Life Science Dictionary P


P generation: The parent individuals from which offspring are derived in studies of inheritance; P stands fro paternal.
P site: One of a ribosome’s three binding sites for tRNA during translation. The P site holds the tRNA carrying the growing peptide chain. P stands for peptidyl tRNA.
p53 gene: The “guardian angel of the genome”, a gene that is expressed when a cell’s DNA is damaged. Its product, p53 protein, functions as a transcription factor for several genes.
Pacemaker: A specialized region of the right atrium of the mammalian heart that sets the rate of contraction; also called the sinoatrial (SA) node.
Paedomorphosis: The retention in an adult organism of the juvenale features of its evolutionary ancestors.
Pain receptor: A kind of interoreceptor that detects pain, also called nociceptor.
Paleoanthropology: The study of human origins and evolution.
Paleontological species concept: Definition of species based on morphological differences known only from the fossil record.
Paleontology: The scientific study of fossils.
Paleoproterozoic: Division of time from 2500 to 1600 Mya.
Palisade mesophyll: one or more layers of elongated photosynthetic cells on the upper part of a leaf; also called palisade parenchyma.
Pancreas: A gland with dual functions: The nonendocrine portion secretes digestive enzymes and an alkaline solution into the small intestine via a duct; the endocrine portion secretes the hormones glucagon and insulin into the blood.
Pangea: The supercontinent formed near the end of the Paleozoic era when plate tectonic movements brought all the and masses of Earth together.
Panmictic: Describes a population in which every individual has the same chance of mating with every other: in other words, where there is no population structure.
Parabasalid: A protist such as trichomonad, with modified mitochondria.
Parabronchus: A site of gas exchange in bird lungs. Parabronchi (plural) allow air to flow past the respiratory surface in just one direction.
Paracentric inversion: A mutation that involves a chromosomal inversion that does not span the centromere.
Parallel evolution: The process by which features that once were different become similar by experiencing the same changes in different evolutionary lineages.
Paralogous genes: Genes that are homologous (share a common ancestry) and have diverged from each other after gene duplication events (e.g. a- and b-globins). Contrast this with orthologous genes.
Paraphyletic: Pertaining to a grouping of species that consists of an ancestral species and some, but not all, of its descendants.
Parapratic: A geographic distribution in which different types are found in different places and meet only in a narrow zone.
Parapratic speciation: The evolution of new species within a spatially extended population in the presence of gene flow.
Paraphyletic: Describes a group of organisms or genes that share a common ancestor to the exclusion of all other entities but in which some members of the group are excluded.
Parareptile: First major group of reptiles to emerge, mostly large, stocky quadrupedal herbivores; die out in the late Triassic period.
Parasite: An organism that benefits by living in or on another organism at the expense of the host.
Parasitism: A symbiotic relationship in which the symbiont (parasite) benefits at the expense of the host by living either within the host (as an endoparasite) or outside of the host (as an ectoparasite).
Parasitoidism: A type of parasitism in which an insect lays eggs on or in a living host; the larvae then feed on the body of the host, eventually killing it.
Parasympathetic division: One of the three divisions of the autonomic nervous system; generally enhances body activities that gain and conserve energy, such as digestion and reduced heart rate.
Parathyroid gland: Any of four small endocrine glands, embedded in the surface of the thyroid gland, that secrete parathyroid hormone.
Parathyroid hormone (PTH):  A hormone secreted by the parathyroid glands that raises blood calcium level by promoting calcium release from bone and calcium retention by the kidneys.
Parazoan: Animal belonging to a grade of organization lacking true tissues; a sponge (Phylum Porifera.
Parenchyma cell: A relatively unspecialized plant cell type that carries out most of the metabolism, synthesizes and stores organic products, and develops into a more differentiated cell type.
Parental type: An offspring with a phenotype that matches one of the parental phenotypes.
Parkinson’s disease: A motor disorder caused by a progressive brain disease and characterized by difficulty in initiating movements, slowness of movement, and rigidity.
