Physiology is the branch of biology that deals with the functions of living organisms and the parts of which they are made. This scientific discipline covers a wide variety of functions, ranging from the cellular and below to the interaction of organ systems that keep the most complex biological machines running.


Plague is an infectious, deadly disease caused by the bacterium Yersinia pestis. Plague pandemics (outbreaks of disease over a wide geographic area and affecting a large number of people) have wiped out populations since A.D.


Plankton are microscopic plants and small animals that live in the surface waters of oceans, lakes, and rivers and drift with the currents. They include bacteria, fungi, algae, protozoa, invertebrates, and some vertebrates.


A plant is any organism in the kingdom Plantae. Kingdoms are the main divisions into which scientists classify all living things on Earth.

Plastic Surgery

Plastic surgery is the branch of medicine concerned with the reconstruction and repair of defects in the body. Reconstructive plastic surgery repairs deformities or disfigurements caused by injuries, disease, or birth defects.


The term plastic can be used as both an adjective and a noun. As an adjective, the term refers to any material that can be shaped or molded, with or without the application of heat.

Plate Tectonics

Plate tectonics is the geologic theory that Earth's crust is made up of rigid plates that "float" on the surface of the planet. Tectonics comes from the Greek word meaning "builder." The movement of the plates toward or away from each other either directly or indirectly creates the major geologic features at Earth's surface.


Pluto, the ninth and farthest planet from the Sun, is one of the least well understood objects in the solar system. It is the smallest of the major planets and has a most unusual orbit.

Poisons and Toxins

A poison is any chemical that kills or injures an organism. The term toxin refers to a poison produced by a living organism, such as a microorganism, a plant, or an animal.


Poliomyelitis, or polio, is a serious infectious disease that attacks muscle-controlling nerves and can eventually cause paralysis. Poliomyelitis, sometimes called infantile paralysis, is caused by one of three related viruses, and it primarily affects children.


Pollution refers to situations in which some material or some form of energy occurs in larger quantity than can be tolerated by humans, plants, or animals without suffering some kind of harm. Probably the best-known forms of pollution are air and water pollution, which are discussed below.

Pollution Control

Pollution control is the process of reducing or eliminating the release of pollutants (contaminants, usually human-made) into the environment. It is regulated by various environmental agencies that establish limits for the discharge of pollutants into the air, water, and land.


A polygon is a geometric figure in two dimensions with three or more sides.


A polymer is a very large molecule in which one or two small units is repeated over and over again. The small repeating units are known as monomers.

Precious Metals

The precious metals—gold, silver, and platinum—have historically been valued for their beauty and rarity. Ancient people considered gold and silver to be of noble birth compared to the more abundant metals.


Pressure is the amount of force applied to a given area. Pressure is expressed in units such as pounds per square inch in the English system or newtons per square meter in the metric system.


The mammals (warm-blooded animals) called primates include the lower primates (lemurs, lorises, and tarsiers) and the higher primates (monkeys, apes, and humans). Mostly occurring in tropical areas, primates first evolved more than 50 million years ago from shrewlike, insect-eating mammals.

Probability Theory

Probability theory is a branch of mathematics concerned with determining the likelihood that a given event will occur. This likelihood is determined by dividing the number of selected events by the number of total events possible.


A proof is a logical argument demonstrating that a specific statement, proposition, or mathematical formula is true. It consists of a set of assumptions (also called premises) that are combined according to logical rules in order to establish a valid conclusion.


Prosthetics (pronounced prahs-THEH-tiks) is the branch of medicine that deals with the artificial replacement of a missing body part. A prosthesis (pronounced prahs-THEE-sis) is the general term for the artificial part itself that replaces the body part usually lost to disease or injury.

Protease Inhibitors

Protease inhibitors (pronounced PRO-tee-ace in-HIH-bi-ters) are a new type of drugs that slow down the spread of HIV (human immunodeficiency virus) inside a person's body. HIV is the virus that causes the disease AIDS (acquired immunodeficiency syndrome).


Proteins are very large molecules consisting of long chains of smaller units known as amino acids. Approximately two dozen different amino acids are used in the production of proteins.


Protozoa are a varied group of single-celled animal-like organisms belonging to the kingdom Protista. More than 50,000 different types of protozoa have been described.


Psychiatry is the branch of medicine concerned with the study, diagnosis, and treatment of mental illnesses. The word psychiatry comes from two Greek words that mean "mind healing." Those who practice psychiatry are called psychiatrists.


Psychology is the scientific study of human and animal behavior, which includes both observable actions (such as eating and speaking) and mental activities (such as remembering and imagining). Psychology tries to understand why a person or animal behaves a certain way and then seeks to predict how that person or animal will behave in the future.


A psychosis is a major psychiatric disorder characterized by the inability to tell what is real from what is not real. Hallucinations, delusions, and thought disorders can accompany psychosis.


In humans, puberty is the period of physical development when sexual reproduction can first take place. It is characterized by maturity of the sexual organs and the development of secondary sexual characteristics (such as the deepening of a boy's voice or the development of a girl's breasts).

Pythagorean Theorem

The Pythagorean theorem is one of the most famous theorems of geometry. It is often attributed to Pythagoras of Samos (Greece), who lived in the sixth century B.C.