Study your flashcards anywhere!

Download the official Cram app for free >

  • Shuffle
    Toggle On
    Toggle Off
  • Alphabetize
    Toggle On
    Toggle Off
  • Front First
    Toggle On
    Toggle Off
  • Both Sides
    Toggle On
    Toggle Off
  • Read
    Toggle On
    Toggle Off

How to study your flashcards.

Right/Left arrow keys: Navigate between flashcards.right arrow keyleft arrow key

Up/Down arrow keys: Flip the card between the front and back.down keyup key

H key: Show hint (3rd side).h key

A key: Read text to speech.a key


Play button


Play button




Click to flip

298 Cards in this Set

  • Front
  • Back
pacemaker the group of specialized heart cells that control the heartbeat
pack ice a floating layer of sea ice that completely covers an area of the ocean surface
pack ice
pahoehoe basaltic lava flow that is characterized by a smooth or wavy surface
paleoanthropologist an anthropologist who studies the fossil remains of humans to trace the development of certain physical characteristics
paleomagnetismthe study of the alignment of magnetic minerals in rock, specifically as it relates to the reversal of Earth's magnetic poles; also the magnetic properties that rock acquires during formation
paleontologist a scientist who studies fossils
paleontology the scientific study of fossils
Paleozoic era the geologic era that followed Precambrian time and that lasted from 542 million to 251 million years ago
Paleozoic era
palisade mesophyll in plants, the layer of vertically elongated cells that contains chloroplasts, that is located beneath the upper epidermis of leaves, and that participates in photosynthesis
palisade mesophyll
pancreas the organ that lies behind the stomach and that produces and secretes insulin, glucagon, and digestive enzymes
Pangaea the supercontinent that formed 300 million years ago and that began to break up 250 million years ago
Panthalassa the single, large ocean that covered Earth's surface during the time the supercontinent Pangaea existed
papilla one of the bumps of tissue on the tongue, between which taste buds are embedded
parallax an apparent shift in the position of an object when viewed from different locations
parallel any circle that runs east and west around Earth and that is parallel to the equator; a line of latitude
parallel describes two or more components of a circuit that provide separate conducting paths for current because the components are connected across common points or junctions
parallel circuit a circuit in which the parts are joined in branches such that the potential difference across each part is the same
parallel circuit
parapodium in polychaetes, one of the two appendages that are used for locomotion or gas exchange
parasite an organism that feeds on an organism of another species (the host) and that usually harms the host; the host never benefits from the presence of the parasite
parasitism a relationship between two species in which one species, the parasite, benefits from the other species, the host, which is harmed
the division of the autonomic nervous system that is responsible for involuntary functions, such as slowing the heart rate and stimulating the digestive tract, while the body is at rest
parasympathetic nervous system
parathyroid gland an endocrine gland that secretes hormones that regulate the concentration of calcium and phosphate ions in the blood
parathyroid gland
parathyroid hormone a hormone that increases the level of blood calcium and decreases the level of phosphate in the blood
parathyroid hormone
parenchyma in higher plants, the fundamental tissue that is composed of thin-walled living cells that function in photosynthesis and storage
parent nuclide a radionuclide that yields a specific daughter nuclide as a later member of a radioactive series
parent nuclide
parent rock a rock formation that is the source of soil
parent rock
the pressure of each gas in a mixture
partial pressure
particulates fine particles that are suspended in the atmosphere and that are associated with air pollution
pascal the SI unit of pressure; equal to the force of 1 N exerted over an area of 1 m2 (symbol, Pa)
Pascal's principle the principle that states that a fluid in equilibrium contained in a vessel exerts a pressure of equal intensity in all directions
Pascal's principle
passive solar heating the use of sunlight to heat buildings directly
passive solar heating
passive transport the movement of substances across a cell membrane without the use of energy by the cell
passive transport
path difference the difference in the distance traveled by two beams when they are scattered in the same direction from different points
path difference
pathogen a microorganism, another organism, a virus, or a protein that causes disease; an infectious agent
pathogenic describes something that causes disease
bacteria that cause disease
pathogenic bacteria
pathology the scientific study of disease
the principle that states that two particles of a certain class cannot be in exactly the same energy state
Pauli exclusion principle
peat the brownish black material that is produced by the partial decomposition of plant remains
pectoral fin one of the two fins located behind the gills of a fish
pectoral fin
