S phase That period of interphase when new DNA is synthesized as part of replication of the chromatin. PICTURE
salivary amylase An enzyme secreted by the salivary glands that begins the breakdown of complex sugars and starches.
salivary glands Glands that secrete salvia into the mouth.
saprophytes Organisms that obtain their nutrients from decaying plants and animals. Saprophytes are important in recycling organic material.
sapwood Layers of secondary xylem that are still functional in older woody plants; visible as the outer lighter areas in the cross section of a tree trunk.
sarcomeres The functional units of skeletal muscle; consist of Þlaments of myosin and actin. PICTURE
saturated fat A fat with single covalent bonds between the carbons of its fatty acids. PICTURE
Schwann cells Specialized glial cells that form the myelin sheath that coats many axons. Cells surrounding the axons of some neurons, thus forming the myelin sheath.
scientific method Systematic apporach of observation, hypothesis formation, hypothesis testing and hypothesis evaluation that forms the basis for modern science.
sclereids Plant cells with thick secondary walls that provide the gritty textures in pears. PICTURE
sclerenchyma One of the three major cell types in plants; have thickened, rigid, secondary walls that are hardened with lignin; provide support for the plant. Sclerenchyma cells include Þbers and sclereids. Plant tissue type consisting of elongated cells with thickened secondary walls for support of the plant. PICTURE 1 | PICTURE 2
scrotum In mammals, a pouch of skin located outside the body cavity into which the testes descend; provides proper temperature for the testes. PICTURE
secondary cell wall In woody plants, a second wall inside the primary cell wall; contains alternating layers of cellulose and lignin.
secondary compounds Plant products that are not important in metabolism but serve other purposes, such as attracting animals for pollination or killing parasites.
secondary extinction The death of one population due to the extinction of another, often a food species.
secondary growth Cells in a plant that are produced by a cambium. Increase in girth of a plant due to the action of lateral meristems such as the vascular cambium. The main cell produced in secondary growth is secondary xylem, better known as wood. PICTURE 1 | PICTURE 2 | PICTURE 3
secondary immunity Resistance to an antigen the second time it appears. Because of the presence of B and T memory cells produced during the Þrst exposure to the antigen, the second response is faster and more massive and lasts longer than the primary immune response.
secondary macronutrients Elements that plants require in relatively small quantities: calcium, magnesium, and sulfur.
secondary (lateral) meristems Plant meristems that produce secondary growth from a cambium.
secondary phloem Phloem produced by the vascular cambium in a woody plant stem or root.
secondary structure The structure of a protein created by the formation of hydrogen bonds between different amino acids; can be a pleated sheet, alpha helix, or random coil. Shape of a protein caused by attraction between R-groups of amino acids. PICTURE
secondary xylem Xylem produced by the vascular cambium in a woody plant stem or root; wood. PICTURE
second law of thermodynamics (entropy) The energy available after a chemical reaction is less than that at the beginning of a reaction; energy conversions are not 100% efficient.
second messenger The mechanism by which nonsteroid hormones work on target cells. A hormone binds to receptors on the cell's plasma membrane activating a molecule&emdash;the second messenger&emdash;that activates other intracellular molecules that elicit a response. The second messenger can be cyclic AMP, cyclic GMP, inositol triphosphate, diacrylglycerol, or calcium. PICTURE 1 | PICTURE 2 | PICTURE 3
secretin A hormone produced in the duodenum that stimulates alkaline secretions by the pancreas and inhibits gastric emptying.
secretion The release of a substance in response to the presence of food or speciÞc neural or hormonal stimulation.
sediment Loose aggregate of solids derived from preexisting rocks, or solids precipitated from solution by inorganic chemical processes or extracted from solution by organisms.
sedimentary rock Any rock composed of sediment, i.e., solid particles and dissolved minerals. Examples include rocks that form from sand or mud in riverbeds or on the sea bottom.
seed Structure produced by some plants in which the next generation sporophyte is surrounded by gametophyte nutritive tissues. An immature sporophyte in an arrested state of development, surrounded by a protective seed coat. PICTURE
seed coat The tough outer layer of the seed, derived from the outer layers of the ovule. PICTURE
segments Repeating units in the body parts of some animals.
segregation Separation of replicated chromosomes to opposite sides of the cell. Distribution of alleles on chromosomes into gametes during meiosis.
