Biology

Abdomen (human anatomy) Abdomen The abdomen is the body region between the chest and pelvis that contains the digestive tract, several vital organs, the abdominal wall and peritoneal cavity; central to digestion…

Acetobacter: aerobic bacteria that oxidize alcohol to acetic acid Acetobacter Acetobacter is a genus of aerobic bacteria known for converting ethanol into acetic acid. They are important in vinegar production, cellulose formation, and as spoilage organisms i…

Acetylcholinesterase — enzyme that terminates cholinergic transmission Acetylcholinesterase Acetylcholinesterase (AChE) is the enzyme that rapidly hydrolyzes acetylcholine at cholinergic synapses and neuromuscular junctions; it is central to muscle control, neural signali…

Acidophile: organisms that thrive in highly acidic environments Acidophile Acidophiles are organisms adapted to live at very low pH. This article explains their defining features, physiological strategies, habitats, examples, ecological importance and ind…

Acoelomorpha Acoelomorpha are small, simple bilaterian animals once grouped with flatworms; molecular evidence places them in the Xenacoelomorpha with debated positions near the base of Bilater…

Actinobacteria (Actinomycetota) Actinobacteria Actinobacteria are a diverse phylum of Gram-positive bacteria found in soil, water, and host-associated microbiomes; they include important decomposers, antibiotic producers, symbi…

Active transport: moving substances across membranes against gradients Active transport Active transport is the energy-dependent movement of molecules across cell membranes from lower to higher concentrations, mediated by protein pumps and carriers and essential to ce…

Adaptation (biology) Adaptation Adaptation is the evolutionary process by which populations become better suited to their environment through heritable changes. Covers mechanisms, types, examples, constraints and…

Adaptive immune system The adaptive immune system is a vertebrate-specific network of lymphocytes and processes that recognize, target, and remember specific pathogens to provide tailored, lasting protec…

Adaptive radiation: rapid diversification of lineages Adaptive radiation Adaptive radiation is the rapid diversification of a lineage into multiple species that occupy different ecological roles, often after new opportunities, innovations, or extinction…

Adelobasileus — an early mammaliaform from the Late Triassic Adelobasileus Adelobasileus is a small, extinct mammaliaform known from a partial skull from Late Triassic Texas. Its braincase and inner ear features bridge cynodonts and true mammals and illum…

Adenosine triphosphate (ATP) — the cellular energy carrier Adenosine triphosphate Adenosine triphosphate (ATP) is the primary molecule cells use to store and transport chemical energy, driving metabolism, transport, movement and signaling across all forms of lif…

Adipocyte: structure, types, functions and clinical significance Adipocyte Adipocytes (fat cells) are connective tissue cells specialized for storing lipids and acting as endocrine and insulating tissue. This article explains their types, structure, devel…

Adipose tissue: types, structure, functions, and medical relevance Adipose tissue Adipose tissue (body fat) is a connective tissue of adipocytes that stores lipids, provides insulation and cushioning, and functions as an endocrine organ producing adipokines like…

Adrenal gland: structure, hormones, and clinical significance Adrenal gland Small paired glands atop or near the kidneys that produce steroid and catecholamine hormones regulating stress, metabolism, blood pressure and electrolyte balance.

Aerobic: oxygen-dependent processes, organisms, and exercise Aerobic Aerobic describes processes and organisms that require oxygen. Covers aerobic respiration, types of aerobes, aerobic exercise and training, environmental uses, and contrast with an…

Aerobic organism An organism that requires or uses molecular oxygen for energy production. Covers types, cellular processes, evolutionary origins, ecological roles, and laboratory distinctions.

Aestivation: summer dormancy and survival strategies in animals Aestivation Aestivation (estivation) is a state of reduced metabolic activity animals use to survive high temperature and dry conditions; found across invertebrates and vertebrates with varied…

Agar plate: composition, uses, and role in microbiology Agar plate Agar plates are nutrient-containing solid media in shallow dishes used to culture, isolate, and study microorganisms in clinical, research, and industrial settings.

Agaricomycetes: the mushroom-forming fungi Agaricomycetes Agaricomycetes is a large class of basidiomycete fungi that includes mushrooms, brackets, puffballs and related forms. They play key roles as decomposers, symbionts and occasional…

Ageing: biological mechanisms, evolutionary explanations, and societal effects Ageing A clear overview of ageing (senescence): its biological basis, evolutionary theories, population patterns, medical relevance, and distinctions between ageing, disease, and non‑agei…

Agnatha: jawless fishes and the origins of vertebrates Agnatha Agnatha are jawless vertebrates — living cyclostomes (lampreys and hagfish) and many extinct armored forms — central to studies of early vertebrate anatomy and the origin of jaws.

Albinism: causes, characteristics, and impacts in humans, animals, and plants Albinism Albinism is a congenital condition marked by reduced or absent melanin production, affecting skin, hair and vision. This article explains causes, types, health issues, and effects…

Algae: diversity, ecology, and human uses Algae An introduction to algae: their biological features, evolutionary origins, ecological roles, major groups, and practical uses including food, industry and environmental application…

Algal bloom A rapid increase or accumulation of algae in water, algal blooms can alter ecosystems, produce toxins, and are driven by nutrients, water conditions and climate factors.

