10Th Chem P

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pacemaker the group of specialized heart cells that control the heartbeat

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pacemaker

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pack ice a floating layer of sea ice that completely covers an area of the ocean surface

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pack ice

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pahoehoe basaltic lava flow that is characterized by a smooth or wavy surface

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pahoehoe

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paleoanthropologist an anthropologist who studies the fossil remains of humans to trace the development of certain physical characteristics

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paleoanthropologist

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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

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paleomagnetism

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paleontologist a scientist who studies fossils

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paleontologist

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paleontology the scientific study of fossils

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paleontology

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Paleozoic era the geologic era that followed Precambrian time and that lasted from 542 million to 251 million years ago

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Paleozoic era

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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

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palisade mesophyll

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pancreas the organ that lies behind the stomach and that produces and secretes insulin, glucagon, and digestive enzymes

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pancreas

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Pangaea the supercontinent that formed 300 million years ago and that began to break up 250 million years ago

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Pangaea

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Panthalassa the single, large ocean that covered Earth's surface during the time the supercontinent Pangaea existed

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Panthalassa

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papilla one of the bumps of tissue on the tongue, between which taste buds are embedded

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papilla

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parallax an apparent shift in the position of an object when viewed from different locations

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parallax

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parallel any circle that runs east and west around Earth and that is parallel to the equator; a line of latitude

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parallel

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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

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parallel

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parallel circuit a circuit in which the parts are joined in branches such that the potential difference across each part is the same

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parallel circuit

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parapodium in polychaetes, one of the two appendages that are used for locomotion or gas exchange

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parapodium

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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

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parasite

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parasitism a relationship between two species in which one species, the parasite, benefits from the other species, the host, which is harmed

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parasitism

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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

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parasympathetic nervous system

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parathyroid gland an endocrine gland that secretes hormones that regulate the concentration of calcium and phosphate ions in the blood

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parathyroid gland

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parathyroid hormone a hormone that increases the level of blood calcium and decreases the level of phosphate in the blood

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parathyroid hormone

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parenchyma in higher plants, the fundamental tissue that is composed of thin-walled living cells that function in photosynthesis and storage

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parenchyma

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parent nuclide a radionuclide that yields a specific daughter nuclide as a later member of a radioactive series

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parent nuclide

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parent rock a rock formation that is the source of soil

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parent rock

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the pressure of each gas in a mixture

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partial pressure

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particulates fine particles that are suspended in the atmosphere and that are associated with air pollution

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particulates

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pascal the SI unit of pressure; equal to the force of 1 N exerted over an area of 1 m2 (symbol, Pa)

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pascal

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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

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Pascal's principle

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passive solar heating the use of sunlight to heat buildings directly

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passive solar heating

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passive transport the movement of substances across a cell membrane without the use of energy by the cell

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passive transport

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path difference the difference in the distance traveled by two beams when they are scattered in the same direction from different points

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path difference

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pathogen a microorganism, another organism, a virus, or a protein that causes disease; an infectious agent

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pathogen

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pathogenic describes something that causes disease

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pathogenic

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bacteria that cause disease

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pathogenic bacteria

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pathology the scientific study of disease

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pathology

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the principle that states that two particles of a certain class cannot be in exactly the same energy state

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Pauli exclusion principle

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peat the brownish black material that is produced by the partial decomposition of plant remains

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peat

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pectoral fin one of the two fins located behind the gills of a fish

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pectoral fin

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pectoral girdle the bony arch that supports the forelimbs of a vertebrate

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pectoral girdle

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pedicellaria on the surface of some echinoderms, very small pincers that are used for protection against ectoparasites

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pedicellaria

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pedigree a diagram that shows the occurrence of a genetic trait in several generations of a family

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pedigree

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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

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pedipalp

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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

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peer review

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pelagic zone the region of an ocean or body of fresh water above the benthic zone

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pelagic zone

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pellicle a protective envelope of nonliving material that covers many protozoans

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pellicle

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pelvic fin in fishes, one of the paired fins positioned in the ventral, posterior portion of the body

