S

S phase: The synthesis phase of the cell cycle; the portion of interphase during which DNA is replicated.
Sac fungus: Member of the phylum Ascomycota. Sac fungi range in size and complexity from unicellular yeasts to minute leafspot fungi to elaborate cup fungi and morels. About half of the sac fungi live with algae or cyanobacteria in the mutualistic associations called lichens.
Saccule: A chamber in the vestibule behind the oval window that participates in the sense of balance.
Salicylic acid: A plant hormone that may be partially responsible for activating systemic acquired resistance to pathogens.
Salivary amylase: A salivary gland enzyme that hydrolyzes starch and glycogen.
Salivary glands: Exocrine glands associated with the oral cavity. The secretions of salivary glands contain substances to lubricate food, adhere together chew pieces into a bolus and begin the process of chemical digestion.
Salt: A compound resulting from the formation of a ionic bond.
Saltatory conduction: Rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelin sheathed regions of membrane.
Saltation: A variation of large effect; also, a major mutation.

Saprobic: Group of fungi that act as decomposers feeding on organic matter like dead trees.
Sapwood: Outer layers of secondary xylem that still transport xylem sap.
Sarcomere: The fundamental repeating unit of striated muscle, delimited by the Z lines.
Sarcoplasmic reticulum:  A specialized endoplasmic reticulum that regulates the calcium concentration in the cytosol.
Satellite DNA: Highly repeated DNA sequence, which was originally detected as a “satellite” component with a density distinct from the rest of the genome.
Saturated fatty acids: A fatty acid in which all carbons in the hydrocarbon tail are connected by a single bond, thus maximizing the number of hydrogen atoms that can attach to the carbon skeleton.
Savanna: A tropical grassland biome with scattered individual trees, large herbivores. and three distinct seasons based primarily on rainfall, maintain by occasionally fires and drought.
Scaffolding protein:  A type of large relay protein to which several other relay proteins are simultaneously attached to increase the efficiency of signal transduction.
Scanning electron microscope: A microscope that uses an electron bean to scan the surface of a sample to study details of its topography.
Scat: The excrement of an animal.
Schizocoelous:  Pattern of formation of the body cavity common to protostome development in which initially solid masses of mesoderm split, forming the body cavity.
Schizophrenia: Severe mental disturbance characterized by psychotic episodes in which patients lose the ability to distinguish reality from hallucination.
Schwann cell: A type of glial cell that forms insulating myelin sheaths around the axons of neurons in the peripheral nervous system.
Scion: The twig grafted onto the stock when mating a graft/
Sclera: A tough. white outer layer of connective tissue that forms the globe of the vertebrate eye.
Sclereid: A short, irregular sclerenchyma cell in nutshells and seed coats and scattered through the parenchyma of some plants.
Sclenchyma cell: A rigid, supportive plant cell type usually lacking protoplast and possessing thick secondary walls strengthened by lignin at maturity.
Scrotum: A pouch of skin outside the abdomen that houses a testis; functions in cooling sperm, thereby keeping them viable.
Scutellum: A specialized type of cotyledon found in the grass family.
Seascape: Several different, primarily aquatic ecosystems linked by exchanges of energy, materials, and organisms.
Second law of thermodynamics: The principle whereby every energy transfer or transformation increases the entropy of the universe.  Ordered forms of energy are at least partly converted to heat, and in spontaneous reaction, the free energy of the system also decreases.
Second messenger: A small, nonprotein, water soluble molecule or ion that relays a signal to cell’s interior in response to a signal received by a signal receptor protein.
Secondary cell wall: A strong a durable matrix often deposited in a several laminated layers for plant cell protection and support.
Secondary consumer: A member of the trophic level of an ecosystem consisting of carnivores that eat herbivores.
Secondary contact: Contact between populations that have previously been geographically separate (i.e. allopatric). It contrasts with primary contact.
Secondary endosymbiosis: A process in eukaryotic evolution in which a heterotrophic eukaryotic cell engulf  a photosynthetic eukaryotic cell, which survived in a symbiotic relationship inside the heterotrophic cell.
Secondary growth: growth produced by lateral meristems, thickening the roots and shoots of woody plants.