Parsimony: General approach to evolutionary reconstructions in which the goal is to identify theories (e.g. evolutionary branching patterns) that require the fewest numbers of evolutionary events and thus might be considered the simplest.
Parthenogenesis: The production of offspring from unfertilized eggs.
Partial pressure: A measure of the concentration of one gas in a mixture of gases; the pressure exerted by a particular gas in a mixture of gases. For example the pressure exerted by oxygen in air.
Parturition: The expulsion of a baby from the mother, also called birth.
Passive immunity: Short-term immunity conferred by the administration of ready-made antibodies or transfer of maternal antibodies to a fetus or nursing infant; last only a few weeks or months because the immune system has not been stimulated by antigens.
Passive transport: The diffusion of a substance across a biological membrane.
Patchiness: Localized variation in environmental conditions within an ecosystem, arranged spatially into a complex of discrete areas that may be characterized by distinctive groups of species or ecosystem processes.
Pathogen: A disease causing agent.
Pathogenicity islands: Contiguous sections of a pathogen’s genome that contain a disproportionate (relative to number of base pairs) number of factors that cause pathogenicity
Patrilineal: Inherited from the father (e.g. the Y chromosome in mammals).
Pattern formation: The ordering of cells  into specific three-dimensional structures, an essential part of shaping an organism and its individual parts during development.
Payoff: In an evolutionary context, the increase in fitness due to a contest.
Peat: Extensive deposits of undecayed organic material formed primarily from the wetland moss Sphagnum.
Pedigree: The family relationship between individuals in a sexual population.
Pelage: The hair, fur, wool, or other soft covering of a mammal.
Pelagic zone:  The area of the ocean past the continental shelf, with areas of open water often reaching to very great depths.
PEP carboxylase: An enzyme that adds carbon dioxide to phosphoenolpyruvate (PEP) to form oxaloacetate.
Pepsin: An enzyme present in gastric juice that begins the hydrolysis of proteins.
Pepsinogen: The inactive form of pepsin that is first secreted by specialized (chief) cells located in the gastric pits of the stomach.
Peptide bond: The covalent bond between two amino acid units, formed by dehydration reaction.
Peptidoglycan: 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: A flowering plant that lives for many years.
Pericarp: The thickened wall of a fruit.
Pericentric inversion: A mutation involving a chromosomal inversion that spans the centromere.
Pericycle:  The outermost layer of the vascular cylinder of a root, where lateral roots originate.
Periderm: The protective coat that replaces the epidermis in plants during secondary growth, formed of the cork and cork cambium.
Pericentriolar material: A mixture of hundreds of proteins which help to nucleate and anchor microtubules.
Periodic table of the elements: A chart of the chemical elements, arranged in three rows, corresponding to the number of electron shells in their atoms.
Peripheral nervous system (PNS): The sensory and motor neurons that connect to the central nervous system.
Peripheral protein:  A protein appendage loosely bound to the surface of a membrane and not embedded in the lipid bilayer.
Peripheral resistance: The impedance of blood flow by the arterioles.
Peristalsis: (1) Rhythmic waves of contraction of smooth muscle that push food along the digestive tract. (2) A type of movement on land produced by rhythmic waves of muscle contraction passing from front to back, as in many annelids.
Peristome: The upper part of the moss capsule (sporangium) often specialized for gradual spore discharge.
Peritubular capillaries: The network of tiny blood vessels that surrounds the proximal and distal tubules in the kidney.
Permafrost: A permanently frozen stratum below the arctic tundra.
Peroxisome: A membrane bound organelle in eukaryotes involved in detoxification. It contains enzymes that transfer hydrogen from various substrates to oxygen, producing and then degrading hydrogen peroxide.
Personalized medicine: Use of information on an individual’s genotype to improve its health.
Petal: A modified leaf of a flowering plant. petals are often the colorful parts of a flower that advertise it to insects and other pollinators.
Petiole: The stalk of a leave, which joins the leaf to a node of the stem.
pH: A measure of hydrogen ion concentration equal to -log [H+] and raging in value from 0 to 14.
