Biology

Paramyxovirus Paramyxoviruses are enveloped, negative-sense RNA viruses in the family Paramyxoviridae that cause a range of respiratory and systemic diseases in humans and animals, including mea…

Paranasal sinuses: structure, function, development, and clinical relevance Paranasal sinuses Paranasal sinuses are air-filled cavities in facial bones that communicate with the nasal cavity; they are lined by mucosa and have roles in respiration, voice resonance, skull lig…

Paraphyly in biological classification Paraphyly Paraphyly describes a group that contains a common ancestor and some, but not all, of its descendants. It contrasts with monophyly and has practical and historical implications in…

Paraneoptera — the insect superorder containing lice, thrips and true bugs Parapneuroptera Paraneoptera is a monophyletic superorder of insects that includes bark lice and true lice, thrips, and hemipterans. Members show diverse mouthparts and feeding strategies and incl…

Parasitism: biological interactions, types, and impacts Parasitism Parasitism is a biological relationship in which one organism benefits at the expense of another. This article explains definitions, types, life cycles, ecological roles, human exa…

Parasitoid (insect life strategy) Parasitoid Parasitoids are animals—most commonly insects—whose immature stages develop by feeding on a single host organism, eventually killing it. They are ecologically important and widely…

Parasympathetic nervous system: function, anatomy, and clinical relevance Parasympathetic nervous system The parasympathetic nervous system is the 'rest-and-digest' branch of the autonomic nervous system. It conserves energy, supports digestion, lowers heart rate, and counterbalances…

Parathyroid glands — anatomy, function, development, and clinical importance Parathyroid glands Small endocrine glands located behind the thyroid that secrete parathyroid hormone to regulate calcium balance; variable in number and clinically important for bone, kidney, and ne…

Paraves: the clade linking birds and their closest dinosaur relatives Paraves Paraves is a branch-based clade that includes modern birds and their closest non-avian relatives, including dromaeosaurids, troodontids and early four-winged species important to t…

Parazoa: the basal animal group including sponges and placozoans Parazoa Parazoa is a small subkingdom of simple animals—chiefly sponges (Porifera) and placozoans—characterized by porous bodies, few cell types, absence of true tissues, and important eco…

Parenthesome (septal pore cap) Parenthesome Parenthesomes, or septal pore caps, are membranous structures that overlie the dolipore septum in many basidiomycete fungi; their forms vary and they help distinguish taxa though t…

Parietal lobe The parietal lobe is a region of the cerebral cortex that integrates sensory information to support spatial awareness, touch perception, and coordination between sensation and acti…

Parotoid gland A parotoid gland is a prominent external skin gland found behind the eyes of many toads and some frogs and salamanders; it secretes toxic compounds used mainly for predator deterre…

Parthenogenesis: asexual reproduction by unfertilized eggs Parthenogenesis Parthenogenesis is reproduction from an unfertilized egg. This article reviews mechanisms, forms, occurrence in plants and animals, examples, and evolutionary consequences.

Pasteurella multocida Pasteurella multocida is a Gram-negative, nonmotile coccobacillus found as a commensal in many animals; it causes important veterinary diseases and can infect humans, often after a…

Pathogen — agents that cause infectious disease Pathogen Overview of pathogens: definitions, types (viruses, bacteria, fungi, parasites), mechanisms of disease, host defenses, historical outbreaks, plant pathogens, and prevention and con…

Pectinase — enzymes that break down plant pectin and their uses Pectinase Pectinases are enzymes that degrade pectin, a plant cell‑wall polysaccharide. They occur in plants, microbes and fungi and are used in food processing, winemaking, protoplast isola…

Pectinina (bivalve suborder) Pectinina Pectinina is a suborder of marine bivalves in the order Ostreoida that includes scallop-like and jingle-shell families. It groups Pectinoidea and Anomioidea; important ecologically…

Pediococcus (genus of lactic acid bacteria) Pediococcus Pediococcus is a genus of gram-positive, homofermentative lactic acid bacteria that form pairs or tetrads. Common in foods, plants and brewing; used in fermentations but can also s…

Pelvis (human anatomy): structure, function and development Pelvis Overview of the human pelvis: its bones, joints, muscles, evolutionary origin, role in support, locomotion and childbirth, sex differences, and common clinical considerations.

Pelycosaurs: early synapsids that preceded therapsids and mammals Pelycosaur Informal group of early synapsids from the late Carboniferous to the end of the Permian. Not reptiles or dinosaurs; important for understanding the rise of therapsids and mammal an…

Penicillium — overview, biology, uses and significance Penicillium Penicillium is a large genus of filamentous ascomycete fungi found worldwide. Species are important decomposers, food-fermenters and sources of antibiotics, enzymes and mycotoxins.