Secondary immune response: The acquired immune response elicited on second or subsequent exposure to a particular antigen.  The secondary immune response is more rapid, of greater magnitude, and of longer duration than the primary immune response.
Secondary oocyte: A haploid cell resulting from meiosis I in oogenesis, which will become an ovum after meiosis II.
Secondary plant body: The tissues produced by the vascular cambium and cork cambium, which thickens the stems and roots of woody plants.
Secondary structure: The localized, repetitive coiling or folding of the polypeptide backbone of a protein due to hydrogen bond formation between peptide linkages.
Secondary succession: A type of succession that occurs where an existing community has been cleared by some disturbance that leaves the soil intact.
Secretion: (1) The discharge of molecules synthesized by a cell. (2) The discharge of waste from the body fluid into the filtrate.
Secretory phase: That portion of the uterine cycle when the endometrium continues to thicken, becomes more vascularized, and develops glands that secrete a fluid rich in glycogen.
Sedimentary rock: Rock formed from sand and mud that once settle in layers on the bottom of seas, lakes, and marshes. Sedimentary rocks are often rich in fossils.
Seed: An adaptation for terrestrial plants consisting of an embryo packaged along with a store of food within a resistant coat.
Seed coat: A tough outer cover of a seed,  formed from the outer coat of an ovule. In flowering plants, the seed coat encloses and protect the embryo and endosperm.
Seedless vascular plants:  The informal collective name for the phyla Lycophyta (club mosses and their relatives) and pteridophyta (ferns and their relatives).
Segmentation gene: A gene of the embryo that directs the actual formation of segments after the embryos axes have been defined.
Segregating sites: Sites that are polymorphic in a sample of sequences.
Segregation: The movement of two homologous chromosomes during meiosis, one to each pole of the cell. Also, the production of different genotypes in the offspring of heterozygotes, as a result of this random meiotic process.
Segregation distortion: Deviation from the expected 1:1 segregation of alleles from a heterozygous at meiosis. It includes both meiotic drive and differential survival of the haploid products of meiosis.
Segregation load: The loss of mean fitness caused by the segregation of homozygotes, when polymorphism is maintained by heterozygous advantage.
Selection coefficient: Difference in relative fitness.
Selection differential: The difference in mean trait value between those that reproduce and the original population.
Selection, direct: A change in genotype frequency that is caused by the effects on fitness on the alleles themselves.
Selection response: The change in mean trait value over one generation.
Selective death: A failure to survive or reproduce; also, a loss of fitness attributable to differences in phenotype.
Selective permeability: A property of biological membranes that allows some substances to cross more easily than others.
Selective reabsorption: The selective uptake of solutes from a filtrate of blood, coelomic fluid, or hemolymph in the excretory organs of animals.
Selective sweep: Increase of neutral alleles by hitchhiking with a favorable mutation. This sweeps variation out of a region of the genome surrounding the favorable mutation.
Self-fertilization:  When a hermaphrodite organism mates with itself.
Self-incompatibility: When individuals cannot self-fertilize. The ability of a seed plant to reject its own pollen and sometimes the pollen of closely related individuals.
Selfish: A gene that increases its own transmission but reduces the fitness of the individual that carries it.
Selfish DNA: Sequences that replicate faster than the rest of the genome and that reduce the fitness of the individual carrying them.
Self-replication: Used to describe a molecule or other structure that can cause its own replication.
Semelparity: A life history in which adults have but a single reproductive opportunity to produce large number of offspring; also know as big-ban reproduction.
Semen: The fluid that is ejaculated by the male during orgasm, contains sperm and secretions from several glands of the male reproductive tract.
Semicircular canals: A tree-part chamber of the inner ear that functions in maintaining equilibrium.
Semiconservative: Describes the replication of double-stranded DNA, where the two new molecules each carry one strand from their parent and a complementary strand that has been synthesized.
Semilunar valve: A valve located at the two exits of the heart, where the aorta leaves the left ventricle and pulmonary artery leaves the right ventricle.
Seminal vesicle: A gland in males that secretes a fluid component of semen that lubricates and nourish sperm.
