Biology

Krebs cycle (citric acid cycle) — central metabolic pathway Krebs cycle The Krebs cycle, also called the citric acid or TCA cycle, oxidizes acetyl-CoA in aerobic organisms to produce reduced cofactors for ATP generation and supplies intermediates for b…

L‑DOPA (levodopa): biological role, medical use, and properties L-DOPA L‑DOPA (levodopa) is a naturally occurring amino acid and the biochemical precursor of dopamine and other catecholamines. It is used medically to treat Parkinsonian motor symptoms…

Labia minora: structure, variation, function, and care Labia minora The labia minora are paired inner folds of the vulva that surround the vaginal opening. This article explains their anatomy, development, functions, common variations, and medical…

Labium: lip-like structures in anatomy, botany, and entomology Labium Labium (plural labia) denotes a lip-like structure. Used in human anatomy for parts of the vulva and in botany and entomology for petal- or mouthpart-like organs; covers form, func…

Medical Research Council Laboratory of Molecular Biology (MRC LMB) Laboratory of Molecular Biology A Cambridge research institute that played a central role in molecular biology, known for major discoveries, a strong Nobel Prize legacy, and interdisciplinary structural and cell…

Labyrinthodonts (Labyrinthodontia): an informal grade of early tetrapods Labyrinthodont Labyrinthodonts is an informal name for a collection of early fossil tetrapods noted for maze-like tooth structure and heavy skull roofs; once a formal taxon, now treated as an evo…

Lachesis (genus) — bushmasters, venomous pit vipers of Central and South America Lachesis (genus) Lachesis, commonly called bushmasters, is a genus of large venomous pit vipers native to forested regions of Central and South America. Notable for their size, secretive habits and…

Lacrimal bone — anatomy, function, and clinical relevance Lacrimal bone Small, thin facial bone forming part of the medial orbital wall; contributes to tear drainage and the nasolacrimal canal. Important in orbital anatomy, development, and certain eye…

Lactase: the enzyme that breaks down lactose in milk Lactase Lactase is an intestinal enzyme that hydrolyzes lactose into simple sugars. This article covers its biological role in mammals, developmental regulation, lactase persistence and de…

Lactation: biology, composition, evolution and role in mammals Lactation Lactation is the production and secretion of milk by mammals for nourishing and protecting offspring; this article explains its physiology, composition, evolution, variation among…

Lactobacillus — lactic acid bacteria important in food, health, and fermentation Lactobacillus Lactobacillus are rod-shaped, Gram-positive lactic acid bacteria used in fermented foods and probiotics. Many species are part of human microbiota and contribute to food preservati…

Lactose intolerance A common digestive condition caused by insufficient lactase enzyme, reducing the ability to digest lactose in milk and many dairy products; covers causes, symptoms, diagnosis and m…

Lamarckism: the theory of inherited acquired characteristics and its legacy Lamarckism Lamarckism is an early evolutionary hypothesis proposing that traits acquired during an organism’s life can be inherited. It is historically important but supplanted by genetics an…

Lamellibrachia: cold‑seep tube worms and their chemosynthetic ecosystems Lamellibrachia Lamellibrachia is a genus of long-lived tube worms that form symbioses with sulfur‑oxidizing bacteria, creating dense cold‑seep communities that sustain diverse deep‑sea life indep…

Lancelet (Cephalochordata): anatomy, evolution, and biological importance Lancelet Lancelets (Cephalochordata) are small, blade-shaped marine chordates that retain a notochord and pharyngeal slits throughout life; they are important for understanding early chorda…

Large intestine: structure, function, and clinical significance Large intestine Overview of the large intestine (colon): anatomy, functions such as water absorption and feces formation, its microbiota, common disorders, and distinctions from the small intestin…

Larva: Immature life stage of animals that undergo metamorphosis Larva Overview of larva, the immature life stage in animals undergoing metamorphosis; covers larval forms, feeding and growth, successive instars, examples (tadpoles, caterpillars, maggo…

Larynx (voice box): anatomy, function, and clinical importance Larynx The larynx, or voice box, is a cartilaginous organ in the neck that protects the airway and produces sound. This article explains its structure, roles in breathing and swallowing,…

Lateral line: the sensory system that lets fish feel water movement Lateral line The lateral line is a mechanosensory system in fish and some aquatic amphibians that detects water motion, pressure gradients and vibrations via hair-cell receptors, aiding orienta…

Law of the Minimum (Liebig's law) Law of the Minimum Principle that growth or production is limited by the scarcest essential factor; used in plant science, ecology and economics and often illustrated by the 'barrel' analogy.