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pelvic fin

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pelvic girdle the bones that support the hind limbs of a vertebrate

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pelvic girdle

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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

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pelvic inflammatory disease

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pelvis the ring of bones that form a bowl-shaped structure to which the thigh bones are attached

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pelvis

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peneplain a low, almost level surface that is produced as a result of erosion

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peneplain

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penicillin an antibiotic drug that combats bacteria by interfering with the synthesis of the bacterial cell wall

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penicillin

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penis the male organ that transfers sperm to a female and that carries urine out of the body

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penis

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penumbra the outer part of the shadow cast by the Earth or the moon in which sunlight is only partially blocked

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penumbra

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pepsin an enzyme that is found in gastric juices and that helps break down proteins into smaller molecules

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pepsin

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peptide bond the chemical bond that forms between the carboxyl group of one amino acid and the amino group of another amino acid

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peptide bond

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peptidoglycan a protein-carbohydrate compound that makes the cell walls of bacteria rigid

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peptidoglycan

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percentage composition the percentage by mass of each element in a compound

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percentage composition

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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

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percentage error

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perched water table the surface of a body of groundwater that is separated from the main body of groundwater below by an impermeable layer

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perched water table

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perennial a plant whose underground vegetative parts live for more than two years and whose upper parts die and regrow seasonally or annually

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perennial

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a collision in which two objects stick together after colliding

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perfectly inelastic collision

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pericardium the tough, saclike membrane that surrounds the heart

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pericardium

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pericycle in plants, the outer portion of the central cylinder of vascular tissue

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pericycle

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perigee in the orbit of a satellite, the point at which the satellite is closest to Earth

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perigee

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perihelion the point in the orbit of a planet at which the planet is closest to the sun

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perihelion

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Period in physics, the time that it takes a complete cycle or wave oscillation to occur

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Period in physics

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period in chemistry, a horizontal row of elements in the periodic table

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period in chemistry,

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period a unit of geologic time that is longer than an epoch but shorter than an era

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period

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periodic describes something that occurs or repeats at regular intervals

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periodic

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periodic law the law that states that the repeating chemical and physical properties of elements change periodically with the atomic numbers of the elements

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periodic law

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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

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periodic table

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periodic wave a wave that changes its movement in a periodic manner over time and space

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periodic wave

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period of revolution the time that it takes a body to make one complete orbit, or revolution, around another body in space

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period of revolution

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periosteum the fibrous tissue that covers bones

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periosteum

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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

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peripheral nervous system

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peristalsis the series of rhythmic muscular contractions that move food through the digestive tract

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peristalsis

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permafrost in arctic regions, the permanently frozen layer of soil or subsoil

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permafrost

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permeability the ability of a rock or sediment to let fluids pass through its open spaces, or pores

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permeability

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permineralization a process in which minerals fill in the spaces of an organism's porous tissues, such as bones or shells

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permineralization

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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

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pest

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pesticide a poison used to destroy pests, such as insects, rodents, or weeds; examples include insecticides, rodenticides, and herbicides

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pesticide

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petal one of the usually brightly colored, leaf-shaped parts that make up one of the rings of a flower

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petal

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petiole the stalk that attaches a leaf to the stem of a plant

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petiole

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petrification a process of fossilization in which organic matter is replaced by inorganic matter

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petrification

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petrochemical an organic compound derived from petroleum

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petrochemical

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petroleum a liquid mixture of complex hydrocarbon compounds; used widely as a fuel source

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petrochemical

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P generation parental generation, the first two individuals that mate in a genetic cross

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P generation

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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

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pH

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phagocyte a cell that ingests and destroys (digests) foreign matter or microorganisms

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phagocyte

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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

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phagocytosis

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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

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pharyngeal pouch

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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

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pharynx

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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

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phase

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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

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phase

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phase change the physical change of a substance from one state (solid, liquid, or gas) to another

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phase change

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phase diagram a graph of the relationship between the physical state of a substance and the temperature and pressure of the substance