Seminiferous tubule: A highly coiled tube in the testis in which sperm are produced.
Senescence: aging
Sensation: An impulse send to the brain from activated receptors and sensory neurons.
Sensitive period: A limited phase in an individual animal development when learning of a particular behavior can take place.
Sensory adaptation: The tendency of sensory neurons to become less sensitive when they are stimulated repeatedly.
Sensory bias: An innate preference for particular male traits, which did not evolve as a result of the sexual selection caused by that preference.
Sensory neuron: A nerve cell that receives information from the internal and external environments and transmits the signals to the central nervous system.
Sensory reception: The detection of the energy of a stimulus by sensory cells.
Sensory receptor: A cellular system that collects information about the physical world outside the body and inside the organism.
Sensory transduction: The conversion of stimulus energy to a change in the membrane potential of a sensory receptor.
Sepal: A modified leave in angiosperms that helps enclose and protect a flower bud before it opens.
Septum: One of the cross-walls that divides a fungal hypha into cells. Septa generally have pores large enough to allow ribosomes, mitochondria, and nuclei to flow from cell to cell.
Serial endosymbiosis: A model of the origin of eukaryotes consisting of a sequence of endosymbiotic events in which mitochondria, chloroplasts, and perhaps the nucleus were derived from small prokaryotes that had been engulfed by larger cells.
Serotonin: A biogenic amine synthesized from the amino acid tryptophan.
Seta: The elongate stalk of a bryophyte sporophyte, such as in a moss.
Sex-biased inheritance: Transmission of genes through one or the other sex (e.g. maternal inheritance or sex linkage).
Sex chromosome: A chromosome that is inherited differently by the two sexes. In mammals, males carry an X and a Y chromosome, and females carry two X chromosomes. In birds and butterflies, males carry two Z chromosomes, and females carry a Z and z W chromosome.
Sex-linked gene: A gene located on a sex chromosome.
Sex ratio: The ration of males to females in a population.
Sexual dimorphism: A special case of polymorphism based on the distinction between the secondary sex characteristics of males and females
Sexual selection: Selection arising from variation in the ability to find a mate.
Sexual reproduction: Production of offspring that are a mixture between two different parental phenotypes.
Sexual selection: Natural selection for mating success.
Shared derived character: An evolutionary novelty that evolve within a particular clade.
Shared primitive character: A character display in species outside a particular taxon.
Shifting balance theory: A theory develop by Sewall Wright, in which species evolve toward the best among many alternative adaptive peaks.
Shoot system: The aerial portion of a plant body, consisting of stem, leaves, and (in angiosperms) flowers.
Short-day plant: A plant that flowers only when the light period is shorter than a critical length; usually late summer, fall, or winter.
Short-term memory: The ability to hold information, anticipations, or goals for a time and then release them if they become relevant.
Short interspersed nucleotide element (SINE): A class of transposable element.
Shotgun sequence: A method of sequencing genomes and environmental samples in which random fragments of DNA are sequenced and then computational methods are used to “reassemble” genomes from the sample.
Sickle cell disease: A human genetic disease caused by a recessive allele that results in the substitution of a single amino acid in the hemoglobin protein; characterized by deformed red blood cells.
Sieve plate: An end wall in a sieve tube member, which facilitates the flow of phloem sap in angiosperm sieve tubes.
Sieve tube member: A living cell that conducts sugar and other organic nutrients in the phloem of angiosperms. The form chains called sieve tubes.
Sign stimulus: An external sensory stimulus that triggers a fixed action pattern.
Signal: A behavior that causes a change in behavior in another animal.
Signal peptide: A stretch of amino acids on a polypeptide that targets the protein  to a specific destination in a eukaryotic cells.
Signal recognition particle:  A protein-RNA complex that recognizes a signal peptide as it emerges from the ribosome.
Signal transduction pathway:  A mechanism liking a mechanical or chemical stimulus to a specific cellular response.
Simple fruit: A fruit derived from a single carpel or several fused carpels.
Simple epithelium:  An epithelium consisting of a single layer of cells that all touch the basal lamina.
Simple sequence repeats (SSR): Tandem repeats of a short sequence.
