Biology

Cell nucleus: structure, function, and biological significance Cell nucleus Summary of the cell nucleus: its structure, main components, functions in gene expression and cell division, historical notes, exceptions and relevance to health and research.

The Cell Concept: Principles, History, and Modern Refinements Cell theory Overview of the cell concept: core principles, 17th–19th century history, and modern molecular refinements explaining cell types, division, genetics, and exceptions like viruses.

Cell wall The cell wall is a rigid or flexible outer layer found in many organisms. This article explains its composition, structural types, functions, evolutionary origins, and relevance in…

Cellular respiration: how cells convert nutrients into ATP Cellular respiration Cellular respiration is the set of biochemical processes by which cells extract usable energy (ATP) from nutrients, primarily glucose, using aerobic or anaerobic pathways.

Central dogma of molecular biology Principle describing directional flow of sequence information among DNA, RNA and proteins (replication, transcription, translation), its history, classifications and notable except…

Central nervous system: structure, function, and protection Central nervous system Comprehensive overview of the central nervous system: anatomy (brain and spinal cord), cells and barriers, development, functions in behaviour and homeostasis, common disorders, an…

Centriole A centriole is a small, cylindrical cellular organelle made of microtubules that helps organize cell division and forms basal bodies for cilia and flagella in many eukaryotes.

Centrosome (animal cell microtubule-organizing center) Centrosome The centrosome is a membrane-less organelle in animal cells that organizes microtubules, contributes to mitotic spindle formation, duplication control, cilia assembly and cellular…

Cephalothorax The cephalothorax (prosoma) is a fused head–thorax body region found in several arthropod groups; it bears sensory organs and limbs and is often protected by a carapace.

Cerebellum: structure, function, development, and clinical significance Cerebellum The cerebellum is a major brain region that coordinates movement, balance, posture and contributes to motor learning and cognitive tasks. This entry explains its anatomy, connectio…

Cerebral cortex: structure, function, development, and clinical significance Cerebral cortex The cerebral cortex is the outer sheet of neural tissue of the cerebrum. This article explains its layers, regional specializations, evolution, roles in cognition and disease, and…

Cerebrospinal fluid: composition, circulation, and clinical importance Cerebrospinal fluid Cerebrospinal fluid (CSF) is a clear bodily fluid that cushions the brain and spinal cord, supports metabolic exchange, and is used in diagnosis of neurological conditions.

Cerebrum: structure, function, and clinical significance Cerebrum The cerebrum is the largest part of the brain, comprising the cerebral hemispheres and cortex. It underlies perception, voluntary movement, language, memory and higher cognition, a…

Cervix: structure, function, and clinical importance Cervix The cervix is the lower, narrow portion of the uterus that opens into the vagina. This article describes its anatomy, roles in reproduction and childbirth, and key clinical conside…

Cetology: the study of whales, dolphins and porpoises Cetology Cetology is the scientific study of cetaceans—whales, dolphins and porpoises—covering their evolution, anatomy, behavior, ecology, and conservation.

Cheek: anatomy, function, and variation in humans and animals Cheek The cheek is the soft, lateral face region that forms the mouth wall. This article explains its anatomy, development, roles in feeding and expression, variations across species, an…

Chemoautotrophs: organisms that build biomass from inorganic chemicals Chemoautotroph Chemoautotrophs are bacteria or archaea that obtain energy by oxidizing inorganic molecules and fix carbon into organic matter, supporting ecosystems such as hydrothermal vents and…

Chimaera (genetics): organisms composed of genetically distinct cell lines Chimaera (genetics) A chimaera is a single organism made from two or more genetically distinct cell populations. This article explains types, causes, detection, examples, and biological and medical si…

Chlamydia (genus of bacteria) Chlamydia (bacterium) Chlamydia is a genus of obligate intracellular bacteria with a unique developmental cycle; species cause sexually transmitted infections, respiratory disease, and trachoma, an impo…

Chlamydia trachomatis: biology, clinical disease, diagnosis and prevention Chlamydia trachomatis Chlamydia trachomatis is an obligate intracellular bacterium causing common sexually transmitted infections, ocular trachoma and lymphogranuloma venereum; it has a biphasic life cy…

Chlamydiae: obligate intracellular bacteria – biology, life cycle, and significance Chlamydiae Chlamydiae are an obligate intracellular phylum of bacteria that infect eukaryotic cells. This article explains their life cycle, diversity, clinical importance, diagnosis, and pre…

Choanoflagellates: collar-bearing unicellular and colonial eukaryotes Choanoflagellate Choanoflagellates are microscopic, collar-bearing flagellates—single cells or colonies—that feed on bacteria and occupy a key evolutionary position as the closest relatives of anim…

Chromalveolate (Chromalveolata): concept, members, and classification issues Chromalveolate Chromalveolata: a historical eukaryote grouping proposed to unite several algal and protist lineages sharing red‑algal derived plastids; later molecular work questioned its unity a…

Chromatid: structure, function and role in cell division Chromatid A chromatid is one of two replicated copies of a chromosome held together at the centromere. This article describes chromatid composition, behavior in mitosis and meiosis, and biol…

Chromatin: structure, states and role in the cell nucleus Chromatin Chromatin is the DNA–protein complex that compacts and organizes genetic material in eukaryotic nuclei and plays central roles in gene regulation, replication and repair.

