Biology

Infraphylum (taxonomic rank) Infraphylum A taxonomic rank in the Linnaean hierarchy that sits below a subphylum and above a class, used to add resolution in complex classifications and sometimes applied differently across…

Inhalation: how air enters the lungs and its roles in respiration Inhalation Inhalation is the intake phase of breathing when air moves into the lungs; it enables gas exchange, supports life, and is the route for inhaled medications and many airborne hazard…

Innate immune system: first-line defenses, cells, and mechanisms Innate immune system An overview of the innate immune system: its components, recognition strategies, evolutionary origins, and role in health and disease, contrasted with adaptive immunity.

Inner ear: anatomy, function, evolution and clinical importance Inner ear The inner ear is the innermost part of the vertebrate ear responsible for hearing and balance. It includes the cochlea and vestibular apparatus housed in the bony labyrinth and con…

Insectivore: organisms that feed primarily on insects Insectivore An insectivore is an animal or plant that eats mainly insects. This article explains definitions, adaptations, examples across taxa, ecological roles, and the historical taxonomy o…

Instar — stage in an insect's post-embryonic development Instar An instar is a discrete growth stage in insects between successive molts. It applies to larval or nymphal phases, varies by species and conditions, and often precedes pupation or t…

Instinct (innate behaviour) Instinct Instinct refers to inherited patterns of behaviour that emerge without prior learning. This article explains definitions, mechanisms, examples, history, and how instincts differ fr…

Integron: bacterial gene-capture systems and their role in antibiotic resistance Integron Integrons are genetic platforms in bacteria that capture and express mobile gene cassettes, enabling rapid acquisition of traits such as antibiotic resistance and contributing to m…

Integumentary system: structure, function, and diversity Integumentary system Summary of the integumentary system: components (skin, hair, scales, glands), layers and cell types, functions in protection, sensation, thermoregulation, diversity across animals…

Intercostal muscles: anatomy, function, and clinical significance Intercostal muscle Intercostal muscles are layers of skeletal muscle between the ribs that support breathing, chest-wall stability and movement. This article covers anatomy, action, innervation and c…

Interferon: Immune signaling proteins that protect against viruses and tumors Interferon Interferons are cytokine proteins released by cells in response to infection or malignancy. They trigger antiviral defenses, modulate immune cells, and have clinical uses and limit…

Intestine (digestive tract) Intestine The intestine is the tubular organ of the digestive system that continues digestion, absorbs nutrients and water, houses microbiota, and compacts waste; it comprises the small and…

Intrinsic factor: role in vitamin B12 absorption and clinical importance Intrinsic factor Intrinsic factor is a stomach-produced glycoprotein essential for vitamin B12 absorption in the ileum; its deficiency causes B12 malabsorption and clinical syndromes such as pernic…

Introduced species: causes, impacts, examples and management Introduced species An introduced species is a non-native organism brought to a new area by people. This article explains pathways, ecological and economic impacts, notable examples, and approaches to…

Intron — non-coding gene sequence and RNA splicing Intron An intron is a non-coding segment of a gene removed from RNA transcripts by splicing. This article explains structure, splicing mechanisms, types, functions, historical discovery,…

Invasive species: causes, impacts and management Invasive species An overview of invasive species: definition, typical traits, how they spread, ecological and economic impacts, notable examples, and approaches for control and prevention.

Ion channel Ion channels are membrane proteins that form pores allowing selective ion flow, underpinning electrical signaling, transport, and many physiological processes; defects cause diseas…

Iris (eye) — structure, function, color, and clinical relevance Iris (eye) The iris is the colored, muscular ring inside the eye that controls pupil size and regulates light reaching the retina. This article explains its anatomy, physiology, pigmentation,…

Irregular bones (anatomy) Irregular bone Bones whose shapes do not fit the long/short/flat/sesamoid categories; provide protection, support, and irregular attachment sites. Includes vertebrae, sacrum, skull bones and the…

Irritation (biology and medicine) Irritation Local tissue response producing redness, discomfort or inflammation from allergens, chemicals, physical damage, or chronic exposure; covers signs, causes, mechanisms, treatment and…

Island dwarfism (insular dwarfism) Island dwarfism Island dwarfism is the evolutionary reduction in body size seen when large animals become isolated in small, resource-limited environments such as islands, caves, or isolated valle…

Islets of Langerhans Clusters of hormone-secreting cells in the pancreas that regulate glucose and metabolism; include beta (insulin), alpha (glucagon), delta (somatostatin) and other cell types with i…

Isoform A protein isoform is one of several distinct molecular forms produced from the same gene or related genes. Isoforms expand protein diversity by alternative splicing, allelic variat…

Isospora belli (Cystoisospora belli): biology, disease, and management Isospora belli Overview of Isospora belli (also Cystoisospora belli): its biology, life cycle, clinical disease isosporiasis, diagnosis, treatment, prevention, and distinctions from other intesti…

Jacobson's Organ (Vomeronasal Organ): Structure, Function and Significance Jacobson's organ Jacobson's organ, or the vomeronasal organ (VNO), is a chemosensory structure in many vertebrates that detects pheromones and social cues, influencing reproduction and social behav…

