es beneÞts at the expense of the population of another species; similar to predation, but differs in that parasites act more slowly than predators and do not always kill the host. A type of symbiosis in which one organism benefits at the expense of the other, for example the influenza virus is a parasite on its human host. Viruses, are obligate intracellular parasites.

parasympathetic system  The subdivision of the autonomic nervous system that reverses the effects of the sympathetic nervous system. Part of the autonomic nervous system that controls heartbeat, respiration and other vital functions.

parenchyma  One of the three major cell types in plants. Parenchyma cells have thin, usually multisided walls, are unspecialized but carry on photosynthesis and cellular respiration and can store food; form the bulk of the plant body; found in the þeshy tissue of fruits and seeds, photosynthetic cells of leaves, and the vascular system. Generalized plant cells whose numerous functions include photosynthesis, storage, bulk of herbaceous stem tissues, lateral transport in woody stems. Parenchyma are variously shaped but are characterized by thin walls and remain alive at functional maturity. PICTURE

parietal lobe  The lobe of the cerebral cortex that lies at the top of the brain; processes information about touch, taste, pressure, pain, and heat and cold. PICTURE

passive transport  Diffusion across a plasma membrane in which the cell expends no energy.

pectin  A substance in the middle lamella that cements adjoining plant cells together.

pectoral girdle  In humans, the bony arch by which the arms are attached to the rest of the skeleton; composed of the clavicle and scapula. PICTURE

pedigree analysis  A type of genetic analysis in which a trait is traced through several generations of a family to determine how the trait is inherited. The information is displayed in a pedigree chart using standard symbols.

pelagic zone  One of the two basic subdivisions of the marine biome; consists of the water above the sea þoor and its organisms.

pelvic girdle  In humans, the bony arch by which the legs are attached to the rest of the skeleton; composed of the two hipbones. PICTURE

pelvis  The hollow cavity formed by the two hipbones. PICTURE

penicillin The first of the so-called wonder drugs; discovered by Sir Alexander Fleming.

pepsin  A