Simulated annealing: An optimization algorithm that chooses random changes that improve the desired trait.
Single-nucleotide polymorphism (SNP): A SNP occurs when members of a population vary in which base they carry at a single nucleotide site. One base-pair variation in the genome sequence.
Single-lens eye: The camera-like eye found in some jellies, polychaetes, spiders, and many molluscs.
Single-strand binding protein: During DNA replication, molecules that line up along the unpaired DNA strands, holding them apart while the DNA strand serve as templates for the synthesis of the complementary strand of DNA.
Sinoatrial (SA) node: A region pf the heart composed of specialized muscle cells that sets the rate and timing at which all cardiac muscle contracts; the pacemaker.
Sinus: Any of the spaces surrounding the organs of the body in animals with open circulatory systems.
Sister chromatids: The two copies of a chromosome after it has been replicated. These replicate forms of a chromosome joining together by the centromere are eventually separated during mitosis and meiosis II.
Skeletal muscle (striated muscle):  Muscle generally responsible for the voluntary movements of the body.
Sliding-filament model: The theory explaining how muscle contracts, based on change within the sarcomere, the basic unit of muscle organization, statin that thin (actin) filaments slide across thick (myosin) filaments, shortening the sarcomere. The shortening of all sarcomeres in a myofibril shortens the entire myofibril.
Slime mold: Eukaryotes, from multiple phyla, that normally exist as single-celled amoeba-like organism but that sometimes gather into “slugs” which move together as a unit.
Slip-strand mispairing (SSM): A process in which a DNA polymerase adds one to many or one to few copies of a repetitive sequence during replication.
Slow block to polyspermy: The formation of the fertilization envelop and other changes in the egg’s surface that prevents fusion of the egg with more than one sperm.
Slow muscle fibers: Muscle cells that can sustain long contractions.
Small intestine: The longest section of the alimentary canal, the principal site of the enzymatic hydrolysis of food macromolecules and the absorption of nutrients.
Small non-coding RNAs (sncRNAs): Short (20-30 nucleotides), non-coding RNAs that can be produced from various precursors and form various classes, including those involved in epigenetics and post transcriptional gene silencing such as microRNAs.
Smooth ER:  That portion of the endoplasmic reticulum that is devoid of ribosomes.
Smooth muscle: A type of muscle lacking the striations of skeletal and cardiac muscle because of the uniform distribution of  myosin filaments in the cell; responsible for involuntary body activities.
Snowball Earths: Two events during the Cryogenian Period where glaciers covered the planet’s landmasses from pole to pole; the first event is called Sturtian glaciation and the second Marinoan Glaciation.
Social Darwinism: The idea that, by analogy with natural selection, societies evolve through competition between individuals or groups.
Social evolution: The study of the evolutionary consequences of interaction between individuals.
Social learning: Modification of behavior through the observation of other individuals.
Sociobiology: The study of social behavior based on evolutionary theory,
Sodium-potassium pump: A special transport protein in the plasma membrane of animal cells that transport sodium out of the cell and potassium into the cell against their concentration gradients.
Solute: A substance that is dissolve din a solution.
Solute potential: A component of water potential that is proportional to the number of dissolved solute molecules in a solution and measures the effect of solutes on the direction of water movement, also called osmotic potential, it can be either zero or negative.
Solution: A liquid that is a homogenous mixture of two or more substances.
Solvent: The dissolving agent in a solution. Water is the most versatile solvent known.
Soma: Those parts of a multicellular organism that will not directly produce gametes. It contrasts with germline.
Somatic cell: Any cell in a multicellular organism that is not an egg or sperm.
Somatic nervous system: The branch of the motor division of the vertebrate peripheral nervous system composed of motor neurons that carry signals to skeletal muscles in response to external stimuli.
Somites: Paired blocks of mesoderm just lateral to the notochord of a vertebrate embryo.
Soredia: In lichens, small clusters of fungal hyphae with embedded algae.
Sorus: A cluster of sporangia on a fern sporophyll. Sori may be arranged in various patterns, such as parallel lines or dots, that are useful for fern identification.
Southern blotting: A hybridization technique that enables researchers to determine the presence of certain nucleotide sequences in a sample of DNA.