Chromatophore: pigment cells and colour change in animals Chromatophore Chromatophores are specialised pigment- or light-reflecting cells that produce skin and eye colour in many cold-blooded animals and enable rapid or long-term colour change.

Chromosome: structure, function, and biological significance Chromosome Chromosomes are DNA–protein complexes in the nuclei of eukaryotic cells that package genes, enable inheritance, and play central roles in cell division, development and genetic var…

Chromosome inversion — types, mechanisms, and biological significance Chromosome inversion Chromosome inversions are structural rearrangements in which a chromosome segment reverses orientation. This article explains types, formation, genetic consequences, detection meth…

Chytridiomycota: the chytrid fungi — features, ecology, and significance Chytridiomycota Chytridiomycota (chytrids) are an early-diverging phylum of fungi with flagellated spores, mainly aquatic saprotrophs and parasites; some species have caused global amphibian decli…

Cilia: structure, function, and biological importance Cilia Cilia are hairlike projections on eukaryotic cells that enable movement, fluid flow, and sensing. This article explains their structure, types, roles, history, and clinical relevan…

Ciliates: structure, life cycles, and ecological roles Ciliate Ciliates are single-celled protists bearing numerous cilia. This article summarizes their morphology, nuclear dimorphism, feeding and reproductive strategies, ecological roles, not…

Circadian Rhythm: The Biological Clock Regulating Daily Physiology Circadian rhythm Circadian rhythms are roughly 24-hour internal cycles found across life. They organize sleep, feeding, hormones, temperature and behavior, and are adjusted by light, temperature, f…

Circulatory system (cardiovascular system) Circulatory system The circulatory system moves blood through the body using the heart and a network of vessels. It transports gases, nutrients, hormones and wastes, supports immunity and helps regul…

Cladistics: principles, methods, and applications Cladistics Cladistics groups organisms by common ancestry into clades. This article summarizes core concepts, data types, analytical methods, history, applications in biology and paleontology…

Class (biology) — taxonomic rank Class (biology) A class is a major rank in biological classification, placed between phylum and order. It groups organisms that share fundamental structural or evolutionary traits and is often div…

Clavicle (Collarbone): Structure, Function, Development and Clinical Notes Clavicle The clavicle (collarbone) is a long bone connecting the shoulder to the trunk. This article covers its anatomy, development, functions, common injuries, and evolutionary and clinic…

Cleidoic egg (amniotic egg) Cleidoic egg A cleidoic (amniotic) egg is a closed, shelled egg produced by amniotes — chiefly birds and most reptiles — that contains specialized membranes allowing embryonic development away…

Climax community (ecology): definition, history and modern perspectives Climax community Overview of the climax community concept in ecology: definition, historical origins, critiques, alternative models, examples, soil development links, and implications for conservat…

Cline (biology) — Gradients of variation across populations Cline A cline is a continuous change in a trait or gene frequency across geographic or environmental space, arising from selection, gene flow, and history. It helps explain adaptation, s…

Clitoris: anatomy, development, function, and significance Clitoris Comprehensive overview of the clitoris: its anatomy, embryology, physiology, role in sexual response, and clinical and cultural considerations.

Cloaca: single posterior chamber for excretion and reproduction in vertebrates Cloaca Cloaca: a single posterior chamber in many vertebrates that expels urine and feces and supports reproductive functions, found in birds, reptiles and monotremes, and contrasted with…

Clone: biological and genetic copies Clone Overview of clones in biology and genetics: natural and laboratory cloning, molecular and cellular methods, examples, limitations, applications and ethical issues in research and m…

Clostridia: anaerobic, spore-forming bacteria of medical and ecological importance Clostridia Clostridia are a class of mostly anaerobic, Gram-positive bacteria (including Clostridium) known for spore formation, diverse metabolism, environmental roles, and some potent human…

Clostridium: overview of the genus, traits, diseases and uses Clostridium Clostridium is a genus of anaerobic, spore-forming, gram-positive rod-shaped bacteria that includes species of environmental importance and notable human pathogens such as C. botul…

Clostridium perfringens Clostridium perfringens is a spore-forming, anaerobic, gram-positive bacterium common in soil and intestinal tracts; causes food poisoning and wound infections through several toxi…

Cnida — the stinging organelles of Cnidaria Cnida Cnida are specialised, single-use capsules in cnidarians used for prey capture, defence and adhesion. This article explains their structure, types (nematocysts, ptychocysts, spiroc…

Coagulation: the physiological process of blood clotting Coagulation Coagulation (blood clotting) is the multi-step biological process that transforms blood into a semi-solid clot to stop bleeding; it involves platelets, clotting factors, fibrin, an…

Coccidia: microscopic apicomplexan parasites Coccidia Coccidia are single-celled, spore-forming parasites that infect the intestines of animals. They cause coccidiosis in livestock and pets and include medically important genera such…