Placebo An inert substance or dosage form that is identical in appearance, flavor, and odor to the active substance or dosage form. It is used as a negative control in a bioassay or in a clinical study.
Placenta A structure that develops in the uterus during pregnancy that provides a blood supply and nutrients for the fetus and eliminates waste; formed from the uterine lining and embryonic membranes. In humans, it is also referred to as the afterbirth because it is ejected after the baby in a normal vaginal birth. If the placenta is abnormally low in the uterus and covering the uterus, a pregnancy-related condition called placenta praevia occurs and usually necessitates delivery by a cesarean section.
Placental mammal Any mammals that bear their young live and are nourished before birth in the mother’s uterus through a placenta. There are about 4,000 species, from bats to cats to humans.
Placoderm A member of an extinct class of early hinged-jaw fishlike vertebrates covered with a tough outer body armor of bony plates with paired fins; lived during the Silurian and Devonian periods (438 to 360 million years ago). Examples include the antiarchs and dinichthyids.
Plankton Plankton includes mostly small-sized plants called phytoplankton (e.g., diatoms) and animals called zooplankton (e.g., radiolarians) that drift and float along with the tides and currents of water bodies. Their name comes from the Greek meaning "drifter" or "wanderer." Phytoplankton produce their own food by photosynthesis and are primary producers and food supply for a host of other organisms. Plankton are also an oxygen producer, generating according to some estimates as much as 80 percent of the Earth’s oxygen supply.
Planula The free-swimming, flat, ciliated larvae of the coelenterates.
Plasma In biology, this term has the following three meanings:
1. Fluid component of blood in which the blood cells and platelets are suspended (blood plasma). Note the distinction between plasma, which describes a part of the blood (the fluid part of blood, outside the blood cells), and serum, which describes a fraction derived from blood by a manipulation (the fluid that separates when blood coagulates).
2. Fluid component of semen produced by the accessory glands, the seminal vesicles, the prostate, and the bulbourethral gland.
3. Cell substance outside the nucleus (cytoplasm).
Plasma cell An antibody-producing B cell that has reached the end of its differentiation pathway. B cells are white blood cells that develop from B stem cells into plasma cells that produce immunoglobulins (antibodies).
Plasma membrane An interface and permeability-limiting membrane composed of lipids and proteins that act as a selective barrier for the cell’s interior cytoplasm.
Plasmid An extrachromosomal genetic element consisting generally of circular double-stranded DNA, which can replicate independently of chromosomal DNA. R plasmids are responsible for the mutual transfer of antibiotic resistance among microbes. Plasmids are used as vectors for cloning DNA in bacteria or yeast host cells.
Plasmodesma Living bridges between cell walls; small tubes or openings lined with plasma membrane between cell walls that connect each cell to one another and are believed to allow molecules to pass through.
Plasmogamy A process of cytoplasm fusion between two cells; the first step in syngamy (sexual reproduction).
Plasmolysis A process caused by diffusion when the cell membrane shrinks away from its cell wall, with the resulting vacuole and cytoplasm shrinking due to the presence of a foreign material, like salt, becoming too abundant; water is drawn from the cell into the extracellular area, and the cell becomes flaccid after losing its internal turgor.
Plastid A type of plant cytoplasmic organelle that develops from a precursor small and colorless undiffer-entiated organelle, the proplastid. During cell differentiation, proplastids differentiate into particular plastid types according to the type of cell in which they are located in response to the particular metabolic demands. Plastids are essential components for plant cell function.
Plastids develop into specialized functional types. They divide by binary fission or budding. Several types of plastids exist: amyloplast or leucoplast (starch synthesis and storage), chloroplast (photosynthesis), chro-moplast (plant color), etioplast (night plants, can develop chloroplasts), proteoplast or proteinoplast (storage), elaioplast (oil storage).
Plastocyanin An electron transfer protein, containing a type 1 copper site, involved in plant and cyanobacterial photosynthesis, which transfers electrons to photosystem i.