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phase diagram

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phenotype an organism's appearance or other detectable characteristic that results from the organism's genotype and the environment

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phenotype

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phenotypic ratio the ratio of phenotypes produced by a cross

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phenotypic ratio

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pheromone a substance that is released by the body and that causes another individual of the same species to react in a predictable way

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pheromone

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phloem the tissue that conducts food (sugars, amino acids, and mineral nutrients) in vascular plants

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phloem

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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

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pH meter

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phospholipid a lipid that contains phosphorus and that is a structural component in cell membranes

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phospholipid

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phosphorescence the emission of light by a mineral that is exposed to radiation; the mineral continues to emit light after the exposure has ceased

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phosphorescence

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phosphorus cycle the cyclic movement of phosphorus in different chemical forms from the environment to organisms and then back to the environment

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phosphorus cycle

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the area of an aquatic system that receives enough light for photosynthesis to occur

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photic zone

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photoautotroph an organism that uses sunlight as its source of energy for photosynthesis

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photoautotroph

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photocell a device that converts light energy into electrical energy

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photocell

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photoelectric effect the emission of electrons from a material when light of certain frequencies shines on the surface of the material

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photoelectric effect

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photon a unit or quantum of light; a particle of electromagnetic radiation that has zero rest mass and carries a quantum of energy

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photon

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photoperiodism the response of plants to seasonal changes in the relative length of nights and days

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photoperiodism

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photosphere the visible surface of the sun

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photosphere

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photosynthesis the process by which plants, algae, and some bacteria use sunlight, carbon dioxide, and water to produce carbohydrates and oxygen

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photosynthesis

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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

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photosystem

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phototropism a plant growth movement that occurs in response to the direction of a source of light

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phototropism

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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

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phycobilin

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phylogenetic tree a branching diagram that shows how organisms are related through evolution

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phylogenetic tree

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phylogeny the evolutionary history of a species or taxonomic group

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phylogeny

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phylum the taxonomic group below kingdom and above class

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phylum

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physical change a change of matter from one form to another without a change in chemical properties

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physic physical property al change

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physical property a characteristic of a substance that does not involve a chemical change, such as density, color, or hardness

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physical property

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physical science the scientific study of nonliving matter

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physical science

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phytochrome a protein (pigment) that regulates flowering in plants in response to light absorption and seed germination

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phytochrome

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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

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phytoplankton

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pigment a substance that gives another substance or a mixture its color

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pigment

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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

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pillow lava

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pilus a short, thick appendage that allows a bacterium to attach to another bacterium

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pilus

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pineal gland an endocrine gland that secretes melatonin, which is thought to regulate daily and annual rhythms, such as sleep and reproductive cycles

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pineal gland

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pinniped an aquatic, fin-footed animal, such as a sea lion, walrus, or seal

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pinniped

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pinocytosis a method of active transport across the cell membrane in which the cell takes in extracellular fluids

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pinocytosis

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pinworm a nematode worm parasite that lives in the intestine of humans and animals; females lay their eggs around the anus, which causes itching

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pinworm

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pioneer an organism that colonizes an uninhabited area and that starts an ecological cycle in which many other species become established

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pioneer

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pioneer species a species that colonizes an uninhabited area and that starts an ecological cycle in which many other species become established

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pioneer species

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pistil the female reproductive part of a flower that produces seeds and consists of an ovary, style, and stigma

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pistil

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pitch a measure of how high or low a sound is perceived to be, depending on the frequency of the sound wave

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pitch

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pith the tissue that is located in the center of the stem of most vascular plants and that is used for storage

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pith

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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

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pituitary gland

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pixel the smallest element of a display image

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pixel

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PKU phenylketonuria, a genetic disorder in which the body cannot metabolize phenylalanine

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PKU

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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

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placenta

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placental mammal a mammal that nourishes its unborn offspring through a placenta inside its uterus

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placental mammal

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placer deposit a deposit that contains a valuable mineral that has been concentrated by mechanical action

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placer deposit

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placoid scale one of the hard scales that resemble vertebrate teeth and cover skin of sharks and rays

