S phase • In the cell cycle, the stage of interphase during which DNA is replicated. (Contrast with G₁ phase, G₂ phase, M phase.)

Saprobe [Gr. sapros: rotten] • An organism (usually a bacterium or fungus) that obtains its carbon and energy directly from dead organic matter.

Sarcomere (sark´ o meer) [Gr. sark: flesh + meros: unit] • The contractile unit of a skeletal muscle.

Saturated fatty acid • A fatty acid usually containing from 12 to 18 carbon atoms and no double bonds.

Savanna • Biome with climate similar to the thorn forest found on the equatorial sides of hot dry tropical and subtropical regions of Africa, South America, and Australia.

Schizocoelous development [Gk. schizo: split + koiloma: cavity] • Formation of a coelom during embryological development by a splitting of mesodermal masses.

Schwann cell • A glial cell that wraps around part of the axon of a peripheral neuron, creating a myelin sheath.

Sclereid [Gr. skleros: hard] • A type of sclerenchyma cell, commonly found in nutshells, that is not elongated.

Sclerenchyma (skler eng´ kyma) [Gr. skleros: hard + kymus: juice] • A plant tissue composed of cells with heavily thickened cell walls, dead at functional maturity. The principal types of sclerenchyma cells are fibers and sclereids.

Second law of thermodynamics • States that in any real (irreversible) process, there is a decrease in free energy and an increase in entropy.

Second messenger • A compound, such as cAMP, that is released within a target cell after a hormone (first messenger) has bound to a surface receptor on a cell; the second messenger triggers further reactions within the cell.

Secondary active transport • Form of active transport which does not use ATP as an energy source; rather, transport is coupled to ion diffusion down a concentration gradient established by primary active transport.

Secondary growth • In plants, growth produced by vascular and cork cambia, contributing to an increase in girth. (Contrast with primary growth.)

Secondary metabolite • A compound synthesized by a plant that is not needed for basic cellular metabolism. Typically has an antiherbivore or antiparasite function.

Secondary structure • Of a protein, localized regularities of structure, such as the _a helix and the b pleated sheet.

Secondary succession • Ecological succession after a disturbance that does not eliminate all the organisms that originally lived on the site.

Secondary wall • Wall layers laid down by a plant cell that has ceased growing; often impregnated with lignin or suberin.

Secretin (si kreet´ in) • A peptide hormone secreted by the upper region of the small intestine when acidic chyme is present. Stimulates the pancreatic duct to secrete bicarbonate ions.

Section • A thin slice, usually for microscopy, as a tangential section or a transverse section.

Seed • A fertilized, ripened ovule of a gymnosperm or angiosperm. Consists of the embryo, nutritive tissue, and a seed coat.

Seed plant • Plants in which the embryo is protected and nourished within a seed; the gymnosperms and angiosperms.

Seedling • A young plant that has grown from a seed (rather than by grafting or by other means.)

Segmentation genes • In insect larvae, genes that determine the number and polarity of larval segments.

Segment polarity genes • Genes that determine the boundaries and front-to-back organization of the segments in the Drosophila larva.

Segregation • In genetics, the separation of alleles, or of homologous chromosomes, from one another during meiosis so that each of the haploid daughter nuclei produced by meiosis contains one or the other member of the pair found in the diploid mother cell, but never both. This principle was articulated by Mendel as his "first law."

Selective permeability • Allowing certain substances to pass through while other substances are excluded; a characteristic of membranes.

Self incompatability • In plants, the rejection of their own pollen; promotes genetic variation and limits inbreeding.

Selfish act • A behavioral act that benefits its performer but harms the recipients.

Semen (see´ men) [L. semin: seed] • The thick, whitish liquid produced by the male reproductive organ in mammals, containing the sperm.

Semiconservative replication • The common way in which DNA is synthesized. Each of the two partner strands in a double helix acts as a template for a new partner strand. Hence, after replication, each double helix consists of one old and one new strand.