Platelet (thrombocyte) Disk-shaped, colorless blood cells produced by the bone marrow (from megakaryocytes); contains numerous proinflammatory mediators and functions to stop bleeding and allow damaged areas to clot. A normal platelet count is 150,000-400,000 mm3 (millimeters cubed).
Platelet-activating factor (PAF) A cytokine mediator of immediate hypersensitivity, perhaps even the most important, which produces inflammation.
Pleated sheet (beta pleated sheet) One type of several secondary structures, the three-dimensional arrangements (folding, twisting, coiling) of polypeptide chains in a protein. They can be in the form of a helix, random coil, or pleated sheet; linked by hydrogen bonds in the peptide backbone; sheets are formed when the polypeptide chains fold back and forth, or when two parts are parallel to each other and bonded; also called the beta pleated sheet.
Pleiotropy The ability of a single gene to affect many phenotypic traits. Jonathan Hodgkin from the MRC Laboratory of Molecular Biology, in Cambridge, United Kingdom, has characterized several types of pleiotropy:
Artefactual Adjacent but functionally unrelated genes affected by the same mutation, e.g., claret. Secondary Simple primary biochemical disorder leading to complex final phenotype, e.g., phenylke-tonurea.
Adoptive One gene product used for quite different chemical purposes in different tissues, e.g., e-crys-tallin.
Parsimonious One gene product used for identical chemical purposes in multiple pathways, e.g., gpb-1. Opportunistic One gene product playing a secondary role in addition to its main function, e.g., sisB/ AS-C.
Combinatorial One gene product employed in various ways, and with distinct properties, depending on its different protein partners, e.g., unc-86. Unifying One gene, or cluster of adjacent genes, encoding multiple chemical activities that support a common biological function, e.g., cha-1 unc-17.
Plesiomorphic character The ancestral character of a homologue; the descendant character is termed the apomorphic character. Two characters in two taxa are homologues if they are the same as the character that is found in the ancestry of the two taxa, or if they have characters that have an ancestor/descendant relationship described as preexisting or novel (plesiomorphic and apomorphic character).
All taxa are mixtures of ancestral and derived characters. Ancestral character states are those similar to the remote ancestor, while derived characters are those that have undergone recent change. Taxa can show either the ancestral (or plesiomorphic) character state or the derived (apomorphic) character state for a particular character.
Pluripotent stem cell (stem cell) Primitive blood cells found in the bone marrow, circulating bloodstream, and umbilical cord that are capable of reproducing and differentiating to make all varieties of mature blood cells (white blood cells, red blood cells, and platelets). This means that all blood cells originate from this single type of cell. Pluripotent stem cells are usually referred to as stem cells.
Pneumatophore (breathing roots) A specialized root structure that has numerous pores or lenticels over its surface, allowing gas exchange, and that grows up into the air, e.g., mangroves.
Pocosin A swamp on the coastal plain of the southeastern United States.
Poikilotherm An organism (e.g., fish or reptile) whose body temperature varies or fluctuates with the temperature of its surroundings; an ectotherm.
Point mutation When the base sequence of a codon is permanently changed. Four types exist: Missense A change in base sequence converts a codon for one amino acid to a codon for a different amino acid.
Nonsense A codon for a specific amino acid is converted to a chain-terminating codon. Silent Conversion of a codon for an amino acid to another codon that specifies the same amino acid. Frameshift A nucleotide is deleted or added to the coding portion of a gene.
Poison plants (poison ivy; poison oak; poison sumac) The poison ivy plant is known as Toxicodendron radi-cans in the eastern United States and T. rydbergii in the midwestern United States. Historically it has been called Rhus toxicodendron. Western poison oak is known as Toxicodendron diversilobum.
These plants can cause a skin reaction. No reaction usually occurs the first time the skin is exposed to the plant. Subsequent contact with the plant or plant resin, however, can result in an allergic skin reaction that usually appears seven to 14 days after contact. Subsequent contact results in a more rapid reaction, usually within two to five days postcontact. The severity of the reaction is related to the amount of plant material that comes in contact with the skin, as well as to the degree of allergic sensitivity of the individual. The allergen (irritant from the plant) is often transferred from the hands or clothing to other parts of the body.
