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Biology LibreTexts

25.7: Glossary: P

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  • pacemaker

    type of cells in the heart that create electrical signals to stimulate heart muscles to contract


    “old stone” stage of human technological development that characterized humans starting about 2-5 million years ago with Homo habilis and continued through early modern Homo sapiens


    scientist who finds and studies fossils to learn about evolution and understand past life

    Paleozoic Era

    age of “old life” from 544-245 million years ago that began with the Cambrian explosion and ended with the Permian extinction


    dual endocrine and exocrine gland near the stomach that secretes insulin and glucagon, which regulate blood glucose, as well as enzymes that aid in digestion

    pancreatic islet

    one of millions of clusters of cells in the pancreas that secrete endocrine hormones such as insulin and glucagon; also called islet of Langerhans


    painful inflammation of the pancreas due to gallstones, chronic alcohol use, or other cause


    infectious disease epidemic that spreads across multiple populations or continents or even worldwide


    supercontinent that formed during the Permian Period and included all of Earth’s major land masses

    papillary layer

    upper layer of the dermis with papillae extending upward into the epidermis

    Pap smear

    medical test in which cells are scraped from the cervix and examined under a microscope in order to detect cancer cells if they are present

    paradigm shift

    radical change in science in which current theories are abandoned and new ideas take their place


    loss of sensation and movement in part of the body, such as may occur with a stroke or spinal cord injury


    species that lives in or on another species, called the host, and causes harm to the host while benefitting from the relationship


    symbiotic relationship between two species in which one species (the parasite) benefits while the other species (the host) is harmed

    parasympathetic division

    division of the autonomic nervous system that returns the body to normal after the fight-or-flight response and maintains homeostasis at other times

    parathyroid gland

    one of a pair of small endocrine glands in the neck that secretes hormones that regulate blood calcium

    parietal lobe

    part of each hemisphere of the cerebrum that controls touch, reading, and arithmetic

    Parkinson’s disease

    degenerative brain disorder caused by progressive death of neurons in the midbrain, resulting in muscular symptoms of tremor, rigidity, slowness of movement, and postural instability

    passive immunity

    short-term immunity to a particular pathogen that results when antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen

    passive transport

    movement of substances across a plasma membrane that does not require energy


    disease-causing agent such as a bacterium, virus, fungus, or protozoan

    pectoral girdle

    paired clavicles (collar bones) and scapulas (shoulder blades) that together form the shoulders and attach the arms to the trunk; also called shoulder girdle


    chart showing how a trait is passed from generation to generation within a family

    pelvic girdle

    paired, fused bones (ilium, pubis, and ischium) that form the hips and attach the legs to the trunk

    pelvic inflammatory disease (PID)

    infection that spreads from the vagina to the upper reproductive organs and may lead to ectopic pregnancies or infertility


    male reproductive organ containing the urethra, through which semen and urine pass out of the body

    peptic ulcer

    sore that develops in the lining of the stomach or duodenum most often caused by infection with the bacterium Helicobacter pylori


    tough, fibrous membrane that covers the outer surface of bones

    peripheral artery disease (PAD)

    narrowing of peripheral arteries, usually in the legs, due to atherosclerosis and generally causing intermittent pain in the legs when walking

    peripheral immune system

    part of the immune system that protects all of the body except for the central nervous system (which is protected by the neuroimmune system)

    peripheral nervous system (PNS)

    one of two major divisions of the nervous system that consists of all the nervous tissue that lies outside the central nervous system


    rapid, involuntary, wave-like contractions of smooth muscles that push food through the gastrointestinal tract and urine through the urinary tract

    permanent teeth

    second set of 32 teeth that emerge mainly during middle childhood and most of which replace deciduous (baby) teeth after they are lost; consist of 8 incisors, 4 canines, 8 premolars, and 12 molars

