Pheromone - Biology Encyclopedia

Pheremones are chemical signals released by an organism that influence the behavior of another. Communication between living cells is often ultimately chemical in nature.

Photoperiodism - Biology Encyclopedia

The term "photoperiodism" was coined to describe a plant's ability to flower in response to changes in the photoperiod: the relative lengths of day and night. Because flowers produce seeds, flowering is crucially important for the plant to complete its life cycle.

Photosynthesis - Biology Encyclopedia

Physical Therapist and Occupational Therapist - Biology Encyclopedia

Physical therapists and occupational therapists are health care professionals who help people with a wide range of diseases, injuries, or disabilities maintain or improve their health and ability to carry out everyday tasks. They assess a patient's overall condition, develop a treatment plan, help the patient carry out the plan, and determine if the plan is working.

Physician Assistant - Biology Encyclopedia

A physician assistant (PA) career has been rated by U.S. News and World Report as one of the fastest growing and most desirable careers for the future.

Physiological Ecology - Biology Encyclopedia

The earth offers a huge variety of possible environments to inhabit: the hot arid environments of the desert, the salty environment of the oceans, the darkness of the deep sea, low oxygen environments of mountain peaks, and the frigid environments of the Arctic and Antarctic poles. This diversity of living conditions is reflected in the intriguing physiological adaptations developed by animals that live in these environments.

Pituitary Gland - Biology Encyclopedia

The pituitary gland is one of the principal glands of the endocrine system. It releases at least nine hormones affecting a wide variety of body functions, including growth, reproduction, and levels of electrolytes and water in the body fluids.

Plankton - Biology Encyclopedia

Plankton are small aquatic organisms that live in both freshwater and marine environments. The word "plankton" is derived from the Greek word Marine plankton.

Plant - Biology Encyclopedia

Plants (of the kingdom Plantae) are multicellular, eukaryotic organisms that develop from an embryo and that have cell walls and chloroplasts. Plants are distinguished from algae (from which they are descended) by a higher degree of multicellular complexity and from fungi by the ability to photosynthesize (those few plants that have lost this ability evolved from others that could).

Plant Development - Biology Encyclopedia

Plant development is an umbrella term for a broad spectrum of processes that include: the formation of a complete embryo from a zygote; seed germination; the elaboration of a mature vegetative plant from the embryo; the formation of flowers, fruits, and seeds; and many of the plant's responses to its environment. Plant development encompasses the growth and differentiation of cells, tissues, organs, and organ systems.

Plant Nutrition - Biology Encyclopedia

Green plants, unlike animals, are able to manufacture their major organic constituents entirely from inorganic raw materials that are obtained from soil, water, or atmosphere using energy provided by photosynthesis. Of over fifty elements found in plant tissues, only sixteen are considered essential nutrients for all plants.

Plant Pathogens and Pests - Biology Encyclopedia

Plants, being immobile, are unable to escape pests (herbivores) that eat them, or microorganisms (pathogens) that cause plant diseases. On a global basis, it is estimated that plant diseases annually cause an 11 to 16 percent reduction in the value of rice, wheat, corn, and potato harvests.

Plant Pathologist - Biology Encyclopedia

Plant pathologists specialize in the study of the nature, cause, and control of the diseases of plants. Plant pathologists are employed by colleges and universities, agricultural businesses, research organizations, government agencies, private enterprises, and as self–employed practitioners.

Plasma Membrane - Biology Encyclopedia

Plasma membranes envelop all plant and animal cells and all single-celled eukaryotes and prokaryotes, separating them from their environments. Structurally, they resemble other cellular membranes, but differ slightly in their lipid composition and more drastically in their protein content from one cell to another and from intracellular membranes.

Platyhelminthes - Biology Encyclopedia

The phylum name Platyhelminthes literally means "flatworms." Members of this phylum are soft, thin-bodied, leaf or ribbonlike worms, including the familiar planaria of ponds and streams, as well as the flukes and tapeworms parasitic in human and other animal bodies. Some defining characteristics of the phylum are that flatworms are acoelomate (they have no body cavity), triploblastic (the body has three tissue layers), and bilaterally symmetric (they have symmetric right and left sides and usually a definite head), and they have organ systems, including an excretory, digestive, reproductive, and nervous system, but no respiratory system.

Poisonous Plants - Biology Encyclopedia

Poisonous plants contain substances that can cause sickness or death if those substances are ingested or come into contact with the body of an animal.

Poisons - Biology Encyclopedia

Poisons are substances that are harmful to living organisms. It is said that "the dose makes the poison" because almost any substance can be poisonous at high enough concentrations, especially many substances used as medicines.

Pollination and Fertilization - Biology Encyclopedia

Pollination is the transfer of pollen to the female organs of seed plants. In flowering plants (angiosperms, or "covered seeds"), immature seeds (ovules) are located within carpels.

Pollution and Bioremediation - Biology Encyclopedia

When a substance is released to the environment at a rate in excess of what can be safely assimilated, that substance becomes an environmental pollutant.

Polymerase Chain Reaction - Biology Encyclopedia

The polymerase chain reaction (PCR) is a process that allows one to make in a short amount of time many copies of a particular deoxyribonucleic acid (DNA) sequence. It was developed in 1985 by Kary Mullis and has provided scientists in diverse fields with a powerful tool for DNA amplification, analysis, and manipulation.

Population Dynamics - Biology Encyclopedia

A population is a collection of individual organisms of the same species that occupy some specific area. The term "population dynamics" refers to how the number of individuals in a population changes over time.

Population Genetics - Biology Encyclopedia

The field of population genetics examines the amount of genetic variation within populations and the processes that influence this variation. A population is defined as a group of interbreeding individuals that exist together at the same time.