Jaw: structure, diversity, development and function Jaw An overview of jaws — biological structures used for grasping and processing food. Covers human anatomy, variation across animals, evolutionary origin, developmental patterns, and…

Jehol Biota: Early Cretaceous Lacustrine Ecosystem of Northeastern China Jehol Biota The Jehol Biota is a Lower Cretaceous lacustrine ecosystem (c.133–120 Ma) in northeastern China famous for exceptionally preserved fossils: feathered dinosaurs, early birds, mammal…

Jejunum: structure, function, and clinical significance Jejunum The jejunum is the middle portion of the small intestine, specialized for nutrient absorption. This article describes its anatomy, histology, function, clinical relevance and the o…

Jelly Fungus Jelly fungus is a loose name for gelatinous mushrooms and related fungi with soft, rubbery fruiting bodies that dry out and revive after rain. Some are edible and used in cooking.

Jeotgalicoccus aerolatus: an environmental Gram-positive bacterium Jeotgalicoccus aerolatus Jeotgalicoccus aerolatus is a Gram-positive, aerobic member of the family Staphylococcaceae, isolated from airborne samples on a turkey farm and encountered in environmental and mi…

Jeotgalicoccus coquinae Jeotgalicoccus coquinae is a gram-positive, coccoid bacterium in the Staphylococcaceae family, known from salty or food‑associated environments and of interest for microbial ecolog…

Jeotgalicoccus pinnipedialis: a marine-associated coccoid bacterium Jeotgalicoccus pinnipedialis Overview of Jeotgalicoccus pinnipedialis, a Gram-positive, coccoid, facultative anaerobic bacterium isolated from a southern elephant seal; taxonomy, morphology, ecology and resear…

Joint (anatomy and classification) Joint An anatomical joint is the contact point between bones that permits movement and provides support. This article explains joint structure, types (structural and functional), common…

Jumping: biology, mechanics, and uses in animals and sport Jumping Jumping is a rapid propulsive movement that temporarily frees a body from the ground. This article explains its mechanics, anatomical adaptations, roles in animals and humans, spor…

Karyotype A karyotype is the complete set of chromosomes of an organism as seen and arranged for study; it informs chromosome number, structure, and clinical or evolutionary changes.

Kelp forest: structure, ecology, uses, threats and conservation Kelp forest Kelp forests are dense, productive coastal marine habitats formed by large brown algae. This article explains their structure, distribution, ecological role, human uses, threats an…

Keratin: structure, functions, and biological roles Keratin Keratin is a family of fibrous proteins that form hair, nails, horns, feathers and other tough, insoluble structures in animals. This article explains its types, formation, propert…

Kidney Two bean-shaped organs that filter blood, remove wastes as urine, regulate fluid and electrolytes, and help control blood pressure and red blood cell production.

Killing: meanings, types, history, and legal and ethical distinctions Killing Overview of killing as a biological, legal, and moral concept; common categories (homicide, suicide, execution, combat, pest control); historical and ecological context; legal and…

Kin selection Evolutionary theory explaining how genes for helping close relatives can spread because relatives share genes; formalized through inclusive fitness and Hamilton’s rule.

Kinetoplastid: single-celled flagellates with a distinctive mitochondrial DNA structure Kinetoplastid Kinetoplastids are single-celled, flagellated eukaryotes in Euglenozoa notable for a DNA-rich kinetoplast. They include free-living species and important parasites such as trypanos…

Kingdom (biology): taxonomic rank and major groups of life Kingdom (biology) A biological kingdom is a high-level taxonomic rank used to group organisms by fundamental traits. This article explains its meaning, history, common systems, and notable distincti…

Kinorhyncha: microscopic marine sediment invertebrates (mud dragons) Kinorhyncha Kinorhyncha are microscopic marine invertebrates (mud dragons) under 1 mm that live between sediment grains worldwide; explore their segmented anatomy, ecology, and species diversi…

Klebsiella (genus of Gram-negative bacteria) Klebsiella Klebsiella is a genus of encapsulated, Gram-negative bacteria in Enterobacteriaceae that cause respiratory, urinary and bloodstream infections; antibiotic resistance among strains…

Klebsiella pneumoniae: biology, ecology, and clinical significance Klebsiella pneumoniae Comprehensive overview of Klebsiella pneumoniae covering its biology, ecology, clinical infections, laboratory identification, antibiotic resistance, prevention and public health i…

Knee (joint) — anatomy, function, common problems and care Knee Overview of the knee joint: major bones and soft tissues, biomechanics, typical injuries and treatments, and practical tips for prevention and rehabilitation.

Knockout mouse: engineered loss-of-function models in biomedical research Knockout mouse A knockout mouse carries one or more deliberately inactivated genes. These engineered mice are key tools for assigning gene function, modelling disease, and testing therapies in ma…

Kollikodon ritchiei — opalised monotreme known from teeth Kollikodon Kollikodon ritchiei is an extinct monotreme known from an opalised tooth fragment found at Lightning Ridge, Australia. Its enlarged, bulbous teeth suggest a specialized feeding hab…