The poison ivy plant and its relatives are common throughout the United States. Poison ivy leaves are coated with a mixture of chemicals called urushiol. When people get urushiol on their skin, it causes allergic contact dermatitis. The body’s immune system treats urushi-ol as foreign and attacks the complex of urushiol derivatives with skin proteins. The irony is that urushiol, in the absence of the immune attack, would be harmless.
Poison ivy can affect two out of three Americans, and of these, 15 percent may have severe allergic reactions that require medical treatment. Millions of Americans yearly seek remedies for the irritation caused by poison ivy, oak, and sumac.
Polar covalent bond A type of chemical bond, based on electron affinity, where electrons shared by atoms spend a greater percentage of time closer to an oxygen nucleus rather than a hydrogen nucleus; bonds are polar, i.e., they have a partial electric charge across the molecule due to their geometry and the electronegativity difference between the two atoms (hydrogen is positive, oxygen is negative); in organisms, they can form weak hydrogen bonds. Water is an example, but peptide bonds and amines also form polar covalent bonds; these molecules can attract each other.
Polar molecule A molecule that has both a positive and a negative end, such as water.
Pollen Microscopic grains produced by plants in order to reproduce. Each plant has a pollinating period that can vary depending on the plant, climate, and region.
Pollen allergy A hypersensitive reaction to pollen. While grass pollens are generally the most common cause of hay fever (seasonal allergic rhinitis), other pollen types are also important. These include tree pollens such as alder, hazel, birch, beech, cypress, pine, chestnut, and poplar, and weed pollens such as plantain, mugwort, and ragweed. The relative importance of the kinds of pollen that can cause hay fever varies between different climatic and vegetation zones. For example, ragweed pollen, although very common in North America, is present in Europe only in the French Rhone valley and some areas of Eastern Europe, while the pollen most associated with seasonal allergy in Mediterranean regions is the olive tree. A person allergic to one pollen is generally also allergic to members of the same group or family (e.g., Betulaceae). Pollen-induced reactions include extrinsic asthma, rhinitis, and bronchitis.
Pollination The first step in plant reproduction. occurs when the male germ cell of a plant, a pollen grain, reaches the female reproductive part, or stigma, of the same species of plant. This happens by wind transportation or by animal carriers, although 90 percent of flowering plants rely on animal delivery.
Polyandry A rare mating system where one female mates with more than one male, although each male mates with only one female. Two types of polyandry exist: simultaneous polyandry, where each female maintains a large territory that contains smaller nesting territories of two or more males who care for the eggs and tend to the young; and sequential polyandry, where a female mates with a male, lays eggs, and terminates the relationship and leaves that male. While the male is left to incubate the eggs, she repeats the sequence with another male. The latter is more common. In human society, it is the practice of a woman to have more than one husband at a time.
Polygenic inheritance The interaction of several genes on a phenotype trait. A series of genes at multiple loci where each contributes a small additive effect on a plant’s phenotype, for example, height in tobacco plants.
Polygyny A mating system where one male mates with more than one female, while each female mates with only one male; believed to be the normal mating system in animals.
Polyhedral symbol The polyhedral symbol indicates the geometrical arrangements of the coordinating atoms about the central atom. It consists of one or more capital italic letters derived from common geometric terms (tetrahedron, square plane, octahedron, etc.), which denote the idealized geometry of the lig-ands around the coordination center, and an Arabic numeral that is the coordination number of the central atom. The polyhedral symbol is used as an affix, enclosed in parentheses, and separated from the name by a hyphen. Examples are T-4, SP-4, TBPY-5, SPY-5, OC-6, and CU-8.
Polymer A macromolecule of high relative molecular mass composed of many similar or identical monomers linked together in chains. Plastics are polymers.
Polymerase chain reaction (PCR) A laboratory technique used to rapidly amplify predetermined regions of double-stranded DNA. Generally involves the use of a heat-stable DNA polymerase.
