Biology

Placenta The placenta is a temporary organ that develops in the uterus during pregnancy to support the developing fetus by exchanging gases, nutrients, wastes and hormones between mother an…

Plankton: drifting life in the sunlit oceans Plankton Plankton are the drifting microscopic and macroscopic organisms of surface waters. This article explains their groups, ecology, distribution, and importance to marine food webs and…

Planococcaceae: family of Gram-positive bacteria Planococcaceae Overview of Planococcaceae, a family of mostly aerobic Gram-positive bacteria: morphology, ecology, taxonomy, uses, and distinguishing features with links to further resources.

Plant cell: structure, functions, and distinguishing features Plant cell An overview of plant cells: their major structures, key functions like photosynthesis and turgor, how they differ from animal cells, and why they matter in ecology and biotechnolog…

Plaque: flat objects, biological films, and arterial deposits Plaque Plaque denotes flat plate-like objects and several biological deposits: bacterial colonies on culture, dental biofilm, lipid deposits in arteries, and commemorative metal or cerami…

Plasmid — extrachromosomal DNA elements, biology and applications Plasmid Plasmids are independently replicating DNA molecules, usually circular, found mainly in bacteria. This article explains their structure, transfer mechanisms, ecological roles, hist…

Plasmodesma: plant cell intercellular channels Plasmodesma Plasmodesmata are membrane-lined channels that traverse plant cell walls, enabling regulated symplastic transport, developmental signaling and coordination between adjacent plant c…

Plasmodiidae: family of blood parasites including Plasmodium Plasmodiidae Plasmodiidae is a family of apicomplexan parasites that infect vertebrate blood, best known for the genus Plasmodium, the cause of malaria. Members have complex life cycles involvi…

Plasmodium: genus of malaria-causing parasitic protozoa Plasmodium Plasmodium is a genus of about 200 parasitic protozoa that cause malaria in vertebrates, including humans. Explore its mosquito-transmitted life cycle, major human species, and glo…

Platelet (thrombocyte): structure, function, and clinical importance Platelet Platelets are small, anucleate blood fragments essential for clotting, vessel repair, and immune responses. This article explains their structure, production, roles in hemostasis,…

Platyzoa: a proposed assemblage of small and flat-bodied protostomes Platyzoa Platyzoa is a proposed higher-level grouping of mostly small, often flat-bodied protostome animals. The composition and relationships of Platyzoa remain debated among taxonomists a…

Pleiotropism (pleiotropy): when one gene affects many traits Pleiotropism Pleiotropism is the phenomenon where a single gene influences multiple distinct phenotypic traits. It is common in development, important in medicine and evolution, and alters how…

Ploidy: chromosome set number and its biological significance Ploidy Ploidy denotes the number of complete chromosome sets in a cell. This article explains terms, life-cycle roles, examples across plants and animals, causes of polyploidy, and its ec…

Pneumoconiosis: chronic lung disease from inhaled mineral and dust particles Pneumoconiosis Pneumoconiosis is a group of noninfectious occupational lung diseases caused by inhaling mineral dusts (coal, silica, asbestos). Progressive, largely irreversible fibrosis with pre…

Point mutation A point mutation is a change affecting a single nucleotide in genetic material. It includes substitutions, single-base insertions or deletions and can alter protein sequence, funct…

Pollen: the male gametophyte and microscopic carrier of plant sperm Pollen Pollen is the microscopic powder composed of male gametophytes in seed plants. It carries sperm cells, enables fertilization, fuels pollinators, and is used in allergy diagnosis, e…

Polydnaviruses Polydnavirus Polydnaviruses are a unique group of insect-associated viruses that live in mutualism with parasitoid wasps and help suppress host immune defenses during parasitism.

Polymerase chain reaction (PCR): principles, history, and applications Polymerase chain reaction PCR is a laboratory method that amplifies specific DNA sequences. This article explains how PCR works, its components, key historical milestones, common variants, and major applica…

Polymorphism (biology): distinct forms within a population Polymorphism Polymorphism is the coexistence of two or more clearly different forms or phenotypes in the same population, maintained by genetic, developmental or ecological processes.

Polypeptide: Structure, Biosynthesis, and Biological Roles Polypeptide A polypeptide is a linear chain of amino acids joined by peptide bonds. This article explains their structure, how cells make them, their roles in biology and medicine, and how the…

Polyphyly: definition, examples, and significance in biological classification Polyphyly Polyphyly is a pattern of grouping organisms that do not share a recent common ancestor within the group. This article explains the concept, examples, history, and how taxonomists…

Pons: structure, function, development and clinical significance Pons The pons is the bulbous part of the brainstem that links the cerebrum, cerebellum and medulla. It contains nuclei, fibre tracts and reticular networks essential for motor control,…

Population genetics: principles, history, and applications Population genetics Field studying genetic variation within and among populations, using evolutionary theory, statistics and modelling to explain heredity, adaptation and genetic change over time.