pectoral girdle the bony arch that supports the forelimbs of a vertebrate
pectoral girdle
pedicellaria on the surface of some echinoderms, very small pincers that are used for protection against ectoparasites
pedigree a diagram that shows the occurrence of a genetic trait in several generations of a family
pedipalp one of the second pair of appendages that are beside the mouth of an arachnid and that are used for chewing and handling prey
peer reviewthe process in which experts in a given field examine the results and conclusions of a scientist's study before that study is accepted for publication
peer review
pelagic zone the region of an ocean or body of fresh water above the benthic zone
pelagic zone
pellicle a protective envelope of nonliving material that covers many protozoans
pelvic fin in fishes, one of the paired fins positioned in the ventral, posterior portion of the body
pelvic fin
pelvic girdle the bones that support the hind limbs of a vertebrate
pelvic girdle
pelvic inflammatory disease a pelvic infection of the upper female reproductive system, including the uterus, ovaries, fallopian tubes, and other structures; it is a sexually transmitted disease
pelvic inflammatory disease
pelvis the ring of bones that form a bowl-shaped structure to which the thigh bones are attached
peneplain a low, almost level surface that is produced as a result of erosion
penicillin an antibiotic drug that combats bacteria by interfering with the synthesis of the bacterial cell wall
penis the male organ that transfers sperm to a female and that carries urine out of the body
penumbra the outer part of the shadow cast by the Earth or the moon in which sunlight is only partially blocked
pepsin an enzyme that is found in gastric juices and that helps break down proteins into smaller molecules
peptide bond the chemical bond that forms between the carboxyl group of one amino acid and the amino group of another amino acid
peptide bond
peptidoglycan a protein-carbohydrate compound that makes the cell walls of bacteria rigid
percentage composition the percentage by mass of each element in a compound
percentage composition
percentage error a figure that is calculated by subtracting the experimental value from the accepted value, dividing the difference by the accepted value, and then multiplying by 100
percentage error
perched water table the surface of a body of groundwater that is separated from the main body of groundwater below by an impermeable layer
perched water table
perennial a plant whose underground vegetative parts live for more than two years and whose upper parts die and regrow seasonally or annually
a collision in which two objects stick together after colliding
perfectly inelastic collision
pericardium the tough, saclike membrane that surrounds the heart
pericycle in plants, the outer portion of the central cylinder of vascular tissue
perigee in the orbit of a satellite, the point at which the satellite is closest to Earth
perihelion the point in the orbit of a planet at which the planet is closest to the sun
Period in physics, the time that it takes a complete cycle or wave oscillation to occur
Period in physics
period in chemistry, a horizontal row of elements in the periodic table
period in chemistry,
period a unit of geologic time that is longer than an epoch but shorter than an era
periodic describes something that occurs or repeats at regular intervals
periodic law the law that states that the repeating chemical and physical properties of elements change periodically with the atomic numbers of the elements
periodic law
periodic table an arrangement of the elements in order of their atomic numbers such that elements with similar properties fall in the same column, or group
periodic table
periodic wave a wave that changes its movement in a periodic manner over time and space
periodic wave
period of revolution the time that it takes a body to make one complete orbit, or revolution, around another body in space
period of revolution
periosteum the fibrous tissue that covers bones
peripheral nervous system all of the parts of the nervous system except for the brain and the spinal cord (the central nervous system); includes the cranial nerves and nerves of the neck, chest, lower back, and pelvis
peripheral nervous system
peristalsis the series of rhythmic muscular contractions that move food through the digestive tract
permafrost in arctic regions, the permanently frozen layer of soil or subsoil
permeability the ability of a rock or sediment to let fluids pass through its open spaces, or pores
permineralization a process in which minerals fill in the spaces of an organism's porous tissues, such as bones or shells
pest any organism that is considered harmful, irritating, or offensive either because of its direct effects on humans or because of its effects on plants and animals
pesticide a poison used to destroy pests, such as insects, rodents, or weeds; examples include insecticides, rodenticides, and herbicides
petal one of the usually brightly colored, leaf-shaped parts that make up one of the rings of a flower
petiole the stalk that attaches a leaf to the stem of a plant
petrification a process of fossilization in which organic matter is replaced by inorganic matter