Seminiferous tubules • The tubules within the testes within which sperm production occurs.

Senescence [L. senescere: to grow old] • Aging; deteriorative changes with aging; the increased probability of dying with increasing age.

Sensory neuron • A neuron leading from a sensory cell to the central nervous system. (Contrast with motor neuron.)

Sepal (see´ pul) [L. sepalum: covering] • One of the outermost structures of the flower, usually protective in function and enclosing the rest of the flower in the bud stage.

Septum [L. saeptum: partition, fence] • A membrane or wall between two cavities.

Sertoli cells • Cells in the seminiferous tubules that nuture the developing sperm.

Sessile (sess´ ul) [L. sedere: to sit] • Permanently attached; not moving.

Set point • In a regulatory system, the threshold sensitivity to the feedback stimulus.

Sex chromosome • In organisms with a chromosomal mechanism of sex determination, one of the chromosomes involved in sex determination.

Sexual reproduction • Reproduction involving union of gametes.

Sex linkage • The pattern of inheritance characteristic of genes located on the sex chromosomes of organisms having a chromosomal mechanism for sex determination.

Sexual selection • Selection by one sex of characteristics in individuals of the opposite sex. Also, the favoring of characteristics in one sex as a result of competition among individuals of that sex for mates.

Shared derived trait • A trait that arose in the ancestor of a phylogenetic group and is present (sometimes in modified form) in all of its members, thus helping define that group. Also called a synapomorphy.

Shoot system • The aerial parts of a vascular plant, consisting of the leaves, stem(s), and flowers.

Short-day plant (SDP) • A plant that requires short days (or long nights) in order to flower.

Sieve tube • A column of specialized cells found in the phloem, specialized to conduct organic matter from sources (such as photosynthesizing leaves) to sinks (such as roots). Found principally in flowering plants.

Sieve tube element • A single cell of a sieve tube, containing cytoplasm but relatively few organelles, with highly specialized perforated end walls leading to elements above and below.

Signal • A chemical (neurotransmitter or hormone) or light message emitted from one cell/cells or organism(s) and received by others to cause some change in function or behavior.

Signal recognition particle (SRP) • A complex of RNA and protein that recognizes both the signal sequence on a growing polypeptide and receptor protein on the surface of the ER.

Signal sequence • The sequence of a protein that directs the protein through a particular cellular membrane.

Signal transduction pathway • The series of biochemical steps whereby a stimulus to a cell (such as a hormone or neurotransmitter binding to a receptor) is translated into a response of the cell.

Silencer sequence • A sequence of eukaryotic DNA that binds proteins that inhibit the transcription of an associated gene.

Silent mutation • A change in gene sequence that, due to the redundancy of the genetic code, has no effect on the amino acid produced, and thus no effect on the protein phenotype. See synonymous mutation.

Silurian • A period in the Paleozoic era in Earth's geologic history (443-417 mya).

Similarity matrix • A matrix to compare the structures of two molecules constructed by adding the number of their amino acids that are identical or different.

Sinoatrial node (sigh´ no ay´ tree al) [L. sinus: curve + atrium: hall, chamber] • The pacemaker of the mammalian heart.

Sink • In plants, any organ that imports the products of photosynthesis, such as roots, developing fruits, immature leaves. Contrast with source.

Sinus (sigh´ nus) [L. sinus: curve, hollow] • A cavity in a bone, a tissue space, or an enlargement in a blood vessel.

Sister chromatid • In the eukaryotic cell, a chromatid resulting from chromosome replication during interphase.

Sister group • Two phylogenetic groups that are each other's closes relative.

Skeletal muscle • See striated muscle.

Sliding filament theory • A proposed mechanism of muscle contraction based on formation and breaking of crossbridges between actin and myosin filaments, causing them to slide together.

Small intestine • The portion of the gut between the stomach and the colon, consisting of the duodenum, the jejunum, and the ileum.

