Science / Biology Glossary

Period: The fundamental unit in the hierarchy of time units; a part of geologic time during which a particular sequence of rocks designated as a system was deposited. Units of geological time that a . . . View Full Definition

Periosteum: A fibrous membrane that covers bones and serves as the site of attachment for skeletal muscles; contains nerves, blood vessels, and lymphatic vessels.

Peripheral Nervous System: The division of the nervous system that connects the central nervous system to other parts of the body. Components of the nervous system that transmit messages to the central nervous system.

Peristalsis: Involuntary contractions of the smooth muscles in the walls of the esophagus, stomach, and intestines that propel food along the digestive tract. Waves of muscle contraction in the esophagus . . . View Full Definition

Permian Period: The last geologic time period of the Paleozoic Era, noted for the greatest mass extinction in earth history, when nearly 96% of species died out.

Peroxisomes: Membrane-bound vesicles in eukaryotic cells that contain oxidative enzymes.

Pesticides: Chemicals that are applied to agricultural crops or domesticated plants and which kill or inhibit growth of insects.

Petals: Usually brightly colored elements of a fiower that may produce fragrant oils; nonreproductive structures that attract pollinators. Sterile leaf-like (white, colorless, but usually colored) s . . . View Full Definition

Petiole: The generally non-leafy part of the leaf that attaches the leaf blade to the stem; celery and rhubarb are examples of a leaf petiole that we use as food. The stalk connecting the leaf blade . . . View Full Definition

Petrifaction: Mode of fossilization where f organic matter is replaced with silica.

PH: The negative logarithm of the H+ ion concentration. The pH is a measure of the acidity or basic character of a solution. Since it measures a fraction, the larger the pH number, the less H io . . . View Full Definition

Phagocytes: White blood cells that can engulf (by phagocytosis) and destroy microorganisms including viruses and bacteria; cells in this category include neutrophils and monocytes.

Phagocytosis: A form of endocytosis in which white blood cells surround and engulf invading bacteria or viruses.

Pharynx: The passageway between the mouth and the esophagus and trachea. Food passes from the pharynx to the esophagus, and air passes from the pharynx to the trachea.

Phenotype: The observed properties or outward appearance of a trait. The physical expression of the alleles posessed by an organism.

Pheromones: Chemical signals that travel between organisms rather than between cells within an organism; serve as a form of communication between animals.

Phloem: Tissue in the vascular system of plants that moves dissolved sugars and other products of photosynthesis from the leaves to other regions of the plant. Phloem tissue consists of cells called . . . View Full Definition

Phosphate Group: A chemical group composed of a central phosphorous bonded to three or four oxygens. The net charge on the group is negative.

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