Phage: A virus that infects bacteria, also called bacteriophage.
Phagocytosis: A type of endocytosis involving large, particulate substances, accomplished mainly by macrophages, neutrophils, and dendritic cells.
Pharyngeal clefts: In chordate embryos, grooves that separate a series of pouches along the sides of the pharynx and may develop into pharyngeal slits.
Pharynx: (1) An area in the vertebrate throat where air and food passage cross. (2) In flat worms, the muscular tube that protrudes from the ventral side of the worm and ends in the mouth.
Phase-change: A shift from one developmental phase to another.
Phenocopy: A phenotype induced by an environmental agent (e.g. a temperature shock), which is similar to that produced by a genetic mutation.
Phenogram: A branching diagram that links entities by estimates of overall similarity.
Phenotype: The appearance of an organism resulting from the interaction between the genotype (genes) and the environment. The observed characteristics of an individual.
Phenotypic polymorphism:  The existence of two or more distinct morphs, each represented in a population in high enough frequencies to be readily noticeable.
Pheromone: In animals and fungi, a small volatile chemical that functions in communication and that in animals acts much like a hormone in influencing physiology and behavior.
Phloem: Vascular plant tissue consisting of living cells arranged into elongated tubes that transport sugar and other organic nutrients throughout the plant.
Phoronids: A tube-dwelling marine lophophorate.
Phosphate group: A functional group important in energy transfer.
Phospholipid: A molecule that is a constituent of the inner bilayer of biological membranes, having a polar hydrophilic head and a nonpolar, hydrophobic tail.
Phosphorylated: Referring to a molecule that has been the recipient of a phosphate group.
Phosphorite: A sedimentary rock rich in phosphate.
Photic zone: The narrow top slice of the ocean, where light permeates sufficiently for photosynthesis to occur.
Photoautotroph:  An organism that harvest light energy to drive the synthesis of organic compounds from carbon dioxide.
Photoheterotroph: An organism that uses light to generate ATP but that must obtain carbon from in organic form.
Photomorphogenesis: Effects of light on plant morphology.
Photon: A quantum, or discrete amount of light energy.
Photoperiod: The interval in a 24-hour period during which a plant or animal is exposed to light. The relative length of night and day.
Photoperiodism: A physiological response to photoperiod.
Photophosphorylation: 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.
Photopsin: One of a family of visual pigments in the cones of the vertebrate eye that absorbs  bright, colored light.
Photoreceptor: An electromagnetic receptor that detects radiation known as visible light.
Photorespiration: 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: 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: Light-capturing unit located in the thylakoid membrane of the chloroplast, consist of a reaction center surrounded by numerous light-harvesting complexes.
Photosystem I: One of two light-capturing units in a chloroplast’s thylakoid membrane: it has two molecules of P700 chlorophyll a at its reaction center.
Photosystem II: One of two light-capturing units in a chloroplast’s thylakoid membrane: it has two molecules of P680 chlorophyll a at its reaction center.
Phototropism:  Growth of a plant shoot toward or away from light.
Phragmoplast: An alignment of cytoskeletal elements and Golgi- derived vesicles across the midline of a dividing plant cell.
Phylogenetic anchor: The use of the phylogeny of a gene to infer the organismal source of a small piece of DNA. It is used in metagenomics.
Phylogenetic species concept (PSC): Defining species as a set of organisms with a unique genetic history.
Phylogenetic tree: A diagram showing the a hypothesis about the evolutionary history of organisms or genes.
Phylogeny: The evolutionary history of organisms or genes.
Phylogenomics: Large-scale phylogenetic analysis using genomic data that include hundreds of genes, large sections of genomes, or whole genomes.
Phylogeography: Inference of population history from the genealogy that connects genes sample from different geographical locations. It is often based on genealogies inferred from mitochondrial DNA.
Phylogram: A phylogenetic tree in which the branch lengths are proportional to the evolutionary distance between nodes. It is also known as an additive tree.
