Biology

Anaerobic organism Organisms that do not require oxygen for growth; includes obligate, facultative, aerotolerant types and microaerophiles, important in ecology, industry and medicine.

Anaerobic respiration Cellular respiration using terminal electron acceptors other than oxygen; common in microbes and anoxic environments, with key roles in biogeochemical cycles, biotechnology and som…

Anapsid (amniote skull type) Anapsid Anapsid: an amniote skull lacking temporal openings behind the eyes. A primitive condition in early Carboniferous amniotes and a secondary trait in some reptiles such as turtles an…

Anatomical terms of location Standardized Latin and Greek–derived words used to describe positions, directions, planes and relationships in animal and human anatomy for clear medical and scientific communicati…

Anatomy: structure, history, and applications of animal and human bodies Anatomy Anatomy is the scientific study of the structure and organization of living bodies. This article explains major branches, historical milestones, practical uses, and key distinction…

Aneuploidy: chromosomal imbalance, causes, consequences and examples Aneuploidy Aneuploidy is the presence of an abnormal number of chromosomes in a cell. This article explains causes, common syndromes, detection methods, links to miscarriage and cancer, and d…

Anfinsen's dogma: the sequence-driven view of protein folding and its limits Anfinsen's dogma Anfinsen's dogma (thermodynamic hypothesis) proposes that a protein's amino acid sequence determines its native structure; many proteins follow this, but chaperones, kinetics, and…

Anguilloidei (suborder of eels) Anguilloidei Anguilloidei is a suborder of Anguilliformes that includes freshwater eels, sawtooth eels and snipe eels. Members share an elongated body and a leptocephalus larva but occupy diver…

Animal colouration: causes, functions, and evolutionary significance Animal colour Overview of how animals produce colour, the biological mechanisms, ecological functions such as camouflage and signalling, evolutionary drivers, research methods, and notable examp…

Animal testing Use of animals in experiments to study biology, develop medicines, or test products; practices, species, history, ethical debate, regulations, and alternatives.

Annelid (segmented worm) Annelid Annelids are a diverse phylum of segmented, mostly soft-bodied worms found in marine, freshwater and terrestrial habitats. They play key ecological roles and include earthworms and…

Anoxia: causes, effects, and ecological and medical significance Anoxia Anoxia is the near absence of oxygen in tissues or environments. This article explains physiological and environmental anoxia, causes, ecological impacts, measurement, historical c…

Antenna (biology): sensory appendages of arthropods Antenna (biology) Antennae are paired sensory appendages on arthropods (insects, crustaceans) that detect touch, chemicals, air movement, sound and temperature. Structure, number and function vary a…

Anthocyanin Anthocyanin: water-soluble plant pigments in the flavonoid family that produce red, purple and blue colors; pH-sensitive, found in many fruits, vegetables and flowers and used as n…

Anthozoa: sea anemones, corals, and related polyp-form cnidarians Anthozoa Anthozoa is the class of cnidarians that includes sea anemones and corals. Anthozoans are exclusively polypoid, use nematocysts to feed and defend, and include reef-building and so…

Antibody (Immunoglobulin): Structure, Function and Medical Uses Antibody Antibodies are Y-shaped proteins of the adaptive immune system that recognize antigens. This article explains their structure, how they are produced, their roles, applications, and…

Antigen: definition, types, immune role and clinical importance Antigen An antigen is a molecule or molecular structure that the immune system recognizes as foreign or distinct; it triggers or participates in immune responses and is central to vaccinat…

Anus (human anatomy and function) Anus The anus is the terminal opening of the digestive tract. This article explains its anatomy, role in continence and defecation, common conditions, and basic care.

Apicomplexa: parasitic single-celled organisms with complex life cycles Apicomplexa Apicomplexa are single-celled, obligate parasites of animals with an apical complex and apicoplast; they include Plasmodium, Toxoplasma, Cryptosporidium and many coccidia important…

Apocrita: the narrow-waisted suborder of Hymenoptera Apocrita Apocrita is a major suborder of Hymenoptera that includes ants, bees, wasps and many parasitoids; characterized by a constricted waist, diverse life histories, and major ecological…

Apomixis: asexual seed formation and clonal reproduction in plants Apomixis Apomixis is a form of asexual seed formation in which embryos develop from maternal tissues without meiosis or fertilization, producing offspring genetically identical to the paren…

Apomorphy (biology): derived traits and their role in phylogeny Apomorphy An apomorphy is a derived character state that distinguishes a clade. This article explains definitions, types (synapomorphy, autapomorphy), examples, phylogenetic use, and complic…

Apoplexy Apoplexy is an older medical term for sudden bleeding or loss of function in an internal organ, formerly used for strokes; today it appears in specific diagnoses and in figurative…

Apoptosis: Programmed cell death in development, health, and disease Apoptosis Apoptosis is the regulated process of programmed cell death essential for development and tissue balance. This article summarizes its mechanisms, roles in development and disease,…

Appendage (anatomy): external projections of animal bodies Appendage An appendage is an external projection from an animal's main body used for locomotion, sensing, feeding or reproduction. Includes limbs, antennae, fins, wings and other specialized…

Appendix (anatomy): structure, function, evolution and clinical importance Appendix (anatomy) The appendix is a small, tube-shaped pouch attached to the cecum. This article explains its anatomy, possible roles, evolutionary background, clinical issues such as appendicitis,…

Aquatic: organisms, habitats, and key characteristics Aquatic Overview of aquatic life and environments, their main types, biological adaptations, ecological roles, examples, and distinctions between freshwater and marine systems.