petrochemical an organic compound derived from petroleum
petroleum a liquid mixture of complex hydrocarbon compounds; used widely as a fuel source
P generation parental generation, the first two individuals that mate in a genetic cross
P generation
pH a value that is used to express the acidity or alkalinity (basicity) of a system; each whole number on the scale indicates a tenfold change in acidity; a pH of 7 is neutral, a pH of less than 7 is acidic, and a pH of greater than 7 is basic
phagocyte a cell that ingests and destroys (digests) foreign matter or microorganisms
phagocytosis the process by which a cell engulfs large particles or whole cells, either as a defense mechanism or as a means to obtain food
pharyngeal pouch one of the lateral sacs that branch from the pharynx of chordate embryos and that may open to the outside as gill slits in adult fishes and invertebrate chordates
pharyngeal pouch
pharynx in flatworms, the muscular tube that leads from the mouth to the gastrovascular cavity; in animals with a digestive tract, the passage from the mouth to the larynx and esophagus
phase in astronomy, the change in the illuminated area of one celestial body as seen from another celestial body; phases of the moon are caused by the changing positions of the Earth, the sun, and the moon
phase in chemistry, one of the four states or conditions in which a substance can exist: solid, liquid, gas, or plasma; a part of matter that is uniform
phase change the physical change of a substance from one state (solid, liquid, or gas) to another
phase change
phase diagram a graph of the relationship between the physical state of a substance and the temperature and pressure of the substance
phase diagram
phenotype an organism's appearance or other detectable characteristic that results from the organism's genotype and the environment
phenotypic ratio the ratio of phenotypes produced by a cross
phenotypic ratio
pheromone a substance that is released by the body and that causes another individual of the same species to react in a predictable way
phloem the tissue that conducts food (sugars, amino acids, and mineral nutrients) in vascular plants
pH meter a device used to determine the pH of a solution by measuring the potential difference between the pH electrode and the reference electrode that are immersed in the solution
pH meter
phospholipid a lipid that contains phosphorus and that is a structural component in cell membranes
phosphorescence the emission of light by a mineral that is exposed to radiation; the mineral continues to emit light after the exposure has ceased
phosphorus cycle the cyclic movement of phosphorus in different chemical forms from the environment to organisms and then back to the environment
phosphorus cycle
the area of an aquatic system that receives enough light for photosynthesis to occur
photic zone
photoautotroph an organism that uses sunlight as its source of energy for photosynthesis
photocell a device that converts light energy into electrical energy
photoelectric effect the emission of electrons from a material when light of certain frequencies shines on the surface of the material
photoelectric effect
photon a unit or quantum of light; a particle of electromagnetic radiation that has zero rest mass and carries a quantum of energy
photoperiodism the response of plants to seasonal changes in the relative length of nights and days
photosphere the visible surface of the sun
photosynthesis the process by which plants, algae, and some bacteria use sunlight, carbon dioxide, and water to produce carbohydrates and oxygen
photosystem in the thylakoid membranes of chloroplasts, a cluster of chlorophyll and other pigment molecules that harvest light energy for the light reactions of photosynthesis
phototropism a plant growth movement that occurs in response to the direction of a source of light
phycobilin a pigment in red algae that can absorb the wavelengths of extremely dim light, which allows red algae to exist in very deep water
phylogenetic tree a branching diagram that shows how organisms are related through evolution
phylogenetic tree
phylogeny the evolutionary history of a species or taxonomic group
phylum the taxonomic group below kingdom and above class
physical change a change of matter from one form to another without a change in chemical properties
physic physical property al change
physical property a characteristic of a substance that does not involve a chemical change, such as density, color, or hardness
physical property
physical science the scientific study of nonliving matter
physical science
phytochrome a protein (pigment) that regulates flowering in plants in response to light absorption and seed germination
phytoplankton the microscopic, photosynthetic organisms that float near the surface of marine or fresh water and that are the basic source of food in many aquatic ecosystems; examples include algae and cyanobacteria
pigment a substance that gives another substance or a mixture its color
pillow lava pillow-shaped masses of lava flow from an underwater eruption; its form is due to the rapid cooling of the magma in sea water
pillow lava
pilus a short, thick appendage that allows a bacterium to attach to another bacterium