Phylotype: The phylogenetic type of an uncultured organism as inferred from analysis of its ribosomal RNA sequence.
Phylum: In classification, the taxonomic category above class.
Physical map: A map that gives the physical location of a genetic variant on the DNA sequence. It contrasts with a genetic map, which is determined using classical genetics.
Physiology: The study of the functions of an organism.
Phytoalexin: An antibiotic, produced by plants, that destroy microorganisms or inhibits their growth.
Phytochromes: A class of light receptors in plants. Mostly absorbing red light, these photoreceptors regulate many plant responses, including seed germination and shade avoidance.
Phytoplankton: Algae and photosynthetic bacteria that drift passively in the pelagic zone of an aquatic environment.
Phytoremediation: An emerging nondestructive technology that seek to cheaply  reclaim contaminated areas by taking advantage of the ability of some plant species to extract heavy metals and other pollutants from the soil and to concentrate them in easily harvested portions of the plant.
Pilus: (plural, pili) A long, hair-like prokaryotic appendage that functions in adherence or in the transfer of DNA during conjugation.
Pineal gland: A small gland on the dorsal surface of the vertebrate forebrain that secretes the hormone melatonin.
Pinocytosis: A type of endocytosis in which the cell ingest extracellular fluid and its dissolved solutes.
Pistil: A single carpel or a group of fused carpels.
Pit: A thinner region in the walls of tracheids and vessels where only primary wall is present.
Pitch: A function of a sound wave’s frequency, or number of vibrations per second, expresses in hertz.
Pith: Ground tissue that is internal to the vascular tissue in a stem; in may monocot roots, parenchyma cells that form the central core of the vascular cylinder.
Pituitary gland: An endocrine gland at the base of the hypothalamus; consist 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: 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 transfer cell: A plant cell that enhances the transfer of nutrients from parent to embryo.
Placoderm:  A member of an extinct class of fishlike vertebrates that had jaws and were enclosed in a tough, outer armor.
Planarian: A free-living flatworm found in unpolluted ponds and streams
Plankton: The aggregate of passively floating, drifting of organisms occurring in a body of water primarily algae and protozoa.
Plantae: The kingdom that consist of multicellular eukaryotes that carry out photosynthesis.
Plasma: The liquid matrix of blood in which the cells are suspended.
Plasma cell:  The antibody secreting effector cell of humoral immunity; arises from antigen-stimulated B cells.
Plasma membrane:  The membrane at the boundary of every cell that acts as a selective barrier, thereby regulating the cell chemical composition.
Plasmid: A genetic element that can replicate autonomously but is usually smaller than the chromosome. It is frequently required only for specialized conditions and is commonly found in bacteria and archaea and sometimes in eukaryotes.
Plasmodesma: An open channel in the cell wall of a plant through which strands of cytosol connect from an adjacent cell.
Plasmodial slime mold: A type of protist that has amoeboid cells, flagellated cells, and a plasmodial feeding stage in its life cycle.
Plasmodium: A single mass of cytoplasm containing many diploid nuclei that forms during the life cycle of some slime molds
Plasmogamy: The fusion of the cytoplasm of cells from two individuals, occurs as one stage of syngamy
Plasmolysis: 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.
Plasticity: Flexible, amenable. In biology it refers to the ability to respond to environmental challenges.
Plastid: A specialized organelle found in plants, algae, and a variety of single-celled eukaryotes. It comes in a variety of forms including chloroplasts and apicoplasts.
Plate tectonics: The mechanism by which the plates that makes up the surface of the earth interact with one another, including the formation and subduction of oceanic crust.
Platelet: A small enucleated blood cell important in blood clotting; derived from large cells in the bone marrow.
Pleiotropy: When one allele affects two or more traits.
Pleistocene: The geological period between 1.8 Mya and about 11,000 years ago. It includes major glaciations.
Ploidy: The number of copies of each chromosome in the organism.
Plumage: The entirely feathery covering of a bird.