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placoid scale

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plane mirror a mirror whose surface lies in a plane

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plane mirror

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planet any of the primary bodies that orbit the sun; a similar body that orbits another star

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planet

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planetary nebula an expanding shell of gases shed by a hot, dying star

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planetary nebula

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planetesimal a small body from which a planet originated in the early stages of development of the solar system

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planetesimal

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plankton the mass of mostly microscopic organisms that float or drift freely in the waters of aquatic (freshwater and marine) environments

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plankton

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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

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Plantae

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plant ecology the scientific study of how plants interact with the environment

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plant ecology

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plant propagation the practice of reproducing plants from seeds or from vegetative parts

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plant propagation

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planula the free-swimming, ciliated larva of a cnidarian

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planula

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plasma in biology, the liquid component of blood

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plasma in biology

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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

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plasma

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plasma cell a type of white blood cell that produces antibodies

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plasma cell

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plasmid a circular DNA molecule that is usually found in bacteria and that can replicate independent of the main chromosome

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plasmid

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plasmodium the multinucleate cytoplasm of a slime mold that is surrounded by a membrane and that moves as a mass

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plasmodium

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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

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plasmolysis

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plastic flowalso the process by which solid rock flows slowly when under pressure, as in the asthenosphere

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plastic flow

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plastid an organelle of plant cells that contains specific substances and performs specific functions for the cell

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plastid

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plastron the bottom, or ventral, portion of a turtle’s shell

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plastron

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plateau a large, flat area that is higher than the surrounding area

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plateau

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platelet a fragment of a cell that is needed to form blood clots

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platelet

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plate tectonics the theory that explains how large pieces of the lithosphere, called plates, move and change shape

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plate tectonics

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plumule the developing shoot above the cotyledons in plant embryos; consists of the epicotyl and young leaves

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plumule

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plumulethe illegal harvesting of fish, game, or other species

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plumule

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point mutation a mutation in which only one nucleotide or nitrogenous base in a gene is changed

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point mutation

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point-source pollution pollution that comes from a specific site

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point-source pollution

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polar describes a molecule in which the positive and negative charges are separated

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polar

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polar climate a climate that is characterized by average temperatures that are near or below freezing; typical of polar regions

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polar climate

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polar compound a compound whose molecules have a negative charge on one side and a positive charge on the other

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polar compound

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polar covalent bond a covalent bond in which a pair of electrons shared by two atoms is held more closely by one atom

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polar covalent bond

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polar easterlies prevailing winds that blow from east to west between 60° and 90° latitude in both hemispheres

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polar easterlies

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polar front the boundary at which cold, polar air meets the warmer air of the middle latitudes

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polar front

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polarity a property of a system in which two points have opposite characteristics, such as charges or magnetic poles

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polarity

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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

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polar nucleus

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polar orbit an orbit in which a satellite passes over a planet's North and South Poles

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polar orbit

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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

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polar stratospheric cloud

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polar zone the North or South Pole and the surrounding region

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polar zone

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pole one of two parts that have opposing qualities, such as the ends of a magnet

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pole

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pollen the tiny granules that contain the male gametophyte of seed plants

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pollen

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pollen grain the structure that contains the male gametophyte of seed plants

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pollen grain

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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

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pollen tube

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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

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pollination

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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

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pollution

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polyatomic ion an ion made of two or more atoms

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polyatomic ion

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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

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polyconic projection

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polygenic trait a characteristic of an organism that is determined by many genes

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polygenic trait

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polymer a large molecule that is formed by more than five monomers, or small units

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polymer

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polymerase chainreaction a technique that is used to make many copies of selected segments of DNA (abbreviation, PCR)

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polymerase chain reaction

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polyp a form of a cnidarian that has a cylindrical, hollow body and that is usually attached to a rock or to another object

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polyp

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polypeptide a long chain of several amino acids

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polypeptide

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polysaccharide one of the carbohydrates made up of long chains of simple sugars; polysaccharides include starch, cellulose, and glycogen