pineal gland an endocrine gland that secretes melatonin, which is thought to regulate daily and annual rhythms, such as sleep and reproductive cycles
pineal gland
pinniped an aquatic, fin-footed animal, such as a sea lion, walrus, or seal
pinocytosis a method of active transport across the cell membrane in which the cell takes in extracellular fluids
pinworm a nematode worm parasite that lives in the intestine of humans and animals; females lay their eggs around the anus, which causes itching
pioneer an organism that colonizes an uninhabited area and that starts an ecological cycle in which many other species become established
pioneer species a species that colonizes an uninhabited area and that starts an ecological cycle in which many other species become established
pioneer species
pistil the female reproductive part of a flower that produces seeds and consists of an ovary, style, and stigma
pitch a measure of how high or low a sound is perceived to be, depending on the frequency of the sound wave
pith the tissue that is located in the center of the stem of most vascular plants and that is used for storage
pituitary gland an endocrine gland that is located at the base of the brain, stores and releases hormones produced by the hypothalamus, and secretes hormones under the control of the hypothalamus
pituitary gland
pixel the smallest element of a display image
PKU phenylketonuria, a genetic disorder in which the body cannot metabolize phenylalanine
placenta the structure that attaches a developing fetus to the uterus and that enables the exchange of nutrients, wastes, and gases between the mother and the fetus
placental mammal a mammal that nourishes its unborn offspring through a placenta inside its uterus
placental mammal
placer deposit a deposit that contains a valuable mineral that has been concentrated by mechanical action
placer deposit
placoid scale one of the hard scales that resemble vertebrate teeth and cover skin of sharks and rays
placoid scale
plane mirror a mirror whose surface lies in a plane
plane mirror
planet any of the primary bodies that orbit the sun; a similar body that orbits another star
planetary nebula an expanding shell of gases shed by a hot, dying star
planetary nebula
planetesimal a small body from which a planet originated in the early stages of development of the solar system
plankton the mass of mostly microscopic organisms that float or drift freely in the waters of aquatic (freshwater and marine) environments
Plantae a classification kingdom made up of eukaryotic, multicellular organisms that have cell walls made mostly of cellulose, that have pigments that absorb light, and that supply energy and oxygen to themselves and to other life-forms through photosynthesis
plant ecology the scientific study of how plants interact with the environment
plant ecology
plant propagation the practice of reproducing plants from seeds or from vegetative parts
plant propagation
planula the free-swimming, ciliated larva of a cnidarian
plasma in biology, the liquid component of blood
plasma in biology
plasma in physical science, a state of matter that starts as a gas and then becomes ionized; it consists of free-moving ions and electrons, it takes on an electric charge, and its properties differ from those of a solid, liquid, or gas
plasma cell a type of white blood cell that produces antibodies
plasma cell
plasmid a circular DNA molecule that is usually found in bacteria and that can replicate independent of the main chromosome
plasmodium the multinucleate cytoplasm of a slime mold that is surrounded by a membrane and that moves as a mass
plasmolysis the contraction or shrinking of the cell membrane of a plant cell in a hypertonic solution in response to the loss of water by osmosis
plastic flowalso the process by which solid rock flows slowly when under pressure, as in the asthenosphere
plastic flow
plastid an organelle of plant cells that contains specific substances and performs specific functions for the cell
plastron the bottom, or ventral, portion of a turtle’s shell
plateau a large, flat area that is higher than the surrounding area
platelet a fragment of a cell that is needed to form blood clots
plate tectonics the theory that explains how large pieces of the lithosphere, called plates, move and change shape
plate tectonics
plumule the developing shoot above the cotyledons in plant embryos; consists of the epicotyl and young leaves
plumulethe illegal harvesting of fish, game, or other species
point mutation a mutation in which only one nucleotide or nitrogenous base in a gene is changed
point mutation
point-source pollution pollution that comes from a specific site
point-source pollution
polar describes a molecule in which the positive and negative charges are separated
polar climate a climate that is characterized by average temperatures that are near or below freezing; typical of polar regions
polar climate
polar compound a compound whose molecules have a negative charge on one side and a positive charge on the other
polar compound
polar covalent bond a covalent bond in which a pair of electrons shared by two atoms is held more closely by one atom
polar covalent bond
polar easterlies prevailing winds that blow from east to west between 60° and 90° latitude in both hemispheres