Aquatic locomotion: principles, modes, and evolutionary context Aquatic locomotion Movement of organisms through water, covering physical principles, major modes (cilia, undulation, jetting, limb paddling), evolutionary origins, and ecological significance.

Aquificaceae: thermophilic bacteria of hot springs and hydrothermal systems Aquificaceae Aquificaceae are a family of heat-loving, chemolithoautotrophic bacteria found in hot springs and hydrothermal vents; they play roles in primary production, biogeochemical cycles a…

Aquificae: Thermophilic, Chemolithoautotrophic Bacteria of Hot Environments Aquificae Aquificae is a phylum of heat-loving, chemolithoautotrophic bacteria that dominate many high-temperature habitats (hot springs, hydrothermal vents) and serve as primary carbon fixe…

Arboreal: living in trees — definition, adaptations, and significance Arboreal Arboreal describes organisms adapted to live and move in trees. This article explains characteristics, common examples, evolutionary history, ecological roles, and distinctions fro…

Archamoebae: mitochondria‑lacking amoeboid protists in the Amoebozoa Archamoebae Archamoebae are a group of primarily anaerobic amoeboid protists within Amoebozoa, notable for lacking conventional mitochondria and including free‑living and animal‑associated gen…

Armillaria gallica — a temperate honey mushroom species Armillaria gallica Armillaria gallica is a widespread honey mushroom that decomposes wood, forms underground rhizomorphs and large colonies, can act as a parasite on trees, and sometimes displays bio…

Arrector pili (hair erector muscle) Arrector pili Small smooth muscles attached to hair follicles in mammals; contraction causes piloerection (goosebumps), aids sebum release and, in furred species, helps thermoregulation and thre…

Artificial insemination Assisted reproductive procedure that places sperm into a female reproductive tract to achieve pregnancy. Used in animal breeding, human fertility care, and conservation, with sever…

Artificial life (Alife): study of life-like systems and emergent behavior Artificial life Artificial life (Alife) investigates how lifelike processes and organization arise in software, hardware, and chemistry, using simulations, robots, and laboratory systems to study…

Artificial selection: intentional breeding of plants and animals Artificial selection Artificial selection (selective breeding) is the deliberate choice of parents to produce offspring with desired traits. This article explains methods, history, examples, benefits,…

Ascomycota (sac fungi): diversity, biology and human relevance Ascomycota Ascomycota, the sac fungi, are the largest fungal phylum. This article explains their defining features, classification, life cycles, ecological roles, uses in food and medicine, a…

Asepsis: principles, methods, and clinical importance Asepsis Asepsis is the practice of preventing contamination by pathogenic microorganisms through techniques and procedures in healthcare, laboratories, and industry to reduce infection and…

Asexual reproduction Biological reproduction in which a single organism or cell produces genetically similar offspring without gamete fusion; includes binary fission, budding, fragmentation, vegetative…

Aspergillus: biology, ecology, and significance of a widespread fungal genus Aspergillus Aspergillus is a large genus of mold-forming fungi in Ascomycota, with several hundred species. This article covers its structure, life cycle, ecology, uses, medical importance, an…

Aspergillus flavus: biology, health risks, and agricultural impact Aspergillus flavus Aspergillus flavus is a common soil fungus and opportunistic pathogen. Some strains produce aflatoxins that contaminate food and pose human and animal health risks; control focuses…

Asphyxia (asphyxiation): causes, mechanisms, and clinical overview Asphyxia Asphyxia is oxygen deprivation that can cause unconsciousness, brain injury, or death. This article reviews physiological mechanisms, common causes, signs, emergency care, forensic…

Assimilation (biology): uptake and incorporation of nutrients Assimilation (biology) Assimilation in biology describes how organisms obtain, transform, and incorporate nutrients — from digestion and absorption to liver processing and cellular synthesis — distinguis…

Astrocytoma — brain tumor originating from astrocytes Astrocytoma Astrocytoma is a primary central nervous system tumor that arises from astrocytes. Types range from slow-growing pilocytic lesions to aggressive glioblastoma; diagnosis and treatme…

ATP synthase: the rotary enzyme that makes cellular ATP ATP synthase ATP synthase is a membrane enzyme complex that produces ATP from ADP and phosphate using a proton (or ion) gradient; it functions as a rotary motor and is essential to cellular ene…

Atrial natriuretic peptide: heart-derived hormone that regulates blood volume and sodium Atrial-natriuretic peptide Atrial natriuretic peptide (ANP) is a cardiac peptide hormone that reduces blood volume and pressure by promoting vasodilation, natriuresis and inhibiting the renin–angiotensin–ald…

Auditory system The auditory system is the biological sensory system for hearing. It includes the ear as the sense organ and neural pathways that transduce sound into perception and enable localiz…