Lazarus taxon A Lazarus taxon is a group that vanishes from the fossil record for one or more intervals and later reappears; causes include sampling bias, local extinction and later recolonizati…

Leaf miner Leaf miners are insect larvae that live and feed within leaves, producing characteristic tunnels or blotches; they occur among moths, flies, sawflies and some beetles and are ecolo…

Leech (Hirudinea): biology, behavior and medical applications Leech Leeches are segmented annelid worms, some of which feed on blood. This article covers their anatomy, feeding strategies, life cycle, ecological role, historical and modern medical…

Leg: structure, function and variation Leg A leg is a weight-bearing limb or support used for stance and movement in animals and objects. This article describes biological forms, evolutionary origins, adaptations, common us…

Legionella: overview, ecology, disease, detection and control Legionella Legionella is a genus of Gram-negative bacteria found in freshwater and man-made water systems; several species, notably L. pneumophila, cause Legionnaires' disease and Pontiac fev…

Leishmania — parasitic protozoa that cause leishmaniasis Leishmania Leishmania is a genus of single-celled parasitic protozoa transmitted by sandflies. It causes cutaneous, mucocutaneous and visceral leishmaniasis and affects millions across many c…

Crystalline lens (eye) — structure, function, development and clinical significance Lens (anatomy) Transparent, biconvex structure behind the iris that refracts light and adjusts focus. Covers anatomy, accommodation, development, age-related change, and common clinical condition…

Leptin: the adipose-derived hormone that regulates appetite and energy balance Leptin Leptin is a hormone produced by fat cells that signals the brain to reduce appetite and adjust energy use. It affects metabolism, reproduction, immunity and is central to understan…

Leptospira — spirochete bacteria that cause leptospirosis Leptospira Leptospira is a genus of thin, motile spirochete bacteria found in water, soil and animal hosts. Some species cause leptospirosis, a zoonotic disease ranging from mild to life‑thre…

Leuconostoc: ecology, characteristics, food uses and clinical importance Leuconostoc Leuconostoc is a genus of Gram‑positive lactic acid bacteria involved in food fermentations and occasional opportunistic human infection. This article summarizes biology, uses, and…

Lichen: symbiotic organisms of fungi and photosynthetic partners Lichen Lichens are composite organisms formed by a fungal partner and a photosynthetic partner (alga or cyanobacterium). This article explains their form, ecology, reproduction, uses and…

Life (biology): definition, characteristics, origin and significance Life An overview of life as a biological concept: definitions, core characteristics, molecular basis, energy and ecology, origins and evolution, lifespan, and key distinctions.

Life (2009 British TV series) Life (British TV series) Life is a 2009 BBC multi-part natural history documentary narrated by David Attenborough, surveying survival strategies, adaptations and behaviours across animals, plants and other…

Life cycle — biological stages of development Life cycle Overview of biological life cycles: patterns, common stages in humans and insects, metamorphosis, ecological and practical importance, and distinctions between gradual and abrupt d…

Life expectancy: definition, causes, trends and global differences Life expectancy Overview of life expectancy: how it is measured, main determinants, historical trends, global and socioeconomic disparities, uses in policy and examples of notable differences betw…

Ligament: structure, function, types, injuries and clinical significance Ligament A ligament is a band of fibrous connective tissue that connects bones or cartilage at joints. This article explains composition, mechanics, common injuries, healing, and clinical c…

Ligand (biochemistry): definition, binding, types and biological roles Ligand A ligand is a molecule that binds to a target (often a protein) to alter its activity. This article explains binding mechanisms, types of ligands, models, measurements and biologic…

Light-Dependent Reactions of Photosynthesis: Photophosphorylation and Electron Transport Light-dependent reaction Concise explanation of light-dependent photosynthesis: how photosystems capture photons, split water, drive electron transport and proton gradients to produce ATP and NADPH in chlo…

Light-independent reaction (Calvin cycle) Light-independent reaction Light-independent reactions are the enzyme-driven steps of photosynthesis that fix CO2 into sugars using ATP and NADPH produced by light reactions; commonly called the Calvin cycle…

Limb: biological appendage in animals and plants Limb A limb is an extension of a living organism's main body (trunk) used for support, movement, manipulation or growth; includes animal appendages (arms, legs, wings) and plant branche…

Limbic system: brain structures for emotion, memory, and motivation Limbic system Overview of the limbic system, its main components, functions, historical development, clinical importance, and contemporary views on this network of brain structures.

Limpet (true limpets and similar shelled gastropods) Limpet Limpets are marine gastropods with simple conical shells. This article covers true limpets (Patellogastropoda), their anatomy, ecology, life cycle, differences from other 'limpets'…

Lineage in Evolutionary Biology Lineage (evolution) Overview of evolutionary lineage: definition, how lineages are reconstructed, applications in conservation and medicine, and complications like hybridization and horizontal gene tr…

Linen: fabric from flax fibers Linen Linen is a textile made from flax (Linum usitatissimum) valued for strength, absorbency, and coolness. It is used for clothing, bedding, and household textiles with a long cultural…

Link reaction The link reaction, or pyruvate oxidation, converts pyruvate into acetyl-CoA, releasing carbon dioxide and producing NADH as the bridge between glycolysis and the Krebs cycle.

Lips Lip The lips are the soft, movable borders of the mouth involved in eating, speech, sensation and social expression; they have distinct anatomy, common disorders, and cultural roles.

Lipase: enzymes that break down and remodel fats Lipase Lipases are enzymes that hydrolyze fats and other lipids. They play key roles in digestion, metabolism, industrial applications, and disease diagnostics across many organisms.

Lipid: structure, types, and biological roles Lipid Lipids are a broad class of mostly hydrophobic organic molecules—fats, oils, waxes, sterols and related compounds—important for energy storage, membranes, signaling and fat‑soluble…