Alicyclobacillaceae A family of acid-tolerant, often thermophilic gram-positive bacteria notable for endospore formation and roles in soil, geothermal environments and occasional spoilage of acidic be…

Alimentary canal (digestive tract) Alimentary canal The alimentary canal is the muscular tube that conveys food through digestion and waste elimination in animals. This article explains its structure, functions, variations across sp…

Alkaliphile — organisms adapted to high-pH environments Alkaliphile Alkaliphiles are organisms that grow best at pH 9.0 or higher. This article explains their definition, strategies for coping with alkalinity, habitats, examples, adaptations and bi…

Allele: variant forms of a gene at a chromosomal locus Allele An allele is a specific variant of a gene at a particular location on a chromosome. Alleles determine genetic variation, influence traits, and are central to inheritance, populatio…

Allele frequency (proportion of gene variants in populations) Allele frequency Proportion of copies of a specific allele at a genetic locus in a population; used to quantify diversity and predict genotype proportions under Hardy–Weinberg; shaped by mutation,…

Allotheria: extinct Mesozoic mammals with multi‑cusped teeth Allotheria Allotheria is a name applied to several extinct mammal groups, noted for two‑row multicusped molariform teeth, a narrow pelvis, and a fossil record that ranges from the Triassic to…

Alpha helix: structure, bonding, and biological roles Alpha helix The alpha helix is a common protein secondary structure: a right-handed helical backbone stabilized by regular hydrogen bonds, important in enzymes, structural proteins, and membra…

Alternation of generations Biological life-cycle pattern in which multicellular haploid and diploid stages alternate; common in plants and some protists, with varying dominance and ecological and evolutionar…

Alternative splicing: how one gene yields multiple mRNA and protein isoforms Alternative splicing Overview of alternative splicing: mechanism, common forms, regulation, biological importance, disease links and quality control in eukaryotes.

Altricial: dependent newborns and hatchlings in animals Altricial Altricial describes offspring born or hatched in a helpless state that require extensive parental care. Explains traits, examples, evolution, distinctions from precocial strategies…

Alveolate (Alveolata) Alveolate Alveolates are a diverse group of single-celled eukaryotes united by membrane-bound cortical sacs. They include ciliates, dinoflagellates and apicomplexans and are important ecolog…

Alveolus: anatomy, pulmonary function, and related cavities Alveolus An alveolus is a small hollow or cavity in anatomy. Most commonly it refers to the pulmonary alveoli — microscopic air sacs in the lungs where gas exchange occurs — but the term ap…

Amanita: overview of the mushroom genus, identification, and medical importance Amanita Amanita is a global genus of basidiomycete mushrooms that includes some of the deadliest species. This article covers appearance, toxins, ecology, history and safety considerations…

Ammonite — extinct spiral-shelled marine cephalopods Ammonite Extinct marine cephalopods of subclass Ammonoidea, known for coiled, chambered shells. Their fossils are widespread and important index fossils; closest living relatives are modern…

Amniotic sac The amniotic sac is the fluid-filled membrane that surrounds and protects the developing fetus during pregnancy, composed of fetal membranes and filled with amniotic fluid with mul…

Amoeba — single-celled organisms with pseudopodia Amoeba Amoebae are single-celled eukaryotes that move and feed using pseudopodia. This article summarizes their structure, movement, diversity, ecology, medical relevance and use in resea…

Amoeboid movement: cell crawling driven by pseudopodia and cytoskeletal dynamics Amoeboid movement Amoeboid movement is a crawling form of cell locomotion using pseudopodia and actin–myosin rearrangements. It occurs in amoebae, slime molds, immune cells and some cancer cells, en…

Amoebozoa: the amoeboid eukaryotes, slime moulds, and their biology Amoebozoa Amoebozoa are a diverse clade of mostly unicellular eukaryotes characterized by lobose pseudopodia, phagocytic nutrition and a close evolutionary relationship to animals and fungi.

Amygdala The amygdala is an almond-shaped group of nuclei in the medial temporal lobe involved in processing emotions (especially fear), emotional memory, social behavior and autonomic resp…

Amylase: enzymes that break down starch into sugars Amylase Amylases are enzymes found across life that hydrolyze starch and related polysaccharides; they vary by mechanism, habitat, and pH preference and have important digestive, industria…

Amyloplast — the starch‑storing plastid in plants Amyloplast Amyloplasts are non‑pigmented plastids in plant cells that synthesize and store starch, contribute to gravity sensing in roots, and are important in seeds, tubers, and food industr…

Anabolism: Biological Synthesis and Metabolic Building Processes Anabolism Anabolism comprises the energy‑consuming metabolic pathways that construct complex molecules from simpler ones, supporting growth, repair, storage, and cellular structure in all li…

Anaerobic digestion: microbial breakdown of organic matter and biogas production Anaerobic digestion Anaerobic digestion is a managed biological process in which microbes decompose organic material without oxygen, producing biogas for energy and a nutrient-rich digestate for soil…