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polysaccharide

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pons the section of the brain stem that is between the medulla oblongata and the midbrain

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pons

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population a group of organisms of the same species that live in a specific geographical area and interbreed

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population

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population density the number of individuals of the same species that live in a given unit of area

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population density

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population genetics the study of the frequency and interaction of alleles and genes in populations

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population genetics

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population growth rat the amount by which a population's size changes in a given period of time

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population growth rate

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porosity the percentage of the total volume of a rock or sediment that consists of open spaces

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porosity

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porphyritic an igneous rock texture in which large crystals are embedded in a fine crystalline matrix

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porphyritic

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positron a particle that has the same mass and spin as an electron but that has a positive charge

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positron

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posterior in animals with bilateral symmetry, refers to the end of the body that is opposite the head; rear

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posterior

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potable suitable for drinking

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potable

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potencythe power of a medicine to produce a desired effect

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potency

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potential difference the voltage difference in potential between two points in a circuit

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potential difference

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potential energy the energy that an object has because of the position, shape, or condition of the object

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potential energy

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power a quantity that measures the rate at which work is done or energy is transformed

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power

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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

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Precambrian time

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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

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precession

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precipitate a solid that is produced as a result of a chemical reaction in solution

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precipitate

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precipitation any form of water that falls to Earth's surface from the clouds; includes rain, snow, sleet, and hail

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precipitation

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precision the exactness of a measurement

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precision

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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

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precocial

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predation an interaction between two species in which one species, the predator, feeds on the other species, the prey

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predation

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predator an organism that eats all or part of another organism

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predator

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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

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prediction

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preen gland in birds, a special gland that secretes oil that a bird spreads over its feathers to clean and waterproof them

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preen gland

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preening in birds, the act of grooming and maintaining their feathers

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preening

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pregnancy the period of time between conception and birth

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pregnancy

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prehensile refers to a limb used for grasping, as in a primate's hand, foot, or tail

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prehensile

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premolar one of the eight teeth that are located between the molars and the canines

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premolar

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pre-mRNA precursor mRNA; the first strand of mRNA produced by gene transcription that contains both introns and exons

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pre-mRNA

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prescription drug a drug that is available only through an authorized, written order by a physician

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prescription drug

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pressure the amount of force exerted per unit area of a surface

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pressure

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prevailing winds winds that blow mainly from one direction during a given period

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prevailing winds

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prey an organism that is killed and eaten by another organism

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prey

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primary amine an organic compound in which one hydrogen atom in an ammonia molecule has been replaced by an alkyl group

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primary amine

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the growth that occurs as a result of cell division at the tips of stems and roots and that gives rise to primary tissue

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primary growth

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primary immune response an immunological response that occurs when an antigen is encountered for the first time

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primary immune response

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primary pollutant a pollutant that is put directly into the atmosphere by human or natural activity

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primary pollutant

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primary pollutantthe total amount of organic material that the autotrophic organisms of an ecosystem produce

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primary pollutant

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Primarysuccessi succs that begins in an area that previously did not support life

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primary succession

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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

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primate

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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

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prime meridian

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primer a short, single-stranded fragment of DNA or RNA that is required for the initiation of DNA replication

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primer

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principal quantunumber the quantum number that indicates the energy and orbital of an electron in an atom

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principal quantum number

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prion an infectious particle that consists only of a protein and that does not contain DNA or RNA

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prion

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prism in optics, a system that consists of two or more plane surfaces of a transparent solid at an angle with each other

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prism

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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

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probability

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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

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probe

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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

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producer

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product a substance that forms in a chemical reaction

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product

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profundal zone the zone in a freshwater habitat to which little sunlight penetrates

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profundal zone

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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

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progesterone

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proglottid one of the many body sections of a tapeworm; contains reproductive organs

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proglottid

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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

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prograde rotation

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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

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projectile motion

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prokaryote a single-celled organism that does not have a nucleus or membrane-bound organelles; examples are archaea and bacteria

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prokaryote

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prokaryotic cell a cell that does not have a nucleus or cell organelles; an example is a bacterial cell

