns that exist in more than one lineage. Often the result of underlying developmental processes.
Parapatric speciation [Gr. para: along side + patria: homeland] • Reproductive isolation between subpopulations arising from some non-geographic but physical condition, such as soil nutrient content. (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.
Parasympathetic nervous system • A portion of the autonomic (involuntary) nervous system. (Contrast with sympathetic nervous system.)
Parathyroid hormone • 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) • A plant tissue composed of relatively unspecialized cells without secondary walls.
Parsimony • The principle of preferring the simplest among a set of plausible explanations of any phenomenon.
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.
Passive transport • Diffusion across a membrane; may or may not require a channel or carrier protein. Contrast to active transport.
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 + genesis: source] • An organism that causes disease.
Pathway • Any sequence of enzyme-catalyzed, chemical reactions in which the product(s) of a reaction is/are the substrate(s) for another reaction.
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 within a family.
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 bond between amino acid residues in a protein. Formed between a carboxyl group and amino group (CO-NH-) with the loss of water molecules.
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: throughout + annus: year] • Refers to a plant that survives 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 single cycle in a cyclical event, such as a circadian rhythm.
Peripheral membrane protein • Membrane protein not embedded in the bilayer. Contrast to integral membrane protein.
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 body cavity among coelomate animals.
Permease • A membrane protein that specifically transports a compound or family of compounds across the membrane.
Permian • A period in the Paleozoic era in Earth's geologic history (290-248 mya).
Peroxisome • An organelle that houses reactions in which toxic peroxides are formed. The peroxisome isolates these peroxides from the rest of the cell.
Petal [Gk. petalon: spread out] • 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. A virus that infects bacteria.
Phagocyte [Gk. phagein: to eat + kystos: sac] • A white blood cell that ingests microorganisms by endocytosis.
Pharynx [Gr. pharynx: 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 resulting from both genetic and environmental factors. (Contrast with genotype.)
Phenotypic plasticity • Refers to the fact that the phenotype of a developing organism is determined by a complex series of processes that are affected by both its genotype and its environment.
Pheromone (feer´ o mone) [Gr. pheros: 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 tissue that transports sugars and other solutes from sources to sinks. 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 • Lipids containing a phosphate group; 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 chemolithotroph, chemoheterotroph, photoheterotroph.)
Photoheterotroph • An organism that obtains energy from light but must obtain its carbon from organic compounds. (Contrast with chemolithotroph, chemoheterotroph, photoautotroph.)
Photon (foe´ ton) [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.
Photoreceptor • (1) A 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 from 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.
Phototropism [Gr. photos: light + trope: 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) • 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 pathogens, produced by plants in response to fungal or bacterial infection.
Phytochrome (fy´ tow krome) [Gr. phyton: plant + chroma: color] • A plant pigment regulating a large number of developmental and other phenomena in plants.
Pigment • A substance that absorbs visible light.
Pilus (pill´ us) [L. pilus: hair] • A surface appendage by which some bacteria adhere to one another during conjugation.
Pistil [L. pistillum: pestle] • The 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.
Pits • Recessed cavities in the cell walls of a plant vascular element where only the primary wall is present. facilitating the movement of sap between cells.
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.
Plant • A member of the kingdom Plantae. Multicellular, gaining its nutrition by photosynthesis.
Planula (plan´ yew la) [L. planum: 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. (2) An accumulation of prokaryotic organisms on tooth enamel. Acids produced by these microorganisms can cause tooth decay. (3) A region of arterial wall invaded by fibroblasts and fatty deposits (see atherosclerosis).
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. plassein: to mold + 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.
Plastoquinone • A mobile electron carrier within the thylakoid membrane of the chloroplast linking photosystems I and II of photosynthesis.
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.
Pleistocene • One of two epochs comprising the current geologic period, the Quaternary.
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 + thermos: 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; can revert to wild type. (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.
Polar nuclei • In flowering plants, the two nuclei in the central cell of the megagametophyte; following fertilization they give rise to the endosperm.
Polarity • In development, the difference between one end and the other. In chemistry, the property that makes a polar molecule.
Pollen [L. pollin: fine, powdery flour] • In seed plants, the microscopic grains containing the male gametophyte (microgametophyte) and gamete (microspore).
Pollination • The process of transferring pollen from the anther to the receptive surface (stigma) of the pistil in plants.
Poly- [Gr. poly: many] • A prefix denoting multiple entities.
Poly(A) tail • A long sequence of adenine nucleotides (50-250) added after transcription to the 3' end of most eukaryotic mRNAs.
Polygenes • Multiple loci whose alleles increase or decrease a continuously variable phenotypic trait.
Polymer [Gr. poly: many + meros: unit] • A large molecule made up of similar or identical subunits called monomers. (Contrast with monomer, oligomer.)