polar easterlies
polar front the boundary at which cold, polar air meets the warmer air of the middle latitudes
polar front
polarity a property of a system in which two points have opposite characteristics, such as charges or magnetic poles
polar nucleus one of the two haploid nuclei in the embryo sac of a seed plant that fuse with a male gamete to form the triploid cell that develops into the endosperm
polar nucleus
polar orbit an orbit in which a satellite passes over a planet's North and South Poles
polar orbit
a cloud that forms at altitudes of about 21,000 m during the Arctic and Antarctic winter or early spring, when air temperatures drop below –80°C
polar stratospheric cloud
polar zone the North or South Pole and the surrounding region
polar zone
pole one of two parts that have opposing qualities, such as the ends of a magnet
pollen the tiny granules that contain the male gametophyte of seed plants
pollen grain the structure that contains the male gametophyte of seed plants
pollen grain
pollen tube a tubular structure that grows from a pollen grain, enters the embryo sac, and allows the male reproductive cells to move to the ovule
pollen tube
pollination the transfer of pollen from the male reproductive structures (the anthers) to the tip of a female reproductive structure (the pistil) of a flower in angiosperms or to the ovule in gymnosperms
pollution an undesirable change in the natural environment that is caused by the introduction of substances that are harmful to living organisms or by excessive wastes, heat, noise, or radiation
polyatomic ion an ion made of two or more atoms
polyatomic ion
polyconic projection a map projection on which the central geographic meridian is a straight line, the other meridians are curved lines, and the parallels are arcs
polyconic projection
polygenic trait a characteristic of an organism that is determined by many genes
polygenic trait
polymer a large molecule that is formed by more than five monomers, or small units
polymerase chainreaction a technique that is used to make many copies of selected segments of DNA (abbreviation, PCR)
polymerase chain reaction
polyp a form of a cnidarian that has a cylindrical, hollow body and that is usually attached to a rock or to another object
polypeptide a long chain of several amino acids
polysaccharide one of the carbohydrates made up of long chains of simple sugars; polysaccharides include starch, cellulose, and glycogen
pons the section of the brain stem that is between the medulla oblongata and the midbrain
population a group of organisms of the same species that live in a specific geographical area and interbreed
population density the number of individuals of the same species that live in a given unit of area
population density
population genetics the study of the frequency and interaction of alleles and genes in populations
population genetics
population growth rat the amount by which a population's size changes in a given period of time
population growth rate
porosity the percentage of the total volume of a rock or sediment that consists of open spaces
porphyritic an igneous rock texture in which large crystals are embedded in a fine crystalline matrix
positron a particle that has the same mass and spin as an electron but that has a positive charge
posterior in animals with bilateral symmetry, refers to the end of the body that is opposite the head; rear
potable suitable for drinking
potencythe power of a medicine to produce a desired effect
potential difference the voltage difference in potential between two points in a circuit
potential difference
potential energy the energy that an object has because of the position, shape, or condition of the object
potential energy
power a quantity that measures the rate at which work is done or energy is transformed
Precambrian time the interval of time in the geologic time scale from Earth’s formation to the beginning of the Paleozoic era, from 4.6 billion to 542 million years ago
Precambrian time
precession a slow, periodic motion of a planet's axis of rotation that is due to changes in the distribution of mass at the planet's surface or the gravitational pull from other celestial bodies
precipitate a solid that is produced as a result of a chemical reaction in solution
precipitation any form of water that falls to Earth's surface from the clouds; includes rain, snow, sleet, and hail
precision the exactness of a measurement
precocial describes a species of birds or mammals whose young are born at an advanced stage of development, with open eyes and the ability to walk and run almost immediately
predation an interaction between two species in which one species, the predator, feeds on the other species, the prey
predator an organism that eats all or part of another organism
prediction a statement made in advance that expresses the results that will be obtained from testing a hypothesis if the hypothesis is supported; the expected outcome if a hypothesis is accurate
preen gland in birds, a special gland that secretes oil that a bird spreads over its feathers to clean and waterproof them
preen gland
preening in birds, the act of grooming and maintaining their feathers
pregnancy the period of time between conception and birth