Small nuclear ribonucleoprotein particle (snRNP) • A complex of an enzyme and a small nuclear RNA molecule, functioning in RNA splicing.

Smooth muscle • One of three types of muscle tissue. Usually consists of sheets of mononucleated cells innervated by the autonomic nervous system.

Sodium–potassium pump • The complex protein in plasma membranes that is responsible for primary active transport; it pumps sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients.

Solute • A substance that is dissolved in a liquid (solvent).

Solute potential • A property of any solution, resulting from its solute contents; it may be zero or have a negative value.

Solution • A liquid (solvent) and its dissolved solutes.

Solvent • A liquid that has dissolved or can dissolve one or more solutes.

Somatic [Gr. soma: body] • Pertaining to the body. Somatic cells are cells of the body (as opposed to germ cells).

Somite (so´ might) • One of the segments into which an embryo becomes divided longitudinally, leading to the eventual segmentation of the animal as illustrated by the spinal column, ribs, and associated muscles.

Source • In plants, an organ exporting photosynthetic products in excess of its own needs. For example, a mature leaf or storage organ. Contrast with sink.

Spatial summation • In the production or inhibition of action potentials in a postsynaptic neuron, the interaction of depolarizations and hyperpolarizations produced by several terminal boutons.

Spawning • The direct release of sex cells into the water.

Speciation (spee´ shee ay´ shun) • The process of splitting one population into two populations that are reproductively isolated from one another.

Species (spee´ shees) [L. specie: kind] • The basic lower unit of classification, consisting of a population or series of populations of closely related and similar organisms. The more narrowly defined "biological species" consists of individuals capable of interbreeding freely with each other but not with members of other species.

Species-area relationship • A relationship between the size of an area and the number of species and is frequently used to estimate the number of species extinctions likely to result from habitat destruction.

Species diversity • A weighted representation of the species of organisms living in a region; large and common species are given greater weight than are small and rare ones. (Contrast with species richness.)

Species richness • The total number of species living in a region. (Contrast with species diversity.)

Specific defenses • Defensive reactions of the immune system that are based on antibody reaction with a specific antigen.

Specific heat • The amount of energy that must be absorbed by a gram of a substance to raise its temperature by one degree centigrade. By convention, water is assigned a specific heat of one.

Sperm [Gr. sperma: seed] • A male gamete (reproductive cell).

Spermatocyte (spur mat´ oh site) [Gr. sperma: seed + kytos: container] • The cell that gives rise to the sperm in animals.

Spermatogenesis (spur mat´ oh jen´ e sis) [Gr. sperma: seed + genesis: source] • Male gametogenesis, leading to the production of sperm.

Spermatogonia • Undifferentiated germ cells that give rise to primary spermatocytes and hence to sperm.

Sphincter (sfink´ ter) [Gr. sphinkter: something that binds tightly] • A ring of muscle that can close an orifice, for example at the anus.

Spindle apparatus • An array of microtubules stretching from pole to pole of a dividing nucleus and playing a role in the movement of chromosomes at nuclear division. Named for its shape.

Spiracle (spy´ rih kel) [L. spirare: to breathe] • An opening of the treacheal respiratory system of terrestrial arthorpods.

Spliceosome • An RNA–protein complex that splices out introns from eukaryotic pre-mRNAs.

Splicing • The removal of introns and connecting of exons in eukaryotic pre-mRNAs.

Spontaneous generation • The idea that life is generated continually from nonliving matter. Spontaneous reaction A chemical reaction which will proceed on its own, without any outside influence. A spontaneous reaction need not be rapid.

Sporangium (spor an´ gee um) [Gr. spora: seed + angeion: vessel or reservoir] • In plants and fungi, any specialized stucture within which one or more spores are formed.

Spore [Gr. spora: seed] • Any asexual reproductive cell capable of developing into an adult organism without gametic fusion. In plants, haploid spores develop into gametophytes, diploid spores into sporophytes. In prokaryotes, a resistant cell capable of surviving unfavorable periods.