Penis: structure, function, and variation across animals Penis Overview of the penis as a reproductive and urinary organ, its anatomical features, variation among species, development and evolution, common health considerations, and notable di…

Peppered moth evolution: industrial melanism, genetics, and scientific significance Peppered moth evolution How the peppered moth (Biston betularia) shifted from light to dark forms during industrialization, the genetics and experiments behind that change, and why it remains a key exampl…

Pepsin: the principal gastric protease Pepsin Pepsin is the main digestive protease of the stomach, active in strongly acidic conditions. This article explains its properties, activation, history, and common uses in food and r…

Peptide (biochemistry): short amino‑acid chains, structure and uses Peptide Peptides are short chains of amino acids joined by peptide (amide) bonds. This article explains their structure, biological roles, synthesis methods, applications, and how they dif…

Peripheral nervous system (PNS): structure, function and clinical overview Peripheral nervous system Overview of the peripheral nervous system: anatomy, subdivisions (somatic, autonomic, enteric), nerve structure, vulnerabilities, common disorders, diagnosis and principles of repa…

Peristalsis: coordinated muscular waves in digestion and other organs Peristalsis Peristalsis is the coordinated contraction and relaxation of muscles that propels contents through tubular organs. This article explains its mechanism, anatomy, examples, control a…

Peroxisome: structure, functions, and biological significance Peroxisome Peroxisomes are single-membrane organelles in eukaryotic cells involved in lipid metabolism, hydrogen peroxide handling, and cellular signaling; defects cause human metabolic disor…

Pest (biology) Organisms that damage human interests—crops, health, property or ecosystems—defined by context. Overview, examples, impacts, management approaches, history and distinctions from re…

Pesticide: Types, Uses, Environmental and Health Considerations Pesticide Comprehensive overview of pesticides: definitions, main types, history, uses, environmental and health impacts, regulation, resistance, safety measures, and alternatives.

Petri dish: design, history, and uses in microbiology and education Petri dish A Petri dish is a shallow, cylindrical laboratory dish used to culture microorganisms and cells. This article explains its design, history, common uses, variants, and safety consid…

Phagocyte A phagocyte is an immune cell that engulfs microbes, dead cells, and debris; central to innate defense, tissue cleanup, and triggering adaptive immunity.

Pharmacokinetics: Principles of How the Body Processes Drugs Pharmacokinetics Pharmacokinetics studies how drugs enter, move through, and leave the body. It covers absorption, distribution, metabolism, excretion, key parameters, models, clinical implications…

Phenotype — observable characteristics and biological expression of organisms Phenotype Phenotype: the full set of an organism's observable traits and characteristics, shaped by genes, environment, and their interaction; central to development, ecology, medicine, and…

Pheromone A pheromone is a chemical signal emitted by an animal that triggers specific behavioral or physiological responses in conspecifics; important in insects, vertebrates and human rese…

Phospholipid: structure, function, and applications Phospholipid Phospholipids are amphipathic lipids containing phosphorus that assemble into membranes. This article explains their structure, biological roles, variety, and common uses in medici…

Photoreceptor cell Specialized sensory neurons that detect light and begin visual signaling. Includes vertebrate rods and cones, diverse in other animals, and central to vision, phototransduction, di…

Photosynthesis: How Organisms Convert Light into Chemical Energy Photosynthesis Photosynthesis is the process by which plants, algae and some bacteria use sunlight to produce sugars and oxygen from water and carbon dioxide, forming the base of most ecosystems.

Phototroph — organisms that use light to make organic matter Phototroph Phototrophs are organisms that capture light energy to synthesize organic compounds from inorganic sources, driving primary production and supporting most ecosystems on Earth.

Phycology: the scientific study of algae Phycology Phycology (algology) is the branch of botany that studies algae — from single-celled microalgae to large seaweeds — their biology, ecology, uses and role in aquatic and terrestrial…

Phylogeny: the study and representation of evolutionary relationships Phylogeny Phylogeny describes hypotheses about how organisms are related through common descent and the methods used to reconstruct those relationships from fossils, anatomy, and molecular d…

Phylum (biology): rank, definition, and role in classifying life Phylum A phylum is a major taxonomic rank used to group organisms that share a basic body plan or common ancestry. This article explains what a phylum is, how phyla are defined, and why t…

Physiology: function, levels, methods, and medical relevance Physiology Physiology examines how living organisms function, from molecular mechanisms to whole-body integration, covering human, animal, and plant systems and their relevance to health, dis…

Pimple (acne): causes, types, prevention and treatment Pimple A pimple is a common skin lesion caused by clogged hair follicles. This article explains causes, types, prevention, treatments, complications and when to see a healthcare professio…

Pineal gland: structure, function and clinical relevance Pineal gland Small pine-cone shaped endocrine organ deep in the brain that secretes melatonin. Regulates circadian rhythms and sleep, has diagnostic and therapeutic importance, and appears in s…

Pithovirus — a giant amoeba-infecting virus discovered in Siberian permafrost Pithovirus A genus of giant double-stranded DNA viruses that infect amoebae, first described from a 30,000-year-old Siberian permafrost sample; notable for its large size and unusual internal…

Pituitary gland: structure, function, development, and clinical relevance Pituitary gland A concise encyclopedia entry on the pituitary gland (hypophysis): its anatomy, lobes and hormones, control by the hypothalamus, development, and key clinical conditions and treatme…