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prokaryotic cell

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prolactin a hormone that is secreted by the anterior lobe of the pituitary gland and that stimulates milk secretion (abbreviation, PRL)

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prolactin

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prominence a loop of relatively cool, incandescent gas that extends above the photosphere and above the sun’s edge as seen from Earth

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prominence

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promoter a nucleotide sequence on a DNA molecule to which an RNA polymerase molecule binds, which initiates the transcription of a specific gene

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promoter

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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

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prophage

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prophase the first stage of mitosis and meiosis in cell division; characterized by the condensation of the chromosomes

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prophase

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prosimian a member of a suborder of primates that are primarily arboreal and nocturnal, such as a lemur, loris, or tarsier

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prosimian

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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

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prostaglandin

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prostate gland a gland in males that contributes to the seminal fluid

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prostate gland

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protein an organic compound that is made of one or more chains of amino acids and that is a principal component of all cells

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protein

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protein synthesis the formation of proteins by using information contained in DNA and carried by mRNA

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protein synthesis

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prothorax the first section of an insect's thorax, where the first pair of legs is located

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prothorax

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protist an organism that belongs to the kingdom Protista

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protist

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Protista a kingdom of mostly one-celled eukaryotic organisms that are different from plants, animals, bacteria, and fungi

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Protista

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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

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proton

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proto-oncogene a gene that regulates normal cell division but that can become a cancer-causing oncogene as a result of mutation or recombination

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proto-oncogene

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protoplanet a planet in the early stage of its formation, when it already has almost all of its mass

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protoplanet

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protostar a contracting, dense cloud of gas that is one of the stages in the formation of a star before thermonuclear reactions take place

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protostar

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protostome an organism whose embryonic blastopore develops into the mouth, whose coelom arises by schizocoely, and whose embryo has determinate cleavage

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protostome

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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

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protozoan

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proventriculus the first of the two chambers in the stomach of a bird

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proventriculus

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provirus viral DNA that has attached to a host cell's chromosome and that is replicated with the chromosome's DNA

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provirus

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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

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pseudocoelom

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pseudocoelomate an animal that has a pseudocoelom, or false body cavity

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pseudocoelomate

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pseudo plasmodium a multicellular mass of amoeboid cells that forms in certain slime molds

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pseudo plasmodium

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pseudopodium a retractable, temporary cytoplasmic extension that functions in food ingestion and movement in certain amoeboid cells

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pseudopodium

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psychoactive drug a substance that has a significant effect on the mind or on behavior

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psychoactive drug

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psychrometer an instrument that is used to measure the moisture content of the atmosphere

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psychrometer

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puberty the stage of human life in which menstruation begins in females, sperm production begins in males, and secondary sex characteristics begin to appear

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puberty

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pulley a simple machine that consists of a wheel over which a rope, chain, or wire passes

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pulley

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pulmonary artery the artery that carries deoxygenated blood from the heart to the lungs

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pulmonary artery

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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

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pulmonary circulation

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pulmonary valve the valve at the exit from the right ventricle of the heart into the pulmonary artery

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pulmonary valve

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pulmonary veinthe vein that carries oxygenated blood from the lungs to the heart

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pulmonary vein

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pulsar a rapidly spinning neutron star that emits pulses of radio and optical energy

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pulsar

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pulse the rhythmic pressure of the blood against the walls of a vessel, particularly an artery

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pulse

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pulse wave a pulse that lasts for a very short period of time

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pulse wave

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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

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punctuated equilibrium

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Punnett square a graphic used to predict the results of a genetic cross

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Punnett square

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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

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pupa

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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

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pupil

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pure substance a sample of matter, either a single element or a single compound, that has definite chemical and physical properties

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pure substance

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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

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purine

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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

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P wave

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pyloric sphincter the circular muscle that controls the flow of chyme from the stomach to the small intestine

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pyloric sphincter

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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

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pyrenoid

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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

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pyrimidine

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fragments of rock that form during a volcanic eruption

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pyroclastic material

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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

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pyruvic acid-