Sporocyte • Specialized cells of the diploid sporophyte that will divide by meiosis to produce four haploid spores. Germination of these spores produces the haploid gametophyte.

Sporophyte (spor´ o fyte) [Gr. spora: seed + phyton: plant] • In plants and protists with alternation of generations, the diploid phase that produces the spores. (Contrast with gametophyte.)

Stabilizing selection • Selection against the extreme phenotypes in a population, so that the intermediate types are favored. (Contrast with disruptive selection.)

Stamen (stay´ men) [L. stamen: thread] • A male (pollen-producing) unit of a flower, usually composed of an anther, which bears the pollen, and a filament, which is a stalk supporting the anther.

Starch [O.E. stearc: stiff] • A polymer of glucose; used by plants to store energy.

Start codon • The mRNA triplet (AUG) that acts as a signal for the beginning of translation at the ribosome. (Compare with stop codons.)

Stasis [Gk. stasis: to stop, stand still] • Period during which little or no evolutionary change takes place within a lineage or groups of lineages.

Statocyst (stat´ oh sist) [Gk. statos: stationary + kystos: cell] • An organ of equilibrium in some invertebrates.

Statolith (stat´ oh lith) [Gk. statos: stationary + lithos: stone] • A solid object that responds to gravity or movement and stimulates the mechanoreceptors of a statocyst.

Stele (steel) [Gr. stylos: pillar] • The central cylinder of vascular tissue in a plant stem.

Stem • Plant structure that holds leaves and/or flowers; it is the site for transporting and distributing material throughout the plant.

Stem cells • In animals, undifferentiated cells that are capable of extensive proliferation. A stem cell generates more stem cells and a large clone of differentiated progeny cells. Compare with initial cells.

Steroid • Any of numerous lipids based on a 17-carbon atom ring system.

Sticky ends • On a piece of two-stranded DNA, short, complementary, one-stranded regions produced by the action of a restriction endonuclease. Sticky ends allow the joining of segments of DNA from different sources.

Stigma [L. stigma: mark, brand] • The part of the pistil at the apex of the style that is receptive to pollen, and on which pollen germinates.

Stimulus [L. stimulare: to goad] • Something causing a response; something in the environment detected by a receptor.

Stolon [L. stolon: branch, sucker] • A horizontal stem that forms roots at intervals.

Stoma (plural: stomata) [Gr. stoma: mouth, opening] • Small opening in the plant epidermis that permits gas exchange; bounded by a pair of guard cells whose osmotic status regulates the size of the opening.

Stop codons • The mRNA codons that signal the end of protein translation at the ribosome: UAG, UGA, UAA.

Stratosphere • The upper part of Earth's atmosphere, above the troposphere; extends from approximately 18 kilometers upward to approximately 50 kilometers above the surface.

Stratum (plural strata) [L. stratos: layer] • A layer or sedimentary rock laid down at a particular time in a past.

Striated muscle • Contractile tissue characterized by multinucleated cells containing highly ordered arrangements of actin and myosin microfilaments. Also known as skeletal muscle.

Stroma • The fluid contents of an organelle such as a chloroplast.

Stromatolites • Composite, flat-to-domed structures composed of successive mineral layers produced by the action of cyanobacteria in water; ancient ones provide evidence for early life on the earth.

Structural gene • A gene that encodes the primary structure of a protein.

Style [Gr. stylos: pillar or column] • In flowering plants, a column of tissue extending from the tip of the ovary, and bearing the stigma or receptive surface for pollen at its apex.

Sub- [L. sub: under] • A prefix often used to designate a structure that lies beneath another or is less than another. For example, subcutaneous (beneath the skin); subspecies.

Suberin • A waxlike lipid that acts as a barrier to water and solute movement across the Casparian strip of the endodermis. Suberin is the waterproofing element in the cell walls of cork.

