Biology

Sternum (breastbone): structure, function, and comparative anatomy Sternum The sternum, or breastbone, is a central flat bone of the chest that protects thoracic organs and anchors ribs and muscles; this article covers its parts, development, variations a…

Steroid: structure, types, biological roles and medical uses Steroid Steroids are organic compounds with a characteristic four‑ring core. This article explains their structure, natural occurrence, major classes (hormones, sterols, synthetic drugs),…

Steropodontidae: Early Cretaceous monotremes of Australia Steropodontidae Steropodontidae is an extinct family of early monotremes known from Early Cretaceous Australian fossils; important for understanding the origins and diversity of egg‑laying mammals…

Stillbirth: causes, diagnosis, and support Stillbirth Overview of stillbirth, how it differs from miscarriage, common causes, diagnosis and management, prevention strategies, and bereavement support; definitions vary by country (20–24…

Stomach (anatomy, function, and clinical notes) Stomach The stomach is a muscular organ of the digestive tract that stores and breaks down food using mechanical action and acid‑activated enzymes; this article covers structure, function,…

Stonefly (Plecoptera): freshwater insects of cold, running water Stonefly Stoneflies (order Plecoptera) are aquatic insects whose nymphs live in well-oxygenated streams and rivers. About 3,500 species exist worldwide; they are important bioindicators and…

Stop codon: signal that ends translation in the genetic code Stop codon A stop codon is a three‑nucleotide signal in mRNA that terminates translation. This article explains the common stop codons, molecular mechanism, biological exceptions, medical rel…

Strain (biology): definition, types, identification and significance Strain (biology) A strain is a distinct genetic or phenotypic lineage within a species used in microbiology, virology, horticulture and laboratory animals. This article explains definitions, identi…

Strangulation: causes, mechanisms, forensic signs and prevention Strangulation Strangulation is neck compression that stops oxygen or blood flow to the brain. This article explains mechanisms, contexts (homicide, sport, erotic practices), forensic findings, f…

Streptococcus: biology, taxonomy, clinical roles and laboratory identification Streptococcus Comprehensive overview of Streptococcus: morphology, physiology, major species, clinical importance, laboratory identification, treatment and public-health measures.

Streptococcus mutans Streptococcus mutans is a Gram-positive oral bacterium strongly associated with dental caries; it forms biofilms, produces acids from sugars, and survives acidic environments. Prev…

Streptomyces: filamentous soil bacteria and prolific producers of antibiotics Streptomyces Streptomyces is a genus of filamentous, high-G+C Gram-positive Actinobacteria common in soil. They form hyphae and spores and are major natural producers of antibiotics and other b…

Stress (biology): physiological and psychological responses Stress (biology) A concise, neutral overview of biological and psychological stress: definitions, physiological pathways, types (acute/chronic, eustress/distress), health effects, and common coping…

Stromules: Dynamic Tubular Extensions of Plastids in Plant Cells Stromule Stromules are dynamic tubular extensions of plastids in plant cells. Present across flowering plants, they reshape rapidly and may aid plastid interactions, molecular exchange or s…

Struggle for Existence: concept, history and role in evolution Struggle for existence The struggle for existence describes how organisms compete and depend on one another for survival. Central to Darwin and Wallace's development of natural selection, it remains a fo…

Subcutaneous tissue (hypodermis) Subcutaneous tissue The subcutaneous tissue, or hypodermis, is the layer beneath the dermis composed of fat and connective tissue. It cushions, insulates, stores energy, hosts vessels and nerves, and…

Suberin: the plant waterproofing polymer Suberin Suberin is a hydrophobic plant biopolymer that forms protective barriers in bark, roots and wounds, limiting water and solute movement and helping resist pathogens and physical dam…

Subgenus — an intermediate taxonomic rank Subgenus A subgenus is an optional taxonomic rank between genus and species used to organize large or diverse genera. This article explains definition, naming, history, uses, and practical…

Sublingual gland The sublingual glands are the smallest of the major salivary glands, located beneath the tongue; they produce mainly mucous saliva and have several small ducts that open into the f…

Submandibular gland One of the paired major salivary glands beneath the lower jaw; produces mixed saliva, drains via Wharton’s duct, and is clinically important for stones, infections, and tumors.

Subphylum — taxonomic rank below phylum Subphylum A subphylum is a taxonomic rank immediately below phylum used to group organisms that share major body plans or evolutionary lineages. It is applied when additional subdivision of…

Substantia nigra: structure, function, and clinical importance Substantia nigra The substantia nigra is a midbrain nucleus essential for movement control and reward, notable for dopamine-producing neurons and its degeneration in Parkinson's disease.

