Parasite An organism that lives on or in another organism (host) and obtains benefits from that host without providing any benefit in return. Read about some protozoan parasites.
Patch Clamp A technique allowing the measurement of electric potential (ion polarization) across a cell membrane. Learn how a patch clamp pipet is made.
Pathogen A bacterium, virus, or other organism that can cause a disease within a host organism.
Penicillinase A bacterial beta-lactamase enzyme capable of destroying penicillin making the bacteria antibiotic resistant. Learn how penicillin kills bacteria.
Periplasmic Space That region between the outer membrane and cytoplasmic membrane of Gram-positive bacteria. The cell wall is located within the space. Learn more about the Gram-positive bacteria envelope.
Peroxidase An enzyme found in eukaryote peroxisomes that uses oxygen from hydrogen peroxide to cause oxidation of a substrate.
Peroxisome A membrane-bound organelle in eukaryotic cells that contains a number of different enzymes. For the peroxisome’s primary role in the production and decomposition of hydrogen peroxide, it contains the enzyme catalase.
pH A measure of acidity or alkalinity of a solution. pH ranges from 0 (highly acidic) to 14 (highly alkaline) with a neutral pH having a value of 7. The value is determined by the negative log of hydrogen ion (-log[H+] concentration.
Phage (see: bacteriophage)
Phagocyte Any cell that engulfs particulate matter. “Professional” phagocytes include neutrophils and macrophages of the immune system which can internalize and kill invading microbes.
Phagocytosis The cellular process of internalization of particles by engulfment forming membrane bound vesicles (phagosomes) containing the swallowed material such as bacteria. See phagocytosis in this time-lapse video.
Phagolysosomes These are formed by the fusion of lysosomes with phagosomes with the release of microbicidal compounds into the phagosomes.
Phase Contrast The illumination for phase contrast is by means of a hollow cone of light. This technique requires a specialized microscope objective that includes a “phase annulus”- a ring shaped coating on one of the lens elements- that must match the incoming cone. Phase contrast greatly increases the apparent contrast between cell organelles. Explore this and other microscope techniques.
Photosynthesis The process used by plants and some bacteria using light energy, water and carbon dioxide to make organic compounds.
Phototaxis Directed migration of a cell or organism to or away from a light source.
Pilus, Pili (fimbriae) Hollow, hairlike structures made of protein that allow bacteria to attach to other cells. A specialized pilus, the sex pilus, allows the transfer of plasmid DNA from one bacterial cell to another. Visit the Bacteria Cell Model.
Plasma The non-particulate fluid of the blood.
Plasma Cell A mature differentiated B lymphocyte that produces a single species of antibody. Read how an antibody is produced.
Plasma Membrane (see cell membrane)
Plasmid Plasmids are small circular DNA fragments found in bacterial cytoplasm that contain code separate from the genophore DNA. Plasmids can replicate independently of the genophore DNA. Some plasmids contain code responsible for antibiotic resistance. Plasmids and the associated traits can be transferred between bacteria of the same or different species. To learn more about bacterial structure, tour the Bacteria Cell Model.
Platelet Small cell fragments in the blood, which contain blood clotting components.
Polarized Light Light waves that oscillate in a single plane.
Polarized Membrane The inside of a nerve or muscle cell membrane has a negative charge relative to the extracellular fluid. Under these conditions the cell is said to be "polarized". The charge is maintained by ion channels.
Polarizer A filter that permits only light in a single plane to pass. Learn about this and other microscope techniques.
Pollen The dust-like male gamete of a flower that can cause allergies in some people. How does the size of a pollen grain compare with other objects? Find out with the interactive"HowBig?" animation.
Polypeptide Chain of amino acids. (also see: protein)
Polysaccharide Chains of simple sugars linked together. Plant cell walls contain polysaccharides including cellulose.
Population Dynamics The biological study of changes in size and distribution of populations of individuals as affected by such things as birth, death and environment.
Programmed Cell Death (see: apoptosis)
Prokaryote, Prokaryotic Cells that lack a true membrane-bound nucleus such as bacteria. To learn about prokaryotes, tour the Bacteria Cell Model.
Prometaphase The stage in eukaryotic mitosis when the nuclear envelope breaks down so there is no longer a recognizable nucleus. In addition, some mitotic spindle fibers elongate from the centrosomes and attach to kinetochores. Kinetochores are protein bundles at the centromere region on the chromosomes where sister chromatids are joined. Other spindle fibers elongate but instead of attaching to chromomosomes, overlap each other at the cell center.
Prophase The stage in eukaryotic mitosis when the nucleolus fades and chromatin condenses into visually distinct chromosomes.
Prophase I The first stage in eukaryotic meiosis when dyad pairs align to create “TETRADS”, non-sister chromatids connect and trade sections at a “CHIASMA”, a process called “CROSSING OVER”.
Prophase II The part of the second stage in eukaryotic meiosis when spindle formation begins and centrosomes begin moving toward poles.
Protease An enzyme that cleaves proteins into smaller peptides.
Protein A macromolecule made up on one or more polypeptide chains.
Protozoa Single-celled non-photosynthetic aquatic eukaryotic organism such as Amoeba, Paramecium, and Euglena.
Provirus The genetic material of a virus incorporated into host DNA where it can replicate without causing the host cell to lyse.