Submucosa (sub mew koe´ sah) • The tissue layer just under the epithelial lining of the lumen of the digestive tract.

Substrate (sub´ strayte) • The molecule or molecules on which an enzyme exerts catalytic action.

Substratum • The base material on which a sessile organism lives.

Succession • In ecology, the gradual, sequential series of changes in species composition of a community following a disturbance.

Sulcus [L. sulcare: to plow] • The valleys or creases between the raised portions of the convoluted surface of the brain. (Contrast with gyrus.)

Summation • The ability of a neuron to fire action potentials in response to numerous subthreshold postsynaptic potentials arriving simultaneously at differentiated places on the cell, or arriving at the same site in rapid succession.

Surface area-to-volume ratio • For any cell, organism, or geometrical solid, the ratio of surface area to volume; this is an important factor in setting an upper limit on the size a cell or organism can attain.

Surfactant • A substance that decreases the surface tension of a liquid. Lung surfactant, secreted by cells of the alveoli, is mostly phospholipid and decreases the amount of work necessary to inflate the lungs.

Suspensor • In the embryos of seed plants, the stalk of cells that pushes the embryo into the endosperm and is a source of nutrient transport to the embryo.

Symbiosis (sim´ bee oh´ sis) [Gr. sym: together + bios: living] • The living together of two or more species in a prolonged and intimate ecological relationship. (Compare with parasitism and mutualism.)

Symmetry • Describes an attribute of an animal body in which at least one plane can divide the body into similar, mirror-image halves. (See bilateral symmetry, biradial symmetry, radial symmetry.)

Sympathetic nervous system • A division of the autonomic (involuntary) nervous system. (Contrast with parasympathetic nervous system.)

Sympatric speciation (sim pat´ rik) [Gr. sym: same + patria: homeland] • Speciation due to reproductive isolation without any physical separation of the subpopulation. (Contrast with allopatric speciation, parapatric speciation.)

Symplast • The continuous meshwork of the interiors of living cells in the plant body, resulting from the presence of plasmodesmata. (Contrast with apoplast.)

Symport • A membrane transport process that carries two substances in the same direction across the membrane. (Contrast with antiport.)

Synapse (sin´ aps) [Gr. syn: together + haptein: to fasten] • The narrow gap between the terminal bouton of one neutron and the dendrite or cell body of another.

Synapsis (sin ap´ sis) • The highly specific parallel alignment (pairing) of homologous chromosomes during the first division of meiosis.

Synaptic vesicle • A membrane-bounded vesicle, containing neurotransmitter, which is produced in and discharged by the presynaptic neuron.

Synapomorphy • See shared derived trait.

Synergids [Gk. syn: together + ergos: performing work] • In flowering plants, the two cells accompanying the egg cell at one end of the megmagametophyte.

Syngamy (sing´ guh mee) [Gr. syn: together + gamos: marriage] • Union of gametes. Also known as fertilization.

Synonymous mutation • A mutation that substitutes one nucleotide for another but does not change the amino acid specified (i.e., UUA -> UUG, both specifying leucine). (Compare with frame-shift mutation, missense mutation, nonsense mutation.)

Synonymous substitution • The situation when a synonymous mutation becomes widespread in a population. Typically not influenced by natural selection, these substitutions can accumulate in a population. (Contrast with nonsynonymous substitution.)

Systematics • The scientific study of the diversity of organisms, and of their relationships. Includes both taxonomy (classification) and phylogeny (evolutionary relationships).

Systemic circulation • The part of the circulatory system serving those parts of the body other than the lungs or gills.

Systemic acquired resistance • A general resistance to many plant pathogens following infection by a single agent.

Systemin • The only polypeptide plant hormone; participates in response to tissue damage.

Systole (sis´ tuh lee) [Gr. systole: contraction] • Contraction of a chamber of the heart, driving blood forward in the circulatory system.