Spatial learning: Modification of behavior based on experience of the spatial structure of the environment.
Spatial summation:  A phenomenon of neural integration in which the membrane potential of the postsynaptic cell  is determined by the combined effect of EPSPs or IPSPs produce nearly simultaneously by different synapses.
Speciation: The process by which new species are formed.
Species: Basic category of biological classification composed of a group of individuals that are evolutionary related and that when mating are able to produce viable and fertile progeny.
Species-area curve: The biodiversity pattern, first note by Alexander von Humboldt, that illustrates that the larger the geographical area of a community, the greater the number of species.
Species diversity: The number and relative abundance of species in a biological community.
Species richness: The number of species in a biological community.
Species selection: Selection between species arising from differences in the rate of speciation and/or extinction of lineages.
Species tree: A phylogenetic tree showing the relationship among species. It is usually shown to contrast with gene trees, which may include events such as gene duplication and lateral gene transfer.
Specific heat: The amount of heat that must be absorbed or lost for 1 g of substance to change its temperature by 1°C.
Specificity: Where individual molecules take up a stable conformation with specific biological functions.
Spectrophotometer: An instrument that measures the proportions of light of different wavelengths absorbed and transmitted by a pigment solution.
Sperm: Tha male gamete.
Spermatheca: A sac in the female reproductive system where sperm are stored.
Spermatogenesis: The continuous production of mature sperm cells in the testis.
Spermatogonia: Stem cells that gives rise to sperm.
Sphenopsid:  A member of a group of plants that include trees in the Carboniferous coal swamp forest as well as the living horsetail.
Sphincter: A ring-like valve consisting of modified muscles in a muscular tube, such as digestive tract; closes of the tube like a drawstring.
Spinal nerve: In the vertebrate peripheral nervous system, a nerve that carries signals to or from the spinal cord.
Spiral cleavage: A type of embryonic development in protostomes, in which the planes of cell division that transform the zygote into a ball of cells occur obliquely to the polar axis. resulting in cells of each tier sitting in the grooves between cells of adjacent tiers.
Spiral valve: A corkscrew-shaped ridge that increases surface area and prolongs the passage of food along that short digestive tract.
Spliceosome: A complex assemblage that interacts with the ends of an RNA intron in splicing RNA, releasing the intron and joining the two adjacent exons.
Sponges: Common name for members of the phylum Porifera, which are thought to be the earliest branching lineage of animals. Sponges feed by moving water through their bodies using specialized cells with cilia called choanocytes.
Spongocoel: The central cavity of a sponge.
Spongy mesophyll: Loosely arranged photosynthetic cells located below the palisade mesophyll cells in a leaf.
Sporangium: A structure containing spores. A capsule in fungi and plants in which meiosis occurs and haploid spores develop.
Spore: In the life cycle of a plant or alga undergoing alternation of generations, a meiotically produced haploid cell that divides mitotically, generating a multicellular individual, the gametophyte, without fusing with another cell.
Sporocyte: A diploid cell, also know as spore mother cell,  that undergoes meiosis and generates haploid spores.
Sporophyll: A leaf specialized for reproduction.
Sporophyte: The diploid phase of the life cycle of plants that gives rise to the production of spores by means of meiosis. It contrasts with gametophyte.
Sporopollenin: A durable polymer that covers expose zygotes of charophycean algae and forms walls of plants spores, preventing them from drying out.
Sporozoite: A tiny infectious cell that represent a stage in the apicomplexan life cycle.
Squamous: The flat, tile-like shape of a type of epithelial cell.
Stabilizing selection: Selection that favors intermediate trait values by acting against extreme phenotypes.
Stamen: The pollen producing reproductive organ of a flower, consisting of an anther and filament.
Standard deviation: Standard deviation is a measure of dispersion; a large value of standard deviation indicates a greater variability and thus the values are more spread out.
Standard metabolic rate: The metabolic rate of a resting, fasting, and non-stressed ectotherm.
Stapes:The third of three middle ear bones.
Star genealogy: A genealogy in which all lineages coalesce in a common ancestor at the same time. It is produced by a population bottleneck or by a selective sweep.