Polymerase chain reaction (PCR) • An enzymatic technique for the rapid production of millions of copies of a particular stretch of DNA.
Polymerization reactions • Chemical reactions that generate polymers by linking monomers.
Polymorphism (pol´ lee mor´ fiz um) [Gr. poly: many + morphe: form, shape] • In genetics, the coexistence in the same population of two distinct hereditary types based on different alleles.
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 (polyribosome) • 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 pattern of bands and bulges seen on these chromosomes 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 bottleneck • See bottleneck.
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 [L. portal: gate] • A vein connecting two capillary beds, as in the hepatic portal system.
Positional cloning • A technique for isolating a gene associated with a disease on the basis of its approximate chromosomal location.
Positional information • Signals by which genes regulate cell functions to locate cells in a tissue during development.
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.
Post [L. postere: behind, following after] • Prefix denoting something that comes after.
Postabsorptive period • When there is no food in the gut and no nutrients are being absorbed.
Postsynaptic cell • The cell whose membranes receive neurotransmitter after its release by another cell (the presynaptic cell) at a synapse.
Postzygotic reproductive barrier • Any mechanism that prevents the hybrid gametes of two different species from developing into viable reproductive adults. (Contrast with prezygotic reproductive barrier.)
Precambrian • An era in Earth's geologic history.
Pre-mRNA (precursor mRNA) • Initial gene transcript before it is modified to produce functional mRNA. Also known as the primary transcript.
Predator • An organism that kills and eats other organisms.
Pressure flow model • An effective model for phloem transport in angiosperms. It holds that sieve element transport is driven by an osmotically driven pressure gradient between source and sink.
Pressure potential • The hydrostatic pressure of an enclosed solution in excess of the surrounding atmospheric pressure.
Presynaptic excitation/inhibition • Occurs when a neuron modifies activity at a synapse by releasing a neurotransmitter onto the presynaptic nerve terminal.
Prezygotic reproductive barrier • Any barrier to gene exchange that operates before mating. (Contrast with postzygotic reproductive barrier.)
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 embryonic organizer • See Organizer.
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 area 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: a prosimian, monkey, ape, or human.
Primer • A short, single-stranded segment of DNA that is 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.
Prion • An infectious protein that can proliferate by converting other proteins.
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 and archaea. 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.
Proteasome • In the eukaryotic cytoplasm, a huge protein structure that binds to and digests cellular proteins that have been tagged by ubiquitin.
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.
Proteolysis [protein + Gk. lysis: break apart] • An enzymatic digestion of a protein or polypeptide.
Proteome • The total of the different proteins that can be made by an organism. Because of alternate splicing of pre-mRNA, the number of proteins that can be made is usually much larger than the number of protein-coding genes present in the organism's genome.
Protobiont [Gr. protos: first, before + bios: life] • 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 the plant epidermis.
Proton (pro´ ton) [Gr. protos: first, before] • (1) A subatomic particle with a single positive charge. The number of protons in the nucleus of an atom determine its element. (2) A hydrogen ion, H+.
Proton pump • An active transport system that uses ATP energy to move hydrogen ions across a membrane generating an electric potential (voltage).
Proton motive force • A force generated across a membrane expressed in millivolts having two components: a chemical potential (difference in proton concentration) plus an electrical potential due to the electrostatic charge on the proton.
Proto-oncogenes • The normal alleles of genes possessing oncogenes (cancer-causing genes) as mutant alleles. Proto-oncogenes encode growth factors and receptor proteins.
Protostome • [Gr. protos: first + stoma: mouth] • 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 (pro to zoe´ ah) [Gk. protos: first, before + zoon: animal) • A term formerly used for a single polyphyletic phylum of single-celled eukaryotic organisms including the flagellates, amoebas, and ciliates. This book does not use the term.
Provirus • Viral DNA inserted into a bacterial host genome. (See Lysogenic cycle.)
Proximal • Near the point of attachment or other reference point. (Contrast with distal.)
Pseudocoelom [Gr. pseudes: false] • A body cavity not surrounded by a peritoneum. Characteristic of nematodes and rotifers.
Pseudogene [Gr. pseudes: false] • A DNA segment that is homologous to a functional gene but contains a nucleotide change that prevents its expression.
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 [L. pulmo: lung] • Pertaining to the lungs.
Punctuated equilibrium • An evolutionary pattern in which periods of rapid change are separated by longer periods of little or no change.
Pupa (pew´ pa) [L. pupa: doll, puppet] • In certain insects (the Holometabola), the encased developmental stage between the larva and the adult.
Pupil • The opening in the vertebrate eye through which light passes.
Purine (pure´ een) • One of the types of nitrogenous bases. The purines adenine and guanine are found in nucleic acids. (Contrast with pyrimidine.)
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 (per 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.