Along with the extracellular þuid, it makes up the internal environment of multicellular organisms.

plasma cells  Cells produced from B cells that synthesize and release antibodies. PICTURE

plasmids  Self-replicating, circular DNA molecules found in bacterial cells; often used as vectors in recombinant DNA technology. Small circles of double-stranded DNA found in some bacteria. Plasmids can carry from four to 20 genes. Plasmids are a commonly used vector in recombinant DNA studies. PICTURE

plasmodesmata  Junctions in plants that penetrate cell walls and plasma membranes, allowing direct communication between the cytoplasm of adjacent cells (sing.: plasmodesma).

plasmolysis Osmotic condition in which a cell loses water to its outside environment.

plastids  Membrane-bound organelles in plant cells that function in storage (of food or pigments) or food production. Term for any double membrane-bound organelle. Chloroplasts contain the chemicals for photosynthesis, amyloplasts (also known as leukoplasts) store starch, chromoplasts contain colorful pigments such as in the petals of a flower or epidermis of a fruit.

platelets  In vertebrates, cell fragments that bud off from the megakaryocytes in the bone marrow; carry chemicals needed for blood clotting. Cell fragment functioning in blood clotting.

plate tectonics  The movement of the plates that make up the surface of the Earth. The revolutionary paradigm in geology that the earth's crust is composed of rigid segments (plates) in constant (although considered slow in a human-scale time frame) motion (tectonics) relative to each other.

pleiotropic A term describing a genotype with multiple phenotypic effects. For example: sickle-cell anemia produces a multitude of consequences in those it affects, such as heart disease, jidney problem, etc.

Pleistocene The first geologic epoch of the Quaternary Period of the Cenozoic Era that ended 10,000 years ago with the retreat of the last glaciers. PICTURE

pleura  A thin sheet of epithelium that covers the inside of the thoracic cavity and the outer surface of the lungs.

pleural cavity  The space between the sheets of pleura (one covering the inside of the thoracic cavity, the other covering the outside of the lungs).

polar covalent bond  A covalent bond in which atoms share electrons in an unequal fashion. The resulting molecule has regions with positive and negati