Pluripotent: Refers to a cell with the developmental capacity to generate any of the three germ layers of the embryo proper (endoderm, mesoderm, and ectoderm).
Point mutation: A mutation that involves a change from one base to another.
Polar covalent bond: A 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.
Polarity: A lack of symmetry. Structural differences in opposite ends of an organism or structure.
Pollen grains: The structures that contain the male gametophyte of seed plants.
Pollination: The transfer of pollen to the parts of a seed plant containing the ovules, a process that is a prerequisite for fertilization.
Policing: Where selfish behavior is prevented by other individuals.
Poly-A-tail: The modified end of the 3′ end of an mRNA molecule consisting of the addition of some 50 to 250 adenine nucleotides.
Polyandrous: Describes females that mate with many males.
Polygenic: Influenced by many genes.
Polymer: A long molecule consisting of many similar or identical monomers linked together.
Polymerase chain reaction (PCR): A method of amplifying as little as a single copy of a specific nuclei acid molecule, which is recognized because it binds to a pair of primer sequences.
Polymorphism: 1. The presence of multiple alleles or inherited phenotypes at appreciable frequencies. 2. The existence of an organism in several forms or color varieties.
Polynucleotide: A polymer consisting of many nucleotide monomers; the two types are DNA and RNA.
Polyp: The sessile variant of the cnidarian body plan. The alternate form is the medusa.
Polypeptide: A polymer (chain) of many amino acids linked together by peptide bonds.
Polyphyletic: Describes a group of organisms or genes that do not share a common ancestor.
Polyploid: A cell or chromosome carrying more than two genomes (e.g. triploid, tetraploid).
Polyribosome (polysome): An aggregation of several ribosomes attached to one messenger RNA (mRNA) molecule.
Polysaccharide: A polymer of up to over a thousand monosaccharides formed by dehydration reactions.
Polyspermy: Fertilization of an egg by more than one sperm.
Polytene chromosome: A chromosome that consist of large numbers of parallel DNA strand, making their structure clearly visible.
Polytomy: A portion of a phylogenetic tree in which more than two branches emerge from a single node.  It can be used to represent radiation events as well as ambiguities in knowledge.
Pons: Portion of the brain that participates in certain automatic, homeostatic functions such as regulating the breathing centers in the medulla.
Population: A localized group of individuals that belong to the same biological species (that are capable of interbreeding and producing fertile progeny).
Population bottleneck: A brief reduction in population size, which causes a burst of random genetic drift. It may be caused by a founder effect.
Population dynamics: The study of how complex  interactions between biotic and abiotic factors  influence variations in population size.
Population ecology: The study of populations in relation to the environment, including environmental influences on population density and distribution, age structure, and variations in population size.
Population genetics: Study of the process that change genetic composition of populations.
Population structure: Any deviation from the ideal state of a single panmictic population. For example, individuals may be more likely to mate with those in the same place or in the same microhabitat.
Positional cloning: Process by which data from genetic crosses are used to identify a DNA fragment that contains a desired gene sequence.
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.
Positive pressure breathing: A breathing system in which air is forced into the lungs.
Posterior: Pertaining to the rear, or tail end, of a bilateral symmetric animal.
Posterior pituitary: Also called neurohypophysis; an extension of the hypothalamus composed of nervous tissue that secretes oxytocin and antidiuretic hormone made in the hypothalamus; a temporary site for these hormones.
Postsynaptic cell: The target of a synapse.
Postzygotic isolation: Reproductive isolation that acts after production of an F1 zygote through hybrid inviability or sterility.
Potential energy: The energy stored by matter as a result of its location or spatial arrangement.
Prebiotic synthesis: The naturally occurring synthesis of organic compounds before there was life on earth.
Predator: An animal that lives by predation, a mode of life in which food is primarily obtained by killing and consuming other animals.
Pregnancy: The condition of carrying one or more embryos in the uterus.
Prenylation: The addition of isoprenoid groups to proteins. It is common in many eukaryotes.
