Biology

Proteobacteria: diversity, characteristics, ecological and medical roles Proteobacteria Proteobacteria are a large, diverse phylum of Gram-negative bacteria defined by rRNA sequences; they include major environmental players, symbionts, and many human pathogens.

Proteome: the complete set of proteins expressed by an organism Proteome A proteome is the full complement of proteins produced by a cell, tissue or organism at a given time. It encompasses sequence variants, modifications and expression dynamics studie…

Protists: a diverse assemblage of mostly single-celled eukaryotes Protist Protists are a varied collection of mostly single-celled eukaryotes with complex cells, diverse modes of nutrition and movement, important ecological roles, and some notable human…

Protoclepsydrops — early Carboniferous amniote of the Joggins cliffs Protoclepsydrops Protoclepsydrops is a fragmentarily known early amniote from the Carboniferous of Nova Scotia, possibly the oldest synapsid; its classification and age have been debated due to lim…

Protocoleoptera: the earliest beetles and their fossil evidence Protocoleoptera Protocoleoptera refers to primitive, stem-group beetles known from Permian and Triassic fossils. Their modified forewings (elytra) obscure venation, complicating classification and…

Protoplasm: the historical concept of living cell substance Protoplasm Protoplasm is an older term for the living material of a cell. This article explains its composition, historical usage, relationship to modern cell concepts, and why biologists now…

Protoplast (wall-less cell): definition, isolation, uses, and distinctions Protoplast A protoplast is a plant, bacterial or fungal cell with its cell wall removed. This article covers structure, methods for producing protoplasts, applications in research and biotech…

Protostomes Protostome Protostomes are a major grouping of bilaterally symmetrical animals distinguished by early embryonic development; they include arthropods, molluscs, annelids, nematodes and several…

Protozoa: single-celled eukaryotes, diversity, ecology and significance Protozoa Protozoa are diverse single-celled eukaryotes—mostly heterotrophs—found in moist habitats. Once a formal group, they are now treated as multiple lineages within the protists and pl…

Pseudogene A pseudogene is a DNA sequence that resembles a gene but has lost its original protein-coding or gene-expression function. It can illuminate genome evolution and sometimes influenc…

Pseudomonadales: biology, ecology, and significance Pseudomonadales Pseudomonadales is an order of mostly aerobic, gram-negative Proteobacteria known for metabolic versatility, roles in nitrogen fixation and bioremediation, and includes opportunist…

Pseudomonas: ecology, biology, and clinical relevance Pseudomonas Pseudomonas is a diverse genus of gram-negative bacteria found in soil, water, plants and animals; it includes opportunistic human pathogens, plant-beneficial strains, and species…

Pseudomonas aeruginosa: biology, clinical impact and control Pseudomonas aeruginosa An overview of Pseudomonas aeruginosa, a Gram-negative opportunistic bacterium known for environmental persistence, biofilm formation, virulence factors and multidrug resistance in…

Pseudopodia: cytoplasmic extensions for movement and feeding Pseudopodia Temporary, cytoplasm-filled cellular projections used by many eukaryotes for locomotion and phagocytosis; includes lobopodia, filopodia, reticulopodia and axopodia and relies on ac…

Pseudosuchia (crocodile-line archosaurs) Pseudosuchia Pseudosuchia are the crocodile-line archosaurs, a major archosaur clade that includes living crocodilians and a diverse suite of extinct Triassic relatives distinguished by a chara…

Pteropoda: pelagic gastropods known as sea butterflies and sea angels Pteropoda Pteropods are small, free-swimming marine gastropods—often transparent—grouped as sea butterflies (shelled) and sea angels (shell-less); they play key roles in open-ocean food webs…

Pterygota: the winged insects and their descendants Pterygota Pterygota is the insect subclass that comprises most winged species and groups that lost flight secondarily. Covers defining features, evolution, main clades and ecological importa…

Puberty: Biological changes, timing, development and social aspects Puberty Puberty is the developmental stage when a child's body matures into an adult body capable of reproduction. This article explains biological processes, typical changes, timing, heal…

Pubis (pubic bone) Pubis (bone) The pubis is the anterior part of the hip bone, forming the front of the pelvis, contributing to the acetabulum and pelvic outlet, and serving as an attachment site for muscles and…

Pulmonary artery: anatomy, function, and clinical significance Pulmonary artery Concise overview of the pulmonary artery: structure, role in pulmonary circulation, development, clinical conditions, and distinguishing features.

Pulmonary fibrosis: causes, symptoms, diagnosis and management Pulmonary fibrosis Pulmonary fibrosis is progressive scarring of the lungs that impairs oxygen transfer. This article summarizes causes, symptoms, diagnostic approaches, treatments and prognosis.

Pupa: the insect stage between larva and adult Pupa A pupa is the often immobile developmental stage in holometabolous insects when larval tissues are reorganized into adult form. Covers types, biology, examples (chrysalis, puparium…

Pupil (eye): structure, function, reflexes and clinical importance Pupil (eye) An overview of the pupil — the central opening of the eye — explaining its anatomy, control, reflexes, variations among species and clinical significance.

