Biology

Sauropsida: the reptile-and-bird clade of amniote vertebrates Sauropsid Sauropsida (sauropsids) are the clade of amniote vertebrates that includes modern reptiles and birds and their fossil relatives, originating early in the Carboniferous Period.

Scallop (Pectinidae): biology, ecology and human uses Scallop Scallops are marine bivalve molluscs (family Pectinidae) known for their ribbed shells, ability to swim by clapping valves, numerous species worldwide, and importance as seafood an…

Scalp: anatomy, function, and clinical significance Scalp The scalp is the soft tissue-covered region of the head between the forehead/face and the neck. This article summarizes its layers, blood and nerve supply, common conditions, and c…

Scar: formation, types, and clinical significance Scar A scar is fibrous tissue that replaces normal tissue after injury. This article explains how scars form, their composition and types, clinical examples, prevention and common treat…

Schistosoma mansoni: intestinal blood fluke and cause of schistosomiasis Schistosoma mansoni Schistosoma mansoni is a trematode (blood fluke) that causes intestinal schistosomiasis in humans. It has a freshwater snail intermediate host, distinctive lateral-spined eggs, and…

Sclera: structure, function, and clinical significance Sclera The sclera is the dense, fibrous white outer coat of the eye. This article explains its anatomy, role in vision and communication, clinical conditions, and differences across speci…

Scleractinia: stony corals and the builders of coral reefs Scleractinia Scleractinia, or stony corals, are cnidarian marine animals that secrete calcium carbonate skeletons. They form reefs, host algal symbionts, and play key ecological and geological…

Scrotum: anatomy, function, and clinical significance Scrotum The scrotum is an external pouch of skin and muscle that contains the testicles in many male mammals. It protects and helps regulate temperature for sperm production and has distin…

Scute: bony external plate or thickened scale Scute Scute — a hardened external plate or scale found in turtles, crocodilians, some birds and fishes. Covers composition, structure, functions, distinctions from scales and osteoderms,…

Sebaceous gland Small skin glands that secrete sebum to lubricate hair and skin. Found across most of the body, they influence skin health, acne, and certain tumors and are regulated by hormones a…

Secondary palate The secondary palate is the anatomical partition that separates the nasal passages from the oral cavity, enabling breathing while feeding and contributing to speech, formed during…

Secondary sex characteristics: definition, examples, and significance Secondary sex characteristic Traits that distinguish the sexes of a species but are not directly part of the reproductive organs; includes examples, development at puberty, evolutionary roles, and human featur…

Secretin: physiology, history, and clinical significance Secretin Secretin is a peptide hormone from the duodenum that stimulates pancreatic bicarbonate, modulates gastric and biliary function, and contributes to fluid/osmotic regulation; discove…

Secretion (biological process) Secretion Biological release of molecules from cells or glands — hormones, enzymes, mucus and neurotransmitters — including cellular routes, types (endocrine/exocrine), mechanisms and biolog…

Segmentation in Biology Segmentation Segmentation is the serial repetition of body units in animals and plants. It shapes form and function across groups such as arthropods, vertebrates, and annelids and influences de…

Selectable marker (molecular biology) Selectable marker A selectable marker is a genetic element introduced with an experimental DNA construct that allows researchers to recover cells that have taken up or maintained the new DNA under s…

Selenocysteine Selenocysteine is a naturally occurring amino acid similar to cysteine but containing selenium. It is co-translationally incorporated into proteins and plays key roles in redox enz…

Self-replication: biological, molecular, digital and cultural processes Self-replication Self-replication is the process by which a system makes a copy of itself. This article explains biological and nonbiological forms, key requirements, history, applications, and imp…

Selfish DNA: noncoding sequences that propagate within genomes Selfish DNA Selfish DNA refers to genetic sequences that replicate within a host genome without providing (and sometimes reducing) host fitness. The term covers transposable elements, satellit…

Semen: composition, biological role, health and cultural aspects Semen Semen is the male reproductive fluid containing sperm and seminal plasma; it plays roles in fertility, clinical testing and cultural contexts, and is produced by testes and accesso…

Seminal vesicle Paired tubular glands in the male reproductive tract that produce most of the seminal fluid, supplying nutrients, prostaglandins and alkaline secretions that support sperm transpor…

Seminiferous tubule — structure, function and clinical significance Seminiferous tubule Seminiferous tubules are coiled tubes in the testes where sperm form. This article explains their anatomy, cell types, role in spermatogenesis, hormonal control, and clinical condi…

Sensory receptor: how organisms detect and convert stimuli Sensory receptor Overview of sensory receptors: definition, transduction, main types, adaptation, organization, examples, medical relevance and research approaches in sensory biology.

Sensory system: how organisms detect and interpret their surroundings Sensory system Overview of the sensory system: sense organs, transduction, classical five senses, additional modalities like balance and proprioception, comparative specializations, neural proces…

Sequence analysis: methods, applications, and interpretation in biology Sequence analysis Overview of sequence analysis in biology: what is sequenced, common methods, data interpretation, applications (phylogeny, clinical genetics, proteomics) and limitations.