Preprophase band: Microtubules in the cortex (outer cytoplasm) of a cell that are concentrated into a ring.
Presynaptic cell: The transmitting cell at a synapse.
Prezygotic barrier: A reproductive barrier that impedes mating between species.
Prey: An animal that is taken by a predator as food.
Prezygotic isolation: Reproductive isolation that stops production of an F1 zygote by preventing cross-mating.
Priapulid: Member of a phylum of worm-like animals (Phylum Priapulida).
Primary cell wall: A relatively thin and flexible layer first secreted by a young plant cell.
Primary consumer: An herbivore; an organism in the trophic level of an ecosystem that eats plants or algae.
Primary contact: When populations have been in contact throughout their divergence. It contrasts with secondary contact.
Primary electron acceptor: A specialized molecule sharing the reaction center with the pair of reaction-center chlorophyll a molecules; it accepts an electron from one of these two chlorophylls
Primary growth: Growth produced by apical meristems, lengthening stems and roots.
Primary immune response: The initial acquired immune response to an antigen, which appears after a lag of about 10 to 17 days.
Primary oocyte: A diploid cell, in prophase I of meiosis, that can be hormonally triggered to develop into an ovum.
Primary plant body: The tissues produced by apical meristems, which lengthen stems and roots.
Primary producers: Any green plant or microorganism that can convert light or chemical energy into organic matter.
Primary production: The amount of light energy converted to chemical energy (organic compounds) by autotrophs in an ecosystem during a given time period.
Primary structure:  The level of protein structure referring to the specific sequence of amino acids.
Primary transcript: An initial RNA transcript before 5′ capping, intron splicing, and poly-A tail addition; also called pre-mRNA when transcribed from a protein-coding gene.
Primary visual cortex: The destination in the occipital lobe of the cerebrum for most of the axons from the lateral geniculate nuclei.
Primase: Enzyme used to initiate replication of DNA, it joins RNA molecules to make a primer.
Primer: A polynucleotide with a free 3′ end, bound by complimentary base pairing to the template strand, that is elongated during DNA replication.
Primitive streak: A groove on the surface of an early avian embryo along the future long axis of the body.
Prion: A protein that can take on alternative stable conformations.  The infectious form of the protein may increase in number by converting related proteins into more prions. It is the infectious agent in diseases such as scrapie and bovine spongiform encephalopathy (BSE).
Probability of fixation: The change of a single copy of an allele will ultimately fix throughout the population.
Product: An ending material in a chemical reaction.
Product efficiency: The fraction of energy stored in food that is not used for respiration.
Progeny: A descendant or offspring, as a child, plant, or animal.
Progestin: One of a family of steroid hormones, including progesterone, that prepares the uterus for pregnancy.
Progymnosperms:  Extinct seedless vascular plants that may be ancestral to seed plants.
Prokaryotes: Organisms that do not have a nucleus. This includes both bacteria and archaea, two distinct phylogenetic domains of life. and thus, the term may represent a polyphyletic grouping of organisms.
Prolactin: A hormone produced and secreted by the anterior pituitary with a great diversity of effects in different vertebrate species. In mammals, it stimulates growth of and milk production by the mammary glands.
Proliferative phase: That portion of the uterine cycle when the endometrium regenerates and thickens.
Prometaphase: The second stage of mitosis, in which discrete chromosomes consisting of identical sister chromatids appear, the nuclear envelop fragments, and the spindle microtubules attach to the kinetochores of the chromosomes.
Promiscuous: A type of relationship in which mating occurs with no strong pair-bonds or lasting relationships.
Promoter: A specific nucleotide sequence in DNA that binds RNA polymerase and indicates where to start transcribing RNA.
Pronuclei: Physically separated paternal and maternal nuclei present in the zygote after fertilization, before pronuclear syngamy (coming together) and the first cleavage division.
Proofreading: Correction of DNA replication mistakes by the DNA polymerase enzyme.
Prophage: A phage genome that has been inserted into a specific site on the bacterial chromosome.