Pore (opening in biology, materials, and geology) Pore A pore is a small opening or cavity that permits passage of fluids, gases or particles; found in skin, plant leaves, soils, rocks and manufactured materials with roles in exchange,…

Pore (skin): structure, function, and clinical significance Pore (skin) A pore is a small opening in the skin that connects to hair follicles and glands. Pores release sebum and sweat, helping lubrication and temperature control; their appearance and h…

Positive-sense single-stranded RNA virus Viruses whose genomes are single-stranded RNA of positive polarity; the genome can act as mRNA, enabling direct translation. Includes many human, animal, and plant pathogens and di…

Poxviridae: family of large double-stranded DNA viruses infecting humans and animals Poxviridae Poxviridae: family of large double-stranded DNA viruses that infect humans and animals, cause skin and mucous lesions, replicate in the cytoplasm, and include smallpox, vaccinia, m…

Precocial (biology): mature and mobile young at birth or hatching Precocial Precocial describes a developmental strategy in which newborn or newly hatched animals are relatively mature and mobile. It contrasts with altricial development and occurs mainly a…

Predation in ecology: interactions, adaptations, and ecological role Predation Predation is the biological interaction in which a predator hunts, kills, or consumes other organisms (prey). This article explains types, adaptations, ecological importance, examp…

Pregnancy Pregnancy is the physiological condition in which a female mammal carries developing offspring; in humans it typically spans about 38 weeks from conception to birth, requiring pren…

Premolar (tooth): form, function, and variation Premolar Premolars are the transitional teeth between canines and molars in mammals, adapted for crushing and grinding; this article covers anatomy, development, variation among species, an…

Prion: proteinaceous infectious agents that cause neurodegenerative disease Prion Prions are misfolded, infectious proteins that induce abnormal folding of normal proteins, causing fatal neurodegenerative conditions in mammals and other organisms; they are disti…

Proboscis: elongated snouts and tubular feeding organs Proboscis An elongate projection at the front of the head—seen as an elephant's trunk, a tapir's snout, or the tubular mouthparts of insects and worms—used for feeding, sensing and manipulat…

Progesterone Progesterone is a natural steroid hormone important for the menstrual cycle, pregnancy maintenance, and reproductive medicine; it is produced from cholesterol and has clinical and…

Prokaryote: overview, structure, classification and ecological importance Prokaryote Prokaryotes are single-celled organisms—bacteria and archaea—characterized by a nucleoid, simple ribosomes and diverse metabolisms. This article covers their structure, evolution,…

Promoter (genetics) A promoter is a DNA region that directs the start and regulation of gene transcription. Promoters determine where RNA polymerase binds and influence when, where and how strongly a…

Propofol: general properties, uses, administration, and safety Propofol An overview of propofol, an intravenous anesthetic agent: pharmacology, clinical uses, administration, common risks, and historical and regulatory context.

Propolis A resinous, wax-like material gathered by bees from plant buds and used to seal and protect hives; valued in folk medicine and studied for antimicrobial and anti-inflammatory prope…

Prosencephalon (Forebrain): Structure, Development, and Clinical Significance Prosencephalon The prosencephalon, or forebrain, is the anterior part of the vertebrate brain. This article outlines its subdivisions, development, functions, and clinical conditions linked to ab…

Prosimian — informal grouping of early primates Prosimian Prosimian describes an informal, paraphyletic assemblage of early or 'primitive' primates that includes lemurs, lorises, tarsiers and several extinct lineages; used for comparison…

Protacanthopterygii Protacanthopterygii is a superorder of mostly temperate soft-rayed teleost fishes including salmon, trout, pike and smelts; composition and relationships vary among classifications…

Protease — enzymes that cleave proteins and their biological roles Protease Proteases are enzymes that hydrolyze peptide bonds, producing peptides and amino acids. Found across life, they form several catalytic classes and function in digestion, regulation…

Protein: structure, synthesis, functions, and significance Protein Proteins are polymers of amino acids that fold into functional molecules. This article outlines their structure, synthesis, roles in biology, medical relevance, industrial uses, an…

Protein biosynthesis: how cells make proteins Protein biosynthesis Overview of protein biosynthesis: stages from genetic information to folded protein, core machinery (ribosomes, tRNA, amino acids), differences between prokaryotes and eukaryotes,…

Protein Data Bank (PDB): archive of macromolecular structures Protein Data Bank The Protein Data Bank (PDB) is the open international archive of experimentally determined three-dimensional structures of proteins, nucleic acids and complexes used across biology…

Protein filament: structure, types, and biological roles Protein filament Protein filaments are elongated polymers of protein subunits that form structural and dynamic elements in cells, from cytoskeleton networks to hair and flagella.

Protein folding: how polypeptides acquire their functional three‑dimensional form Protein folding Overview of how linear amino acid chains fold into functional protein structures, the forces and pathways involved, cellular helpers, misfolding and disease, and modern experimenta…

Protein structure and folding: architecture, dynamics, and methods of study Protein structure Overview of how proteins are built and folded, levels of structural organization, forces that shape them, experimental and computational methods, biological roles, and examples of…