    Permian extinction

    extinction event at the end of the Paleozoic Period that was the biggest mass extinction that had occurred on Earth until then


    naturally occurring nonrenewable liquid fossil fuel found in reservoirs and rocks below Earth’s surface that is the world’s primary fuel source for transportation; also called crude oil


    scale that is used to measure acidity, on which 7 is neutral, less than 7 is acidic, and greater than 7 is basic


    process in which certain leukocytes engulf and break down solid particles such as pathogens or debris


    study of how genetic variation affects individual responses to therapeutic drugs


    tubular organ that connects the mouth and nasal cavity with the larynx and through which air and food pass


    characteristics of an organism that depend on how the organism’s genotype is expressed

    phenotypic plasticity

    ability to change the phenotype in response to changes in the environment

    phenylthiocarbamide (PTC)

    harmless compound that is used to test people for taster status because it tastes bitter to tasters and has no taste to nontasters


    type of lipid that contains a phosphate group and is the main component of the cell membranes of all living things

    phospholipid bilayer

    double layer of phospholipid molecules that makes up a plasma membrane

    phosphorus cycle

    biogeochemical cycle in which phosphorus is continuously recycled through biotic and abiotic components of ecosystems

    photic zone

    area in an aquatic biome extending to a maximum depth of 200 meters that receives enough sunlight for photosynthesis


    producer that uses energy from sunlight to produce organic molecules by photosynthesis


    type of sensory receptor that responds to light


    process of using light energy to make organic molecules from inorganic substances

    phylogenetic tree

    diagram that shows how species are related to each other through common ancestors


    evolutionary history of a group of related organisms

    physical exercise

    any bodily activity that enhances or maintains physical fitness and good health


    study of the functioning of the human organism


    any of a large group of plant chemicals that may be needed in small amounts for good health but are not (yet) classified as nutrients

    pineal gland

    endocrine gland that secretes the hormone melanin, which regulates the sleep-wake cycle

    pioneer species

    type of species that first colonizes a disturbed area

    pituitary gland

    master gland of the endocrine system that secretes many hormones, the majority of which regulate other endocrine glands


    “fake” treatment that actually has no effect on health and is included in a study to control for the placebo effect

    placebo effect

    psychologically based reaction to a treatment that occurs just because the subject is treated, even if the treatment is a placebo that has no physiological effect on the patient


    temporary organ that consists of a large mass of maternal and fetal blood vessels through which the mother’s and embryo’s or fetus’s blood exchange substances

    placental insufficiency

    problem with the placenta that causes inadequate transfer of oxygen and nutrients to the fetus, causing intrauterine growth restriction


    straw-yellow fluid part of blood that contains many dissolved substances and blood cells

    plasma membrane

    double layer of phospholipids that surrounds and encloses the contents of a cell; also called cell membrane

    Plasmodium falciparum

    protozoan parasite that causes malaria when it infects a human host


    cell fragment in blood that helps blood clot


    situation in which a single gene affects more than one trait


    disease in which the alveoli of the lungs become inflamed and filled with fluid, usually as a result of infection, causing symptoms such as shortness of breath, coughing, chest pain, and fever

    point mutation

    change in a single nucleotide base in a gene

    point-source pollution

    contamination of the environment that occurs when pollutants enter the environment from a single source


    difference in electrical charge between different parts of the same molecule


    fertilization in plants in which pollen is transferred from one flower to another, often with the help of a pollinator such as a bee


    anything that causes pollution, either a substance or a form of energy


    introduction of contaminants into the natural environment, leading to adverse changes in ecosystems and potential damage to human health

    polycystic kidney disease (PKD)

    genetic disorder in which multiple abnormal cysts develop and grow in the kidneys

    polygenic trait

    trait controlled by more than one gene, each of which may have two or more alleles


    large molecule that consists of many small repeating units called monomers

    polymerase chain reaction (PCR)

    biotechnology process that makes many copies of a gene or other DNA segment


    situation in which a gene has two or more alleles in a population at frequencies greater than 1 percent