Substrate: biological, chemical, geological, and technological contexts Substrate An overview of substrates across biology, chemistry, geology and technology: definitions, types, roles, and distinctions including enzyme substrates and material bases used in indu…

Sue (Tyrannosaurus rex): discovery, history and scientific importance Sue (dinosaur) Sue (FMNH PR 2081) is one of the largest and most complete Tyrannosaurus rex specimens. Discovered in 1990, it became the center of a legal battle, record auction, and a major exhi…

Superphylum (informal taxonomic grouping) Superphylum A superphylum is an informal taxonomic grouping above phylum and below subkingdom used to assemble multiple phyla that likely share a common ancestry, especially in animal classifi…

Suprachiasmatic nucleus: mammalian master circadian pacemaker and light-entrained clock Suprachiasmatic nucleus Overview of the suprachiasmatic nucleus, the hypothalamic master circadian clock in mammals: its neuronal organization, peptide and neurotransmitter signaling, light entrainment vi…

Survival of the Fittest: Meaning, History, and Common Misunderstandings Survival of the fittest An accessible overview of the phrase 'survival of the fittest': its scientific meaning, origins, examples, and the frequent confusions and misuses surrounding it.

Survivor: meaning, contexts, and notable distinctions Survivor A survivor is an individual or organism that continues to live after an event that could have caused death. Covers definitions, contexts, history, uses, and related concepts such a…

Swallowing (Deglutition): Process, Structures, and Clinical Importance Swallowing Swallowing is the coordinated sequence that moves food and liquid from the mouth through the pharynx and esophagus into the stomach. Covers mechanics, phases, development, and clin…

Sweat (Perspiration): Function, Composition, Glands, and Health Sweat Sweat, or perspiration, is a body fluid produced by skin glands to cool the body. This article explains composition, gland types, how evaporation cools, causes of odor, and clinica…

Swimming: movement in water, skills, history, and modern uses Swim Swimming is the controlled movement of organisms or people through water. This article covers its mechanics, history, practical uses, sports, safety, and training methods.

Swim bladder: structure, function, and evolutionary significance Swim bladder A gas-filled internal organ in many bony fishes that controls buoyancy, aids stability and hearing, and is evolutionarily related to lungs. Variations include open (pneumatic duct)…

Swine influenza (swine flu): overview, strains, transmission and prevention Swine influenza Swine influenza is an influenza A virus group common in pigs that can infect humans. This article explains virology, strains, symptoms, diagnosis, treatment, prevention and public‑…

Symbiosis: close and long-term biological associations between species Symbiosis Symbiosis describes prolonged, intimate interactions between different organisms—mutualistic, commensal, or parasitic—that shape ecology, evolution and many practical systems from…

Symmetry in Biology Symmetry (biology) Overview of how symmetry appears in living organisms: types, development, functions, examples, and notable exceptions including asymmetry and approximate symmetry.

Sympathetic nervous system The sympathetic nervous system is the vertebrate autonomic branch that mobilizes the body for action. It controls cardiovascular, respiratory, thermoregulatory and metabolic respon…

Synapomorphy: shared derived traits in cladistics Synapomorphy A synapomorphy is a shared, derived trait used to infer common ancestry. This article defines the concept, gives examples, outlines its history and importance, and distinguishes it…

Synapsid: the lineage that produced mammals Synapsid Synapsids are an amniote lineage distinguished by a single temporal skull opening; they include extinct groups (pelycosaurs, therapsids) and all mammals, and document the gradual o…

Syncytium: multinucleate tissues and their biological roles Syncytium A syncytium is tissue in which many nuclei share a continuous cytoplasm without full cellular boundaries; found in animals, some plants and fungi and important in development and p…

Synovial fluid: composition, function, and clinical importance Synovial fluid Synovial fluid is the viscous, lubricating fluid in movable joints. This article describes its composition, production, mechanical properties, clinical uses, and role in joint dise…

Synthetic genomics: design, assembly and uses of artificial genomes Synthetic genomics Synthetic genomics is the design and laboratory construction of genomes and large DNA constructs that do not occur in nature, enabling research, biotechnology products, and new bio…

Systema Naturae: Linnaeus's Classification of Plants, Animals and Minerals Systema Naturae Systema Naturae is Carl (Carolus) Linnaeus's foundational work that organized known living organisms and minerals into a consistent hierarchical system and introduced binomial name…

Systematics: study of organismal diversity, relationships, and evolutionary history Systematics Systematics investigates the diversification of life and relationships among organisms, producing classifications and phylogenies used across biology, conservation, and biogeograph…

Systole (cardiac contraction) Systole Systole is the phase of the cardiac cycle when the heart's chambers contract to eject blood. It includes mechanical steps, electrical control, clinical significance and distinction…

T cell (T lymphocyte): development, types, functions, and clinical importance T cell T cells are thymus-derived lymphocytes central to cell-mediated immunity. This article describes their development, main subtypes, functions, and roles in infection, cancer, vaccin…

T helper cell (CD4+ T cell): role, types and clinical importance T helper cell T helper cells (CD4+ T cells) coordinate adaptive immune responses by recognizing antigen on MHC II, secreting cytokines, and activating B cells and cytotoxic T cells. They are cen…

Enterobacteria phage T2 (T2 bacteriophage) T2 phage T2 phage is a lytic, tailed bacteriophage that infects Escherichia coli. A classical model in molecular biology, it helped demonstrate DNA as the hereditary material and illustrate…

Tactopoda (proposed clade within Ecdysozoa) Tactopoda Tactopoda is a proposed grouping of some molting animals (ecdysozoans). The hypothesis links certain microscopic and arthropod lineages; support and interpretation vary across mole…