prehensile refers to a limb used for grasping, as in a primate's hand, foot, or tail
premolar one of the eight teeth that are located between the molars and the canines
pre-mRNA precursor mRNA; the first strand of mRNA produced by gene transcription that contains both introns and exons
prescription drug a drug that is available only through an authorized, written order by a physician
prescription drug
pressure the amount of force exerted per unit area of a surface
prevailing winds winds that blow mainly from one direction during a given period
prevailing winds
prey an organism that is killed and eaten by another organism
primary amine an organic compound in which one hydrogen atom in an ammonia molecule has been replaced by an alkyl group
primary amine
the growth that occurs as a result of cell division at the tips of stems and roots and that gives rise to primary tissue
primary growth
primary immune response an immunological response that occurs when an antigen is encountered for the first time
primary immune response
primary pollutant a pollutant that is put directly into the atmosphere by human or natural activity
primary pollutant
primary pollutantthe total amount of organic material that the autotrophic organisms of an ecosystem produce
primary pollutant
Primarysuccessi succs that begins in an area that previously did not support life
primary succession
primate a member of the order Primates, the group of mammals that includes humans, apes, monkeys, and prosimians; typically distinguished by a highly developed brain, forward-directed eyes and binocular vision, opposable thumbs, and varied locomotion
prime meridian the meridian, or line of longitude, used as the origin in a system of coordinates; it passes through Greenwich, England, and it is designated as 0º longitude
prime meridian
primer a short, single-stranded fragment of DNA or RNA that is required for the initiation of DNA replication
principal quantunumber the quantum number that indicates the energy and orbital of an electron in an atom
principal quantum number
prion an infectious particle that consists only of a protein and that does not contain DNA or RNA
prism in optics, a system that consists of two or more plane surfaces of a transparent solid at an angle with each other
probability the likelihood that a possible future event will occur in any given instance of the event; the mathematical ratio of the number of times one outcome of any event is likely to occur to the number of possible outcomes of the event
probe a strand of RNA or single-stranded DNA that has been labeled with a radioactive element or fluorescent dye and that is used to bind with and identify a specific gene in genetic engineering
producer an organism that can make organic molecules from inorganic molecules; a photosynthetic or chemosynthetic autotroph that serves as the basic food source in an ecosystem
product a substance that forms in a chemical reaction
profundal zone the zone in a freshwater habitat to which little sunlight penetrates
profundal zone
progesterone a steroid hormone that is secreted by the corpus luteum of the ovary, that stimulates changes in the uterus to prepare for the implantation of a fertilized egg, and that is produced by the placenta during pregnancy
proglottid one of the many body sections of a tapeworm; contains reproductive organs
prograde rotation the counterclockwise spin of a planet or moon as seen from above the planet's North Pole; rotation in the same direction as the sun's rotation
prograde rotation
projectile motion the curved path that an object follows when thrown, launched, or otherwise projected near the surface of Earth; the motion of objects that are moving in two dimensions under the influence of gravity
projectile motion
prokaryote a single-celled organism that does not have a nucleus or membrane-bound organelles; examples are archaea and bacteria
prokaryotic cell a cell that does not have a nucleus or cell organelles; an example is a bacterial cell
prokaryotic cell
prolactin a hormone that is secreted by the anterior lobe of the pituitary gland and that stimulates milk secretion (abbreviation, PRL)
prominence a loop of relatively cool, incandescent gas that extends above the photosphere and above the sun’s edge as seen from Earth
promoter a nucleotide sequence on a DNA molecule to which an RNA polymerase molecule binds, which initiates the transcription of a specific gene
prophage the viral genome (DNA) of a bacteriophage that has entered a bacterial cell, has become attached to the bacterial chromosome, and is replicated with the host bacterium's DNA
prophase the first stage of mitosis and meiosis in cell division; characterized by the condensation of the chromosomes
prosimian a member of a suborder of primates that are primarily arboreal and nocturnal, such as a lemur, loris, or tarsier
prostaglandin a type of hormone that is synthesized in the body tissues and that usually acts locally; prostaglandins have a variety of effects, such as the dilatation of blood vessels, the contraction and relaxation of smooth muscle, and the regulation of the kidney function
prostate gland a gland in males that contributes to the seminal fluid
prostate gland
