Biology

On the Origin of Species — Charles Darwin's 1859 work On the Origin of Species Charles Darwin's On the Origin of Species (1859) introduced evolution by natural selection, reshaping biology, science, and public debate. This article summarizes its content, publ…

Oncogene: genes that promote uncontrolled cell growth Oncogene An oncogene is a gene whose altered activity can drive uncontrolled cell proliferation and contribute to cancer; this article explains mechanisms, examples, history, and clinical r…

Online Mendelian Inheritance in Man (OMIM): a concise reference for human genes and phenotypes Online Mendelian Inheritance in Man OMIM is a curated catalog of human genes and genetic phenotypes, linking genetic variants to clinical features and literature. It is searchable through major biomedical resources.

Oomycete (water mold): biology, life cycle, and impact Oomycete Oomycetes are fungus-like microorganisms in the heterokont lineage. This article explains their biology, differences from true fungi, life cycle, historical classification, and eco…

Oophorectomy Oophorectomy is the surgical removal of one or both ovaries. This article explains types, indications, surgical approaches, short- and long-term effects, historical context, and et…

Opisthokonts: animals, fungi and their unicellular relatives Opisthokont Opisthokonts are a major eukaryotic clade that includes animals, fungi and related single-celled lineages, united by shared cellular traits and supported by molecular phylogenetics…

Opsin — light-sensitive proteins that mediate photoreception Opsin Opsins are light-activated G protein‑coupled proteins that convert photons into cellular signals, enabling vision, colour discrimination and non-image responses such as circadian e…

Optic nerve (cranial nerve II) Optic nerve The optic nerve transmits visual signals from retinal ganglion cells to central visual centers. It is developmentally and structurally part of the central nervous system and essent…

Optic tectum: structure, function, and evolutionary roles Optic tectum An overview of the optic tectum, a layered midbrain center for vision and orienting behavior in vertebrates, its structure and function, species differences, and relevance to resea…

Optic tract (visual pathway segment) Optic tract The optic tract is a paired bundle of nerve fibers in the brain that relays information from the eyes to central visual nuclei; it carries signals representing the contralateral vi…

Order (biology): taxonomic rank in classification Order (biology) An order is a principal taxonomic rank in biological classification, below class and above family. Orders group related families and are used in naming, identification, and biodive…

Organ (biology, musical instrument, and institutional uses) Organ An organ is a distinct functional unit: in biology, a structure composed of tissues serving a role in an organism; in music, a keyboard instrument; also a term for governmental bod…

Organ (biology) Organ (anatomy) An organ is a structure of tissues in multicellular organisms that carries out one or more specialized functions; found in animals, plants and fungi and combined into organ systems…

Organ donation and transplantation Organ donation Organ donation transfers organs or tissues from living or deceased donors to recipients by transplant surgery; it saves lives, raises legal and ethical issues, and intersects with…

Organ system An organ system is a collection of organs that cooperate to carry out broad physiological functions; this article explains organization, examples, development, and significance in…

Organelle (cell biology): structure, types, and function Organelle An organelle is a specialized subunit within a cell that performs a distinct function. This article explains types, structures, origins, roles in health and key distinctions betwee…

Organic: Concepts in Chemistry, Biology, and Agriculture Organic Concise guide to the term "organic": meanings in chemistry, biology and ecology, organic agriculture and food standards, history, common distinctions, and practical importance.

Organism: definition, features, organization, and borderline cases Organism An organism is an individual living entity. This article explains defining features, structural levels, examples from single-celled to multicellular life, viruses and spores, and e…

Organotroph: organisms that use organic molecules as electron donors Organotroph Organotrophs obtain electrons from organic compounds to support respiration or fermentation. Term used in microbial physiology to classify organisms by their electron source; may b…

Origin of Birds: Evolutionary Transition from Theropod Dinosaurs Origin of birds Summary of how birds originated from theropod dinosaurs, key fossil and anatomical evidence, historical milestones, hypotheses about the origin of flight, and the significance of a…

Origin of life: hypotheses, evidence, and open questions Origin of life Overview of scientific ideas about how life began on Earth, reviewing major hypotheses, steps toward cells, supporting evidence, experimental approaches, and remaining uncertaintie…

Ornithology: the scientific study of birds Ornithology Ornithology is the branch of zoology devoted to the study of birds: their anatomy, behavior, ecology, evolution, and conservation, using fieldwork, lab research, and modern technol…

Orthomyxoviridae: family of segmented negative-strand RNA viruses Orthomyxoviridae Orthomyxoviridae are enveloped, segmented negative-sense RNA viruses including influenza A, B and C, thogotoviruses and isaviruses. They infect birds, mammals, fish and some arthro…

Orthonectida — simple multicellular parasites of marine invertebrates Orthonectida Orthonectida are tiny, simple parasitic animals that infect marine invertebrates. They have a reduced body plan, a brief life cycle with ciliated stages, and are important for stud…

Osmoregulation: how organisms control water and salt balance Osmoregulation Osmoregulation is the biological control of water and dissolved solutes across membranes. This article explains mechanisms, examples in microbes, plants and animals, physiological…