selective breeding The selection of individuals with desirable traits for use in breeding. Over many generations, the practice leads to the development of strains with the desired characteristics.
selectively permeable Term describing a barrier that allows some chemicals to pass but not others. The cell membrane is such a barrier.
semen A mixture of sperm and various glandular secretions.
semiconservative replication Process of DNA replication in which the DNA helix is unwound and each strand serves as a template for the synthesis of a new complementary strand, which is linked to the old strand. Thus, one old strand is retained in each new molecule. PICTURE
semilunar valve A valve between each ventricle of the heart and the artery connected to that ventricle. PICTURE
seminal vesicles Glands that contribute fructose to sperm. The fructose serves as an energy source. The structures that add fructose and hormones to semen. PICTURE
seminiferous tubules Tubules on the interior of the testes where sperm are produced. PICTURE
sensor In a closed system, the element that detects change and signals the effector to initiate a response.
sensory cortex A region of the brain associated with the parietal lobe.
sensory input Stimuli that the nervous system receives from the external or internal environment; includes pressure, taste, sound, light, and blood pH.
sensory neurons Neurons that carry signals from receptors and transmit information about the environment to processing centers in the brain and spinal cord. Neurons carrying messages from sensory receptors to the spinal cord. Sometimes referred to as an afferent neuron.
sensory (afferent) pathways The portion of the peripheral nervous system that carries information from the organs and tissues of the body to the central nervous system.
sepals Modified leaves that protect a flower's inner petals and reproductive structures. Small, leaf-like structures in flowers that enclose and protect the developing flower. These are often green, but in many monocots they are the same color as the petals (in which case the term tepal is applied since sepals and petals look so much alike).
separation Splitting of the cytoplasm by cytokinesis (= cytokinesis).
severe combined immunodeficiency (SCID) A genetic disorder in which afþicted individuals have no functional immune system and are prone to infections. Both the cell-mediated immune response and the antibody-mediated response are absent.
sex chromosomes The chromosomes that determine the sex of an organism. In humans, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Chromosome that determines the gender (sex) of the individual. Human males have a large X and a smaller Y sex chromosomes, while human females have two X sex chromosomes. PICTURE
sex hormones A group of steroid hormones produced by the adrenal cortex. Hormones that are produced in the gonads and promoted development and maintainence of the secondary sex characteristics and structures, prepare the female for pregnancy, and aid in development of gametes. Males produce testosterone, while females produce estrogen and progesterone.
sex linkage The condition in which the inheritance of a sex chromosome is coupled with that of a given gene; e.g., red-green color blindness and hemophilia in humans. Traits located on the X-chromosome.
sexual reproduction A system of reproduction in which two haploid sex cells (gametes) fuse to produce a diploid zygote. PICTURE
shoot The plant stem; provides support for the leaves and þowers; one of the three major plant organs; also referred to as the shoot system. PICTURE
short-day plants Plants that þower during early spring or fall when nights are relatively long and days are short; e.g., poinsettia and dandelions.
sickle cell anemia Human autosomal recessive disease that causes production of abnormal red blood cells that collapse (or sickle) and cause circulatory problems.
sieve cells Conducting cells in the phloem of vascular plants. See sieve elements
sieve elements Tubular, thin-walled cells that form a system of tubes extending from the roots to the leaves in the phloem of plants; lose their nuclei and organelles at maturity, but retain a functional plasma membrane. PICTURE 1 PICTURE 2
sieve plates Pores in the end walls of sieve elements that connect the sieve elements together. The end walls of sieve tube cells that are perforated (sieves). PICTURE
sieve tube members Phloem cells that form long sieve tubes. See sieve elements.