Sericulture: the cultivation and production of silk Sericulture Sericulture is the practice of rearing silkworms and processing their cocoons to produce silk fibers, threads, and fabrics. It includes biological, technical and cultural aspects f…

Serosa (serous membrane): structure, function, and clinical relevance Serosa A serosa is a thin serous membrane of mesothelial cells and connective tissue that lines closed body cavities (pleura, pericardium, peritoneum), reducing friction and participating…

Serotonin (5‑Hydroxytryptamine): Roles, Biology, and Clinical Significance Serotonin An accessible overview of serotonin (5-HT): its chemistry, synthesis, distribution, physiological and clinical roles, historical discovery, and notable distinctions within the nerv…

Sertoli cell: supporting cells of the testis and guardians of spermatogenesis Sertoli cell Sertoli cells are somatic cells in seminiferous tubules that nurture developing germ cells, form the blood–testis barrier, secrete regulatory factors and maintain the stem cell nic…

Serum (blood component): composition, preparation, and uses Serum (blood) Serum is the clear fluid remaining after blood has clotted. This article explains its composition, how it is prepared, laboratory and research uses, and how it differs from plasma.

Sex (biological reproduction) Sex Sex, or sexual reproduction, is the biological process by which organisms combine hereditary material from different individuals to produce genetically varied offspring.

Sex hormones (sex steroids): overview, synthesis, functions, and clinical significance Sex hormone Sex hormones are steroid signaling molecules produced by gonads and adrenals that regulate reproduction, development, metabolism, and are used therapeutically in contraception and…

Sex linkage: inheritance of genes on sex chromosomes Sex linkage Sex linkage covers inheritance patterns for genes on sex chromosomes, chiefly X-linked traits in XX/XY systems, how dosage compensation and pseudoautosomal regions alter expression…

Human Genitalia: Anatomy, Function, and Cultural Context Sex organ Comprehensive guide to human genitalia: male and female anatomy, reproductive and urinary functions, sexual health roles, and cultural and legal perspectives on nudity and public e…

Sexual conflict (evolutionary biology) Sexual conflict Sexual conflict occurs when evolutionary interests of males and females differ, producing opposing selective pressures that shape behaviour, morphology and genomes across sexually…

Sexual dimorphism Sexual dimorphism describes consistent physical differences between males and females of a species. It covers size, shape, coloration and ornamentation and reflects reproductive ro…

Sexual intercourse Physical act by which individuals engage for reproduction, pleasure, bonding or social reasons; includes penile–vaginal sex and a range of other consensual practices across species…

Sexual reproduction Biological process in which offspring arise from the fusion of gametes, combining genetic material from two parents; common across animals, plants, fungi and many protists and impo…

Sexual selection: mechanisms, examples, and evolutionary significance Sexual selection Sexual selection is an evolutionary process where traits evolve because they increase mating success. This article explains mechanisms, examples, history, hypotheses, costs, and on…

Shell (biological coverings, casings, computing and uses) Shell Overview of the term “shell”: protective biological coverings (molluscs, eggs, turtles), manufactured casings and ammunition, computing shells, corporate shells, plus history, stru…

Shigella: bacterial genus that causes shigellosis and severe diarrheal disease Shigella Shigella is a gram-negative, non-motile bacterial genus that causes shigellosis, a highly infectious diarrheal illness. Spread by the fecal–oral route; prevention emphasizes hygien…

Sibling species: cryptic close relatives separated by reproductive isolation Sibling species Sibling species are distinct species that are morphologically similar but reproductively isolated. They matter for evolution, disease transmission, pest control, and biodiversity a…

Sight (Vision): How Eyes and the Brain Create Visual Perception Sight An overview of sight: biological components, how visual perception works, variation across animals, practical uses, and common impairments and corrections.

Signal recognition particle (SRP) — cellular protein-targeting complex Signal recognition particle The signal recognition particle (SRP) is a conserved ribonucleoprotein that recognizes signal peptides on nascent proteins and directs them to membranes for insertion or secretion.

Signal transduction: cellular communication and response mechanisms Signal transduction Signal transduction describes how cells detect external or internal cues and convert them into biochemical responses that change cell behavior, gene expression, or metabolism.

Silkworm (Bombyx mori) — the domesticated silk-producing caterpillar Silkworm Overview of the silkworm (Bombyx mori): biology, life cycle, domestication in China, sericulture practices, uses of silk, wild relatives and current research and practical notes fo…

Single-nucleotide polymorphism (SNP) Single nucleotide polymorphism A SNP is a single-base DNA variation found in populations. SNPs mark genetic diversity, can affect traits and disease risk, and are widely used in research, medicine, and forensics…

Molluscan siphons: form and function in gastropods, bivalves, and cephalopods Siphon (mollusc) Overview of molluscan siphons: tubular mantle extensions in gastropods, bivalves and cephalopods—their forms, roles in breathing, feeding, locomotion and sensing, and how designs v…