    chain of small molecules called nucleotides that alone or with a complementary chain makes up a nucleic acid (DNA or RNA)


    chain of amino acids that alone or with other such chains makes up a protein


    chain of monosaccharides (simple sugars) that makes up a complex carbohydrate (such as starch or cellulose)


    all the organisms of the same species that live in the same area

    population density

    average number of individuals in a population per unit of area

    population distribution

    how the individuals in a population are arrayed over the area they inhabit

    population genetics

    science focusing on the forces of evolution at the level of allele frequencies in populations

    population growth rate (r)

    net change in population size per year per 100 people already present in the population that occurs because of births, deaths, and migrations

    population projection

    prediction of future population growth or size based on assumptions about future birth and death rates

    population pyramid

    bar graph that represents the age-sex structure of a population

    positive feedback loop

    control mechanism that serves to intensify a response until an endpoint is reached

    posterior pituitary

    back lobe of the pituitary gland that stores and secretes hypothalamic hormones


    stage of the human organism between middle childhood and adolescence that covers the ages 11 to 12 years, during which many children start going through puberty


    water that falls from clouds in the atmosphere to Earth’s surface in the form of rain, snow, sleet, hail, or freezing rain


    relationship between two species in which members of one species (the predator) consume members of the other species (the prey)


    species that consumes the other species (the prey) in a predatory relationship


    carrying of one or more offspring from fertilization until birth


    one of eight cusped teeth in the sides of the jaws between the canine teeth and molars that are used for crushing food


    common vision problem associated with aging in which the eye gradually loses its ability to focus on close objects

    preschool stage

    second part of early childhood that covers the ages 4 to 5 years


    species that is consumed by the other species (the predator) in a predatory relationship

    primary pollutant

    contaminant that is released directly into the environment from the source(s) of pollution

    primary succession

    change over time in the numbers and types of species that live in an area that was never before colonized by living things


    any member of the Primate Order of mammals, in which the human species is placed


    infectious agent composed entirely of proteins that may cause a fatal neurodegenerative disease such as mad cow disease or kuru


    organism that produces food for itself and other organisms


    substance that forms as the result of a chemical reaction


    female sex hormone secreted mainly by the ovaries that helps maintain a successful pregnancy


    single-celled organism that lacks a nucleus

    prokaryotic cell

    living cell without a nucleus


    region of a gene where RNA polymerase binds to initiate transcription of the gene


    first phase of mitosis during which chromatin condenses into chromosomes, the nuclear envelope breaks down, centrioles separate, and a spindle begins to form


    any non-anthropoid primate; lemur, loris, or tarsier

    prostate cancer

    tumor in the prostate gland of the male reproductive system that is the most common type of cancer in men

    prostate gland

    gland in the male reproductive system that secretes fluid into semen and provides nourishing substances to sperm


    class of biochemical compounds made of amino acids, with complex structures and diverse functions

    protein synthesis

    process in which cells make proteins that includes transcription of DNA and translation of mRNA


    any member of the Protist Kingdom, the kingdom in the Eukarya Domain that includes all eukaryotes except plants, animals, and fungi


    gene that normally promotes growth and reproduction of cells but may cause cancer if it mutates and becomes an oncogene

    protozoan (plural, protozoa)

    animal-like, single-celled protist such as Giardia or Plasmodium species, both of which are common human parasites


    claim, belief, or practice that is presented as scientific but does not adhere to the standards and methods of science

    psychoactive drug

    drug that affects the central nervous system, generally by influencing neurotransmitters in the brain


    period during which humans become sexually mature

    pulmonary circulation

    part of the cardiovascular system that carries blood between the heart and lungs

    punctuated equilibrium

    model of the timing of evolution in which long periods of little evolutionary change are interrupted by bursts of rapid evolutionary change

    Punnett square

    chart for determining the expected percentages of different genotypes in the offspring of two parents

    pyruvate kinase (PK)

    enzyme needed for ATP production in red blood cells, deficiency in which is a genetic adaptation to malaria