Partition Coefficient see Octanol/water partition coefficient. Passive Transfer The movement across a membrane by simple diffusion.  This is the most common way lipophilic xenobiotics pass through cell membranes. Percutaneous absorption The transfer of a substance from the outer surface of the skin through the corneum and outer layers and into the systemic circulation. Phagocytosis The engulfing of particles by certain cells of the circulatory and lymphatic systems, known as phagocytes.  Phagocytosis is a primary cellular defense mechanism against foreign particles or organisms. Pharmacokinetics Quantitation of the time course of chemical absorption, distribution, metabolism, and elimination. Phospholipids Molecules containing phosphates and lipids found in the cell membrane.  The phosphate head is hydrophilic, whereas the lipid tail is hydrophobic. Pinocytosis The process whereby a liquid is engulfed and taken into a cell by an inward folding of the cell membrane, which detaches and moves into the cytoplasm. Plasma The non-cellular, fluid portion of whole blood. Plasma membrane The membrane composed of phospholipids, proteins, and cholesterol that forms the outer boundary of a cell and regulates the movement of substances into and out of the cell.  Same as "cell membrane." Polar A term used to describe a molecule which is charged or ionized.  Polar substances are usually the easiest for the body to excrete. Portal circulation The term applied to the venous circulation draining the tissues of the gastrointestinal tract into the liver. PPM Parts per million - the number of units of a substance in a million units.  PPM is a common concentration unit for dilute samples of dissolved substances or airborne substances. Protein A complex nitrogenous substance which constitutes the main building material in cells.