phragmoplast   Structure made of microtubules and actin filaments that forms in the prospective plane of division of a plant cell and guides formation of the cell plate. (Figure 17–49) phytochrome   Plant photoprotein that senses light via a covalently attached light-absorbing chromophore, which changes its shape in response to light and then induces a change in the protein’s conformation. Plant phytochromes are dimeric, cytoplasmic serine/threonine kinases, which respond differentially and reversibly to red and far-red light to alter cell behavior.
PI-3-kinase–Akt pathway Intracellular signaling pathway that stimulates animal cells to survive and grow. (Figure 15–53) pinocytosis   Literally, “cell drinking.” Type of endocytosis in which soluble materials are continually taken up from the environment in small vesicles and moved into endosomes along with the membrane-bound molecules. Compare phagocytosis. (Figure 13–48) piRNAs (piwi-interacting RNAs) A class of small noncoding RNAs made in the germ line that, in complex with Piwi proteins, keep in check the movement of transposable elements by transcriptionally silencing transposon genes and destroying RNAs produced by them.
planar cell polarity   Type of cellular asymmetry seen in some epithelia, such that each cell has a polarity vector oriented in the plane of the epithelium. (Figure 21–51) plant growth regulator (plant hormone) Signal molecule that helps coordinate growth and development. Examples are ethylene, auxins, gibberellins, cytokinins, abscisic acid, and the brassinosteroids.
plasma membrane   The membrane that surrounds a living cell. (Figure 10–1) plasmid vector   Small, circular molecules of double-stranded DNA derived from plasmids that occur naturally in bacterial cells; widely used for gene cloning.
plasmodesma (plural plasmodesmata) Plant equivalent of a gap junction. Communicating cell–cell junction in plants in which a channel of cytoplasm lined by plasma membrane connects two adjacent cells through a small pore in their cell walls.
platelet   Cell fragment, lacking a nucleus, that breaks off from a megakaryocyte in the bone marrow and is found in large numbers in the bloodstream. Helps initiate blood clotting when blood vessels are injured. (Figure 22–29) pleckstrin homology domain (PH domain) Protein domain found in some intracellular signaling proteins. Some PH domains in intracellular signaling proteins bind to phosphatidylinositol 3,4,5-trisphosphate produced by PI 3-kinase, bringing the signaling protein to the plasma membrane when PI 3-kinase is activated.
pluripotent Describes a cell that has the potential to give rise to all or almost all of the cell types of the adult body.
polarized   In epithelia, that the basal end of a cell, adherent to the basal lamina below, differs from the apical end, exposed to the medium above; thus, all epithelia and their individual cells are structurally polarized.
Polycomb group   Set of proteins critical for cell memory for some genes. They form complexes as part of the chromatin of the Hox complex, where they maintain a repressed state in cells where Hox genes have not been activated.
polymerase chain reaction (PCR) Technique for amplifying specific regions of DNA by the use of sequence-specific primers and multiple cycles of DNA synthesis, each cycle being followed by a brief heat treatment to separate complementary strands. (Figure 8–36) polymorphisms Describes genome sequences that coexist as two or more sequence variants at high frequency in a population.