Osmosis: solvent movement through semipermeable membranes Osmosis Osmosis is the spontaneous flow of solvent molecules across a semipermeable membrane toward higher solute concentration; it underlies water balance in cells and technologies such a…

Ossicles (malleus, incus, stapes) — the ear's tiny bones Ossicles The three small middle-ear bones (malleus, incus and stapes) transmit and amplify sound in mammals. Their integrity is essential for hearing; damage or fixation causes conductive h…

Osteichthyes — the bony fishes Osteichthyes Osteichthyes, or bony fishes, are the largest group of vertebrates, including ray‑finned and lobe‑finned fishes; they show diverse adaptations and have major ecological and human i…

Osteoarthritis Osteoarthritis is a degenerative joint condition marked by cartilage breakdown, pain, stiffness and reduced mobility. This article explains causes, pathology, symptoms, diagnosis,…

Osteology: the scientific study of bones and the skeleton Osteology Osteology is the branch of anatomy concerned with bones, teeth and bony structures; it informs forensic, archaeological, paleontological and medical investigations of vertebrate re…

Osteoporosis — causes, symptoms, diagnosis, prevention and treatment Osteoporosis Overview of osteoporosis: what it is, causes and risk factors, common symptoms, how it is diagnosed, prevention strategies, treatment options and public-health impact.

Ostreoida: the bivalve order including oysters and scallops Ostreoida Ostreoida is an order of marine bivalve mollusks that traditionally groups oysters and related families such as scallops; important ecologically and commercially as filter feeders…

Outbreak (epidemiology): causes, characteristics, and public health response Outbreak An outbreak is a rise in cases of a health-related event above expected levels. This article explains definitions, features, history, detection, control measures, and differences f…

Outcrossing (genetics) Outcrossing Outcrossing: mating between unrelated individuals. Covers definition, genetic effects including heterosis, historical use, applications in breeding and conservation, and contrasts…

Ovary — structure, function, and variation in animals Ovary Ovaries are female reproductive organs that produce ova and sex hormones; this article explains their anatomy, development, role in reproduction, variation among animals, and clini…

Ovipositor: insect egg-laying organ and its forms Ovipositor An ovipositor is a rear appendage in female insects (and some other animals) used to lay eggs. It varies from a simple tube to a piercing or saw-like tool and is sometimes modified…

Ovum (egg) — the female reproductive cell Ovum An ovum (plural ova) is the haploid female reproductive cell or gamete produced by animals and land plants; it is formed by meiosis and, when fertilized, gives rise to a zygote.

Oxytocin: the peptide hormone and neuropeptide Oxytocin Oxytocin is a nonapeptide hormone and neuropeptide that regulates childbirth, lactation, social bonding and stress-related behavior; it has clinical uses and is widely studied for…

p53 (TP53): the tumor suppressor and guardian of the genome p53 p53 (TP53) is a widely conserved tumor suppressor gene encoding a DNA-binding protein that controls cell cycle arrest, DNA repair, senescence and apoptosis; frequently mutated in c…

Palaeoptera — primitive winged insects that do not fold their wings Palaeoptera Palaeoptera is a traditional grouping of primitive winged insects (mayflies and dragonflies) that cannot fold their wings; its monophyly is debated and much of the group is known o…

Palate (anatomy): structure, function, development and clinical notes Palate The palate is the roof of the mouth separating oral and nasal cavities. This article explains hard and soft palates, development, functions in speech and swallowing, comparative an…

Palatine bone The palatine bone is a paired L-shaped bone at the back of the nasal cavity and hard palate, contributing to the orbit floor, nasal wall and serving as passage for neurovascular st…

Paleobiology: study of ancient life and its biological context Paleobiology Paleobiology investigates fossil organisms and their biology, ecology and evolution by combining methods from biology and geology to reconstruct past life and environments.

Paleozoology: study of ancient animal life and fossils Paleozoology Paleozoology examines animal fossils to reconstruct anatomy, behavior, environments and evolutionary history using comparative anatomy, stratigraphy, and modern analytical methods.

Palynology: study of pollen, spores and microscopic particles Palynology Palynology examines microscopic particles such as pollen, spores and other organic/inorganic debris to reconstruct past environments, aid in forensics, archaeology and petroleum ex…

Pancreas: anatomy, functions, and clinical importance Pancreas Comprehensive overview of the pancreas: anatomy, exocrine and endocrine functions, regulation, common diseases (diabetes, pancreatitis, cancer), diagnosis, treatment and current re…

Pandoravirus: giant amoeba-infecting viruses with exceptionally large genomes Pandoravirus Pandoraviruses are giant viruses that infect amoebae, notable for genomes of roughly 1.9–2.5 Mb, a high proportion of unique genes, unusual particle shape, and implications for vir…

Paramecium: a common ciliate protist Paramecium Paramecium is a freshwater ciliate genus of single-celled organisms known for ciliary locomotion, visible macronucleus and micronucleus, osmoregulation, and roles in ecology and ce…