Starch: A storage polysaccharide in plants consisting  entirely of glucose.
Statistical power: The chance that the null hypothesis will be rejected when the data are generated by a different model.
Statocyst: A type of mechanoreceptor that functions in equilibrium in invertebrates through the use of statoliths, which stimulates hair cells in relation to gravity.
Statolith: (1) In plants, a specialized plastid that contains dense starch grains and may play a role in detecting gravity. (2) In invertebrates, a grain or other dense granule that settles in response to gravity and is found in sensory organs that function in equilibrium.
Stele: The vascular tissue of a stem or root.
Stem: A vascular plant organ consisting of an alternating system of nodes and internodes that support the leaves and reproductive structures.
Stem cell: Any relatively unspecialized cell that can divide during a single division into one identical daughter  cell and one more specialized daughter cell, which can undergo further differentiation.
Stem group: The series of extinct organisms within a clade of living and fossil organism that lie below the crown group.
Stem-loop structure: A hairpin structure in an RNA molecule that is maintained by complementary base pairing.
Stenohaline: Referring to organisms that cannot tolerate substantial changes in external osmolarity.
Steranes: Chemical derivatives of sterols that have been used as chemical fossils.
Stereoisomers: Molecules whose atoms are connected with each other in the same way but are arranged differently in space. This include enantiomers, which are mirror-image reflections of each other.
Steroid: A type of lipid characterized by a carbon skeleton consisting of four rings with various functional groups attached.
Sterol: Amphipathic molecules found in the membranes of many organisms, specially eukaryotes.
Sticky end: A single-stranded end of a double stranded DNA restriction fragment.
Stigma: The female reproductive organ in a flowering plant, which receive pollen.
Stipe: A stemlike structure of a seaweed.
Stock: The plant that provides the root system when making a graft.
Stoma (plural stomata): A microscopic pore surrounded by guard cells in the epidermis of leaves and stems that allows gas exchange between the environment and the interior of the plant.
Stomach: An organ of the digestive system that stores food and performs preliminary steps of digestion.
Stratified epithelium: An epithelium consisting of more than one layer of cells in which some but not all cells touch the basal lamina.
Stream: A flowing body of water that is generally small, cold, and clear.
Stress-induced proteins: Molecules, including heat shock proteins, that are produced within cells in response to exposure marked increases in temperature and to other forms of severe stress, such as toxins, rapid pH changes, and viral infections.
Stretch-gated ion channel: Protein pore in a cell’s plasma membrane that opens when the membrane is mechanically deformed. allowing the passage of certain ions.
Striated muscle: see skeletal muscle.
Strobili: The technical term for clusters of sporophylls known commonly as cones, found in most gymnosperms and some seedless vascular plants.
Stroke: The death of nervous tissue in the brain, usually resulting from rupture or blockage of arteries in the head.
Stroke volume: The amount of blood pumped by the left ventricle in each contraction.
Stroma: The fluid of the chloroplast surrounding the thylakoid membrane; involved i the synthesis of organic molecules from carbon dioxide and water.
Stromatolite: Rocklike structure composed of layers of prokaryotes and sediment.
Structural formula: A type of molecular notation in which the constituents atoms are joined by lines representing covalent bonds.
Structural isomer: One of several organic compounds that have the same molecular formula but differ in the covalent arrangement of their atoms.
Style: The long structure between the stigma and ovule in a flower. The pollen must grow through the style in order to fertilize the plant.
Styptic: Tendency to constrict; as to constrict blood vessel and stop bleeding.
Substance P: A neuropeptide that is a key excitatory signal that mediates our perception of pain.
Substitution: The replacement in a population of one nucleotide or amino acid by another.
Substitution load: The total loss of mean fitness caused because favorable alleles substitute gradually by selection rather than instantaneously.
Substrate: The reactant on which an enzyme works.
Substrate feeder: An organisms that lives in or on its food source, eating its way through the food.
Substrate level phosphorylation: Formation of ATP by directly transferring a phosphate group to ADP from an intermediate substrate in catabolism.