protein an organic compound that is made of one or more chains of amino acids and that is a principal component of all cells
protein synthesis the formation of proteins by using information contained in DNA and carried by mRNA
protein synthesis
prothorax the first section of an insect's thorax, where the first pair of legs is located
protist an organism that belongs to the kingdom Protista
Protista a kingdom of mostly one-celled eukaryotic organisms that are different from plants, animals, bacteria, and fungi
proton a subatomic particle that has a positive charge and that is located in the nucleus of an atom; the number of protons in the nucleus is the atomic number, which determines the identity of an element
proto-oncogene a gene that regulates normal cell division but that can become a cancer-causing oncogene as a result of mutation or recombination
protoplanet a planet in the early stage of its formation, when it already has almost all of its mass
protostar a contracting, dense cloud of gas that is one of the stages in the formation of a star before thermonuclear reactions take place
protostome an organism whose embryonic blastopore develops into the mouth, whose coelom arises by schizocoely, and whose embryo has determinate cleavage
protozoan a single-celled protist that can be aquatic or parasitic, that has organelles enclosed by a membrane, and that can move independently; examples include amoebas and paramecia
proventriculus the first of the two chambers in the stomach of a bird
provirus viral DNA that has attached to a host cell's chromosome and that is replicated with the chromosome's DNA
pseudocoelom the type of body cavity, derived from the blastocoel and referred to as a "false body cavity," that forms between the mesoderm and the endoderm in rotifers and roundworms
pseudocoelomate an animal that has a pseudocoelom, or false body cavity
pseudo plasmodium a multicellular mass of amoeboid cells that forms in certain slime molds
pseudo plasmodium
pseudopodium a retractable, temporary cytoplasmic extension that functions in food ingestion and movement in certain amoeboid cells
psychoactive drug a substance that has a significant effect on the mind or on behavior
psychoactive drug
psychrometer an instrument that is used to measure the moisture content of the atmosphere
puberty the stage of human life in which menstruation begins in females, sperm production begins in males, and secondary sex characteristics begin to appear
pulley a simple machine that consists of a wheel over which a rope, chain, or wire passes
pulmonary artery the artery that carries deoxygenated blood from the heart to the lungs
pulmonary artery
pulmonary circulation the flow of blood from the right ventricle of the heart to the lungs and back to the left atrium of the heart through the network of pulmonary arteries, capillaries, and veins
pulmonary circulation
pulmonary valve the valve at the exit from the right ventricle of the heart into the pulmonary artery
pulmonary valve
pulmonary veinthe vein that carries oxygenated blood from the lungs to the heart
pulmonary vein
pulsar a rapidly spinning neutron star that emits pulses of radio and optical energy
pulse the rhythmic pressure of the blood against the walls of a vessel, particularly an artery
pulse wave a pulse that lasts for a very short period of time
pulse wave
punctuated equilibrium a model of evolution in which short periods of drastic change in species, including mass extinctions and rapid speciation, are separated by long periods of little or no change
punctuated equilibrium
Punnett square a graphic used to predict the results of a genetic cross
Punnett square
pupa the immobile, nonfeeding stage between the larva and the adult of insects that have complete metamorphosis; as a pupa, the organism is usually enclosed in a cocoon or chrysalis and undergoes important anatomical changes
pupil the opening that is located in the center of the iris of the eye and that controls the amount of light that enters the eye
pure substance a sample of matter, either a single element or a single compound, that has definite chemical and physical properties
pure substance
purine a nitrogenous base that has a double-ring structure; one of the two general categories of nitrogenous bases found in DNA and RNA; either adenine or guanine
P wave a primary wave, or compression wave; a seismic wave that causes particles of rock to move in a back-and-forth direction parallel to the direction in which the wave is traveling; P waves are the fastest seismic waves and can travel through solids, liquids, and gases
P wave
pyloric sphincter the circular muscle that controls the flow of chyme from the stomach to the small intestine
pyloric sphincter
pyrenoid a small, protein structure that is found within the chloroplasts of algae and some liverworts and that plays a part in the synthesis of starch
pyrimidine a nitrogenous base that has a single-ring structure; one of the two general categories of nitrogenous bases found in DNA and RNA; thymine, cytosine, or uracil
fragments of rock that form during a volcanic eruption
pyroclastic material
pyruvic acid the three-carbon compound that is produced during glycolysis and needed for both the aerobic and anaerobic pathways of cellular respiration that follow glycolysis
pyruvic acid-