Purple bacteria Purple bacteria are phototrophic Proteobacteria that perform anoxygenic photosynthesis using bacteriochlorophylls and carotenoids; they inhabit aquatic, sediment and microbial mat…

Putrefaction: the microbial breakdown of organic matter Putrefaction Putrefaction is the breakdown of dead organic material by microorganisms, producing gases, liquids and nutrients. It is central to decomposition, nutrient cycling, and forensic and…

Qingjiang biota: exceptional Cambrian fossil lagerstätte in Hubei, China Qingjiang biota The Qingjiang biota is a richly preserved Cambrian fossil site in Hubei, China, notable for abundant soft‑bodied animals and exceptional soft‑tissue preservation that illuminates e…

Quadruped walk: mechanics, variations, and evolutionary context Quadruped walk The quadruped walk is the four-limbed mode of terrestrial locomotion used by many vertebrates. This article explains its mechanics, common gaits, evolutionary origins, and distinct…

Race (biology): populations, differences, and taxonomic use Race (biology) Biological race describes distinct populations within a species with modest morphological or genetic differences. The term contrasts with subspecies and has ecological, geographic,…

Radiata: historical grouping of radially symmetric animals Radiata Radiata is a historical term for animals with radial or biradial body plans (notably cnidarians and ctenophores). Modern phylogenetics treats it cautiously and often rejects it as…

Radiolaria: silica-skeleton protists of the open ocean Radiolaria Radiolaria are predominantly planktonic, single-celled protists that build intricate mineral skeletons, contribute to marine silica and carbon cycles, and leave abundant microfossi…

Radioresistance: biological resistance to radiation Radioresistance Radioresistance is the ability of cells or organisms to withstand damaging effects of ionizing and certain non-ionizing radiation; mechanisms include DNA repair, antioxidants, spor…

Radius (bone) The radius is the lateral forearm bone that enables wrist motion and rotation of the hand. This article covers its anatomy, function, development, evolutionary context and clinical…

Radula — the toothed feeding organ of molluscs Radula A chitinous, toothed ribbon used by most molluscs to rasp, cut or scrape food. Present in snails and cephalopods but absent from bivalves; important in feeding, taxonomy and materi…

Range (biology): species distribution, patterns, measurement, and conservation Range (biology) An overview of a species' geographic range: definitions, types of distribution patterns, measurement methods, causes of change, and conservation implications.

Range of motion (ROM): joint and muscle movement capacity Range of motion Range of motion (ROM) is the span of movement at a joint or muscle group. It is measured clinically, affects function and performance, and guides rehabilitation and injury manageme…

Rapid eye movement (REM) sleep Rapid eye movement sleep REM sleep is a distinct stage of sleep marked by rapid eye movements, vivid dreaming, and characteristic brain and muscle activity. It cycles with non‑REM sleep and appears across…

Recapitulation theory (ontogeny recapitulates phylogeny) Recapitulation theory Historical biological idea that an individual’s embryonic development mirrors the species’ evolutionary history; once influential, now replaced by more nuanced developmental-evolut…

Receptor (cell biology) A receptor is a protein molecule that detects chemical signals and initiates cellular responses. Covers types, structure, mechanisms of signal transduction, physiological roles, an…

Receptor antagonist A receptor antagonist is a molecule that binds to a cellular receptor and prevents activation, used widely in physiology and medicine to modulate signalling and treat disease.

Recognition (cognitive process) Recognition Recognition is the cognitive ability to identify and classify sensory input based on past experience. It spans perception, memory, learning, animal behavior, human social uses, and…

Rectum (anatomy, function, and clinical importance) Rectum Overview of the rectum: structure, role in defecation, temperature measurement, common conditions, diagnostic procedures, and key anatomical relationships.

Red algae (Rhodophyta): biology, ecology and human uses Red algae Red algae (phylum Rhodophyta) are mostly marine, photosynthetic eukaryotes notable for red phycobiliproteins, unique cellular traits, complex life cycles and important ecological a…

Red blood cell (erythrocyte) Red blood cell Red blood cells (erythrocytes) are the oxygen-carrying cells of blood; they have a distinctive biconcave shape, contain hemoglobin, and are produced in bone marrow with a lifespan…

Reflex (automatic physiological response) Reflex A reflex is a rapid, involuntary response to a stimulus mediated by a reflex arc. This article explains mechanisms, types, development, clinical relevance and distinctions from fix…

Regeneration (biological tissue regrowth) Regeneration Regeneration is the biological process by which organisms restore lost or damaged tissues and structures, from cell renewal to whole-limb regrowth; it is studied to inform medicine…

Regurgitation: biological process, causes, and ecological roles Regurgitation Regurgitation is the return of partly digested food from the digestive tract to the mouth. It occurs in many species for feeding, cleaning, communication, or as a medical sign and…

Relaxin: peptide hormone in reproduction, tissue remodeling and cardiovascular function Relaxin Relaxin is an insulin-family peptide hormone that modulates connective tissue remodeling, vascular tone and reproductive processes. Multiple relaxin peptides and receptors produce…

Renin: kidney-derived enzyme that controls blood pressure Renin Renin is an enzyme produced by the kidneys that initiates the renin–angiotensin–aldosterone system, regulating blood pressure, fluid balance and electrolyte homeostasis.