Sugar sink: A plant organ that is a net consumer or storage of sugar. Growing roots, shoot tips, stems, and fruits are sugar sinks supplied by phloem.
Sugar source: A plant organ in which sugar is being produced by either photosynthesis or the breakdown of starch. Mature leaves are the primary sugar sources of plants.
Sulfhydryl group: A functional group consisting of a sulfur atom bonded to a hydrogen bond (_SH).
Suprachiasmatic nuclei (SNC): A pair of structures in the hypothalamus of mammals that functions as a biological clock.
Supercoiling: Higher-order twisting of DNA strands.
Supergene: A cluster of tightly linked genes, which allow distinct alternative morphs to be maintain as a polymorphism within one population.
Suppressor mutation: A secondary mutation that can cancel the effect of a primary mutation, resulting in a wild-type phenotype.
Surface tension: A measure of how difficult it is to stretch or break the surface of a liquid.  Water has a high surface tension because of the hydrogen bonding of surface molecules.
Survivorship curve: A plot of the numbers of a cohort that are still alive at each age; one way to represent age-specific mortality.
Survival: The act or fact of surviving under adverse or unusual circumstances.
Survive: To remain alive after the cessation of something or the occurrence of some event. To continue to live.
Surroundings: All that is around, environment.
Suspension feeder: An aquatic animal, such as a clam or a baleen whale, that sifts small food particles from the water.
Sustainable development: The long-term prosperity of human societies and the ecosystems that support them.
Swim bladder: In aquatic osteichthyans , an air sac that enables the animal to control its buoyancy in the water.
Symbiont: The smaller participant in a symbiotic relationship.
Symbiosis: The system in which members of different species live in physical contact with one another.
Sympathetic division: One of three divisions of the autonomous nervous system of vertebrates generally increasing energy expenditure and prepares the body for action.
Sympatric speciation: The separation of a single population into two or more reproductively isolated species in the absence of any geographical barriers.
Sympatry: Coexistence in the same place.
Symplats: In plants, the continuum of cytoplasm connected by plasmodesmata between cells.
Synapomorphy: A trait present in an ancestral species and shared exclusively by its evolutionary descendants.
Synapse: The locus where one neuron communicates with another neuron in a neural pathway; a narrow gap between a synaptic terminal of an axon and a signal-receiving portion (dendrite or cell body) of another neuron or effector cell. Neurotransmitter molecules released by synaptic terminals diffuse across the synapse, relaying messages to the dendrite or effector.
Synapsid: Member of an amniote clade distinguished by a single hole on each side of the skull, including the mammals.
Synapsis: The lining up of homologous chromosomes during prophase I of  meiosis. Describes the state in which the homologs are fully paired by the synaptonemal complex.
Synaptic cleft: A narrow gap separating the synaptic knob of a transmitting neuron from a receiving neuron or an effector cell.
Synaptic plasticity: A process whereby synapses, the connections between two neurons, get strengthened by experience or practice.
Synaptic terminal: A bulb at the end of an axon in which neurotransmitter molecules are stored and released.
Synaptic vesicle: Membranous sac containing neurotransmitter molecules at the tip of the presynaptic axon.
Sync: Synchronization.  To harmonize. To match, to occur at the same time.
Syngamy: The union of two genomes, which leads to doubling the ploidy level.
Synonymous mutation: A point mutation in a protein-coding region that changes a codon such that it does not alter the resulting amino acid sequence of the protein.
Syntrophy: One species living off of the metabolic products of another species.
System: A more complex organization formed from a combination of components.
Systematics: Taxonomy. The analytical study of the diversity and relationship of organisms, both present-day (extant) and extinct.
Systemic acquired resistance (SAR): A defense response in infected plants that helps protect healthy tissue from pathogenic invasion.
Systemic circuit: A branch of the circulatory system that supplies all body organs and that returns oxygen-poor blood to the right atrium via the veins.
Systemic circulation: Movement of blood through the systemic circuit.
Systems biology: An approach to studying biology that aims to model the dynamic behavior of whole biological systems.
Systole: The stage of the heart cycle in which the heart muscle contracts and the chambers pump blood.
Systolic pressure: Blood pressure in the arteries during contraction of the ventricles.