Prophase: The first stage of mitosis, in which the chromatin is condensing and the mitotic spindle begins to form, but the nucleolus and nucleus still intact.
Prostaglandin: One of a group of modified fatty acids secreted by virtually all tissues and functioning as local regulators.
Prostate gland: A gland in human males that secretes an acid neutralizing component of semen.
Proteasome: A giant protein complex that recognizes and destroy proteins tagged for elimination by the small protein ubiquitin.
Protein: A three dimensional biological polymer constructed from a set of 20 different monomers called amino acids.
Protein electrophoresis: A method of analyzing a mixture of proteins by separating the molecules based on physical characteristics such as size, shape, or isoelectric point.
Protein kinase: An enzyme that transfers phosphate groups  from ATP to a protein.
Protein phosphatase: An enzyme that removes phosphate groups from proteins, usually reversing the effects of a protein kinase.
Proteoglycan: A glycoprotein in the extra cellular matrix of animal cells, rich in carbohydrate.
Proteomics: The systematic study of the full protein set (proteome) encoded by genomes.
Proterozoic: Geological eon lasting from 25000 to 541 Million years ago.
Protist: An informal term applied to any eukaryote that is not a plant, animal or fungus. Most protist are unicellular, but some are colonial or multicellular.
Protobiont: An aggregate of abiotically produced molecules surrounded by a membrane or membrane-like structure.
Proton: A subatomic particle with a single positive electrical charge found in the nucleus of an atom.
Proton pump: An active transport mechanism in cell membranes that uses ATP to force hydrogen ions out of cell generating a membrane potential in the process.
Protonema: A mass of green, branched, one-cell thick filaments produce by germinating moss spores.
Proton-motive force: The potential energy stored in the form of an electrochemical gradient, generated by the pumping of hydrogen ion across biological membranes during chemiosmosis.
Proto-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.
Protoplast: The contents of a plant cell exclusive to the cell wall.
Protostome: Member of one of two large groups of bilaterian animals including ecdysozoans and lophotrocozoans. In this group, the initial embryonic opening becomes the mouth.
Protozoan: A protist that lives primarily by ingesting food. an animal-like mode of nutrition.
Provirus: Viral DNA that inserts into a host genome.
Proximal tube: In the vertebrate kidney, the portion of a nephron immediately downstream from Bowmans’ capsule that coveys and helps refine filtrate.
PR (pathogenesis relates) protein: A protein involved in plant responses to pathogens.
Pseudocoelomate: An animal whose body cavity is not completely lined by mesoderm.
Pseudogene: A gene that has lost its function and is degenerating under mutation and drift.
Pseudopodium:  A cellular extension of ameboid cells used in moving and feeding.
Pterophyte: An informal name for any member of the phylum Pterophyta, which includes ferns, horsetails, whisk ferns, and the genus Tmesipteris.
Pterosaur: Winged reptile that lived during the time of the dinosaurs.
Psychrophiles: Organisms that prefer to grow at low temperatures.
Pulmocutaneous circuit:  The route of circulation that directs blood to the skin and lungs.
Pulmonary circuit: The branch of the circulatory system that supplies the lungs.
Pulse: The rhythmic stretching of the arteries caused by the pressure of blood forced through the arteries by contractions of the ventricles during systole.
Punctuated equilibrium: A theory devised by Eldredge and Gould (in 1972) based on the observation that in the fossil record species often show long periods of stasis, punctuated by rapid morphological change associated with speciation.
Punnet square: A diagram used in the study of inheritance to show the results of random fertilization in genetic crosses.
Pupil: The opening in the iris, which admits light into the interior of the vertebrate eye. Muscles in the iris regulate its size.
Purifying selection: Selection against deleterious alleles.
Purines: A class of nuclei acid bases including adenine (A) and guanine (G).
Pyloric sphincter: In the vertebrate digestive tract, a muscular ring that regulates the passage of food out of the stomach and into the small intestine.
Pyrimidine: A class of nuclei acid bases including Thymine (T), cytosine (C), and uracil (U).