S phase • In the cell cycle, the stage of interphase during which DNA is replicated. (Contrast with G1 phase, G2 phase.)
Saprobe [Gr. sapros: rotten + bios: life] • 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: a part] • The contractile unit of a skeletal muscle.
Saturated hydrocarbon • A compound consisting only of carbon and hydrogen, with the hydrogen atoms connected by single bonds.
Schizocoelous development • 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.
Secondary active transport • Form of active transport in which ions or molecules are transported against their concentration gradient using energy obtained by relaxation of a gradient of sodium ion concentration rather than directly from ATP. (Contrast with primary active transport.)
Secondary compound • A compound synthesized by a plant that is not needed for basic cellular metabolism. Typically has an antiherbivore or antiparasite function.
Secondary growth • In plants, growth produced by vascular and cork cambia, contributing to an increase in girth. (Contrast with primary growth.)
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.
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 cyclic AMP, that is released within a target cell after a hormone or other "first messenger" has bound to a surface receptor on a cell; the second messenger triggers further reactions within the cell.
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 crop • The number of seeds produced by a plant during a particular bout of reproduction.
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 (genetic) • 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.
Selective permeability • A characteristic of a membrane, allowing certain substances to pass through while other substances are excluded.
Selfish act • A behavioral act that benefits its performer but harms the recipients.
Semelparous organism • An organism that reproduces only once in its lifetime. (Contrast with iteroparous.)
Semen (see´ men) [L.: seed] • The thick, whitish liquid produced by the male reproductive organ in mammals, containing the sperm.
Semicircular canals • Part of the vestibular system of mammals.
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) • 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.: partition] • A membrane or wall between two cavities.
Sertoli cells • Cells in the seminiferous tubules that nuture the developing sperm.
Serum • That part of the blood plasma that remains after clots have formed and been removed.
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.
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.
Shoot • The aerial part of a vascular plant, consisting of the leaves, stem(s), and flowers.
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 member • 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.
Sign stimulus • The single stimulus, or one out of a very few stimuli, by which an animal distinguishes key objects, such as an enemy, or a mate, or a place to nest, etc.
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.
Signature principle • States that because of continuity, prebiotic processes should leave some trace in contemporary biochemistry. (Compare with principle of continuity.)
Silencer • A sequence of eukaryotic DNA that binds proteins that inhibit the transcription of an associated gene.
Silent mutations • Genetic changes that do not lead to a phenotypic change. At the molecular level, these are DNA sequence changes that, because of the redundancy of the genetic code, result in the same amino acids in the resulting protein. See synonymous mutation.
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) • The pacemaker of the mammalian heart.
Sinus (sigh´ nus) [L. sinus: a bend, hollow] • A cavity in a bone, a tissue space, or an enlargement in a blood vessel.
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.
Society • A group of individuals belonging to the same species and organized in a cooperative manner; in the broadest sense, includes parents and their offspring.
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, or body cells (rather than 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.
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.: 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 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 number of species of organisms living in a region. (Contrast with species diversity.)
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 reproductive cell.
Spermatocyte (spur mat´ oh site) [Gr. sperma: seed + kytos: cell] • 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 (sfingk´ ter) [Gr. sphinkter: that which binds tight] • 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.
Spiteful act • A behavioral act that harms both the actor and the recipient of the act.
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. Usually distinguished from the current idea that life evolved from nonliving matter under primordial conditions at an early stage in the history of earth.
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 plant without gametic fusion. Haploid spores develop into gametophytes, diploid spores into sporophytes. In prokaryotes, a resistant cell capable of surviving unfavorable periods.
Sporophyte (spor´ o fyte) [Gr. spora: seed + phyton: plant] • In plants 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.: 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] • An a-linked polymer of glucose; used by plants as a means of storing energy and carbon atoms.
Start codon • The mRNA triplet (AUG) that acts as signals for the beginning of translation at the ribosome. (Compare with stop codons. There are a few mnior exceptions to these codons.)
Stasis • 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: pouch] • 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. stele: pillar] • The central cylinder of vascular tissue in a plant stem.
Stem cell • A cell capable of extensive proliferation, generating more stem cells and a large clone of differentiated progeny cells, as in the formation of red blood cells.
Step cline • A sudden change in one or more traits of a species along a geographical gradient.
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.: mark, brand] • The part of the pistil at the apex of the style, which is receptive to pollen, and on which pollen germinates.
Stimulus • Something causing a response; something in the environment detected by a receptor.
Stolon • 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 • Triplets (UAG, UGA, UAA) in mRNA that act as signals for the end of translation at the ribosome. (See also start codon. There are a few mnior exceptions to these codons.)
Stratosphere • The part of the atmosphere above the troposphere; extends upward to approximately 50 kilometers above the surface of the earth; contains very little water.
Stratum (plural strata) • 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.
Stromatolite • A composite, flat-to-domed structure composed of successive mineral layers. Some are known to be produced by the action of bacteria in salt or fresh water, and some ancient ones are considered to be evidence for early life on the earth.
Structural formula • A representation of the positions of atoms and bonds in a molecule.
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.: under] • A prefix often used to designate a structure that lies beneath another or is less than another. For example, subcutaneous, subspecies.
Submucosa • (sub mew koe´ sah) • The tissue layer just under the epithelial lining of the lumen of the digestive tract. (Contrast with mucosa.)
Substrate (sub´ strayte) • (1) The molecule or molecules on which an enzyme exerts catalytic action. (2) The base material on which an organism lives.
Substrate level phosphorylation • ATP formation resulting from direct transfer of a phosphate group to ADP from an intermediate in glycolysis. (Contrast with oxidative phosphorylation.)
Succession • In ecology, the gradual, sequential series of changes in species composition of a community following a disturbance.
Sulcus (plural: sulci) [L. sulcare: to plow] • The valleys or creases between the raised portions of the convoluted surface of the brain. (Contrast to gyrus.)
Sulfhydryl group • The —SH group.
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.
Symbiosis (sim´ bee oh´ sis) [Gr.: to live together] • The living together of two or more species in a prolonged and intimate ecological relationship. (See parasitism, commensalism, mutualism.)
Symmetry • In biology, the property that two halves of an object are mirror images of each other. (See bilateral symmetry and biradial symmetry.)
Sympathetic nervous system • A division of the autonomic (involuntary) nervous system. Its activities include increasing blood pressure and acceleration of the heartbeat. The neurotransmitter at the sympathetic terminals is epinephrine or norepinephrine. (Contrast with parasympathetic nervous system.)
Sympatric speciation (sim pat´ rik) [Gr. sym: same + patria: homeland] • The occurrence of genetic reproduction isolation and the subsequent formation of new species 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.
Syngamy (sing´ guh mee) [Gr. sun-: 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 Æ 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.
Systemic circulation • The part of the circulatory system serving those parts of the body other than the lungs or gills.
Systemin • The only polypeptide plant hormone; participates in response to tissue damage.
Systole (sis´ tuh lee) [Gr.: contraction] • Contraction of a chamber of the heart, driving blood forward in the circulatory system.