PDZ domain   Protein-binding domain present in many scaffold proteins, and often used as a docking site for intracellular tails of transmembrane proteins. (Figure 19–22) pectin   Mixture of polysaccharides rich in galacturonic acid which forms a highly hydrated matrix in which cellulose is embedded in plant cell walls. (Figure 19–63) peripheral (secondary) lymphoid organ   Lymphoid organ in which T cells and B cells interact and respond to foreign antigens. Examples are spleen, lymph nodes, and mucosalassociated lymphoid organs. (Figure 24–12) peroxins   Form a protein translocator that participates in the import of proteins into peroxisomes.
peroxisome Small membrane-bounded organelle that uses molecular oxygen to oxidize organic molecules. Contains some enzymes that produce and others that degrade hydrogen peroxide (H2O2). (Figure 12–27) pH scale   Common measure of the acidity of a solution: “p” refers to power of 10, “H” to hydrogen. Defined as the negative logarithm of the hydrogen ion concentration in moles per liter (M). pH = –log [H+]. Thus a solution of pH 3 will contain 10–3 M hydrogen ions. pH less than 7 is acidic and pH greater than 7 is alkaline.
phagocytosis   Process by which unwanted cells, debris, and other bulky particulate material is endocytosed (“eaten”) by a cell. Prominent in carnivorous cells, such as Amoeba proteus, and in vertebrate macrophages and neutrophils. From Greek phagein, to eat.
phagosome Large intracellular membrane-enclosed vesicle that is formed as a result of phagocytosis. Contains ingested extracellular material. (Figure 13–61) phase variation The random switching of phenotype and expression of proteins involved in infection at frequencies much higher than mutation rates.