Polymorphic Refers to a phenotypic expression occurring in a number of forms appearing within an interbreeding population, such as fur coloration.
Polymorphism Difference in DNA sequence among individuals in a plant or animal population expressed as two or more distinct forms of individuals in the same population. Polymorphisms can be inherited or environmentally created (polyphenism). Examples include sickle cell anemia and the caste system of bees.
Polymorphonuclear leukocyte (PMN; granular leukocyte; granulocyte; inflammatory granulocyte; polymorphonuclear cell) A subgroup of leukocytes (white blood cells) filled with granules of toxic chemicals that enable them to digest microorganisms by phagocytosis. Examples of granulocytes are neu-trophils, eosinophils, and basophils.
Polymerase chain reaction (PCR) is a fast, inexpensive technique for making an unlimited number of copies of any piece of DNA. Sometimes called "molecular photocopying," PCR has had an immense impact on biology and medicine, especially genetic research.
Polyp A cnidarian body form that is the sessile reproductive stage; the alternate is the mobile medusa stage. In medicine, a polyp is a small stalked benign growth or tumor protruding from a mucous membrane; can be precursors of cancer.
Polypeptide A polymer chain of amino acids linked by covalent peptide bonds. one or more polypeptides form proteins. Each polypeptide has two terminal ends; one, called the amino terminal or N-terminal, has a free amino group, while the other end is called the carboxyl terminal or C-terminal with a free carboxyl group.
Polyphyletic Refers to a group of organisms that might have some similarities but that do not include the most recent common ancestor of all the member organisms due to that ancestor lacking some or all characteristics of the group. Polyphyletic groups are not recognized in accepted taxonomies.
Polyploidy When the number of chromosomes in a cell gets doubled; two complete chromosome sets; a mutation. Polyploidy is very common in plants, where 30 percent to 70 percent of modern angiosperms are believed to be polyploids. Polyploidy is rare in animals but is found in some insects, amphibians, reptiles, and one mammal (a rat in Argentina).
Polyribosome (polysome) A cluster of ribosomes translating on one messenger RNA molecule.
Polysaccharide A carbohydrate (polymer) made by polymerizing any of more than 1,000 monosaccha-rides; a complex sugar.
Population Any group of interbreeding individuals of a particular species living in a specific geographic area.
Population viability analysis (PVA) A process to evaluate the likelihood of a population surviving and to identify threats facing the species. Used in endangered species recovery and management.
Porins A class of proteins that create water-filled channels across cell membranes.
Porphyrin A macrocyclic molecule that contains four pyrrole rings linked together by single carbon atom bridges between the alpha positions of the pyrrole rings. Porphyrins usually occur in their dianionic form coordinated to a metal ion.
Positional information Cells send signals indicating their locations relative to each other in the embryo development process; positional information is communicated through gap junctions, specific cell-cell adhesions, or diffusible signal molecules; part of the process of determining cell fate.
Positive feedback When a change occurs in a variable during homeostasis, the response is to reinforce the change in the variable. Examples include nerve impulse conduction, blood clotting, female ovarian cycles, labor and birth, and immune responses. A resulting negative feedback may need to take over to halt the process.
Postsynaptic membrane The presynaptic membrane is at the tip of each nerve ending. A small cleft called the synaptic cleft separates the presynaptic membrane from the postsynaptic membrane, a special area on the receiving cell. When the presynaptic nerve ending receives nerve impulses, it sends neurotransmit-ters stored in synaptic vesicles into the synaptic cleft (the synapse), which in turn diffuse across and transmit the signal to the postsynaptic membrane receptor molecules, which are specialized molecules that open or close certain ion channels when activated by the correct neurotransmitter.
Postzygotic barrier A species-isolating mechanism preventing hybrids that are produced by two different species from developing into fertile and viable adults.
Potency A comparative rather than an absolute expression of drug activity. Drug potency depends on both affinity and efficacy. Thus, two agonists can be equipotent but have different intrinsic efficacies, with compensating differences in affinity. Potency is the dose of drug required to produce a specific effect of given intensity as compared with a standard reference.