Silurian Period The geological time period of the Paleozoic Era following the Ordovician, between 435 and 395 million years ago, when plants colonized the land. PICTURE
simple leaf A leaf in which the blade does not form leaflets.
sink A body or process that acts as a storage device or disposal mechanism; e.g., plants and the oceans act as sinks absorbing atmospheric carbon dioxide. Also, a location in a plant where sugar is being consumed, either in metabolism or by conversion to starch.
sinoatrial (SA) node A region of modiÞed muscle cells in the right atrium that sends timed impulses to the heart's other muscle cells, causing them to contract; the heart's pacemaker. PICTURE
sister chromatids Chromatids joined by a common centromere and carrying identical genetic information (unless crossing-over has occurred). PICTURE
sleep movement In legumes, the movement of the leaves in response to daily rhythms of dark and light. The leaves are horizontal in daylight and folded vertically at night.
skeletal muscle Muscle that is generally attached to the skeleton and causes body parts to move; consists of muscle Þbers. Voluntary muscle cells that have a striated appearance. These muscles control skeletal movements and are normally under conscious control. PICTURE
skeletal system One of eleven major body organ systems in animals; supports the body, protects internal organs, and, with the muscular system, allows movement and locomotion. PICTURE
skin One of eleven major body organ systems in animals; the outermost layer protecting multicellular animals from the loss or exchange of internal þuids and from invasion by foreign microorganisms; composed of two layers: the epidermis and dermis. PICTURE
sliding filament model Model of muscular contraction in which the actin Þlaments in the sarcomere slide past the myosin Þlaments, shortening the sarcomere and therefore the muscle. PICTURE 1 | PICTURE 2 | PICTURE 3 | PICTURE 4
slime molds Protistans that may represent a transition between protistans and fungi.
small intestine A coiled tube in the abdominal cavity that is the major site of chemical digestion and absorption of nutrients; composed of the duodenum, jejunum, and ileum. PICTURE
smog A local alteration in the atmosphere caused by human activity; mainly an urban problem that is often due to pollutants produced by fuel combustion.
smooth muscle Muscle that lacks striations; found around circulatory system vessels and in the walls of such organs as the stomach, intestines, and bladder. Involuntary, not striated cells that control autonomic functions such as digestion and artery contraction. PICTURE
social behavior Behavior that takes place in a social context and results from the interaction between and among individuals.
societies The most highly organized type of social organization; consist of individuals that show varying degrees of cooperation and communication with one another; often have a rigid division of labor.
sodium-potassium pump The mechanism that uses ATP energy to reset the sodium and potassium ions after transmission of a nerve impulse.
soil Weathered rocks and minerals combined with air, water and organic matter that can support plants.
somatic Relating to the non-gonadal tissues and organs of an organism's body.
somatic cell A cell that is not or will not become a gamete; the cells of the body.
somatic senses All senses except vision, hearing, taste, and smell; include pain, temperature, and pressure.
somatic nervous system The portion of the peripheral nervous system consisting of the motor neuron pathways that innervate skeletal muscles.
somatostatin Pancreatic hormone that controls the rate of nutrient absorption into the bloodstream.
somites Mesodermal structures formed during embryonic development that give rise to segmented body parts such as the muscles of the body wall.
special senses Vision, hearing, taste, and smell.
species One or more populations of interbreeding or potentially interbreeding organisms that are reproductively isolated in nature from all other organisms. Populations of individuals capable of interbreeding and producing viable, fertile offspring. The least inclusive taxonomic category commonly used. PICTURE
species diversity The number of living species on Earth.
species packing The phenomenon in which present-day communities generally contain more species than earlier communities because organisms have evolved more adaptations over time.
species richness The number of species present in a community.
sperm The male gamete. PICTURE
spermatogenesis The development of sperm cells from spermatocytes to mature sperm, including meiosis. PICTURE
spicules Needle-shaped skeletal elements in sponges that occur in the matrix between the epidermal and collar cells.
spinal cord A cylinder of nerve tissue extending from the brain stem; receives sensory information and sends output motor signals; with the brain, forms the central nervous system. Nerve cell collections extending from the base of the brain to just below the last rib vertebrae.
spindle apparatus Microtubule construction that aligns and segregates chromosomes during eukaryotic cell division. PICTURE
spleen An organ that produces lymphocytes and stores erythrocytes.
spongy bone The inner layer of bone; found at the ends of long bones and is less dense than compact bone. Some spongy bone contains red marrow.
spongy mesophyll Parenchyma cells found in plant leaves that are irregularly shaped and have large intracellular spaces. PICTURE 1 | PICTURE 2
sporangia The structures in which spores are produced (sing.: sporangium).
spores Impervious structures formed by some cells that encapsulate the cells and protect them from the environment; haploid cells that can survive unfavorable conditions and germinate into new haploid individuals or act as gametes in fertilization.
sporophyte The diploid stage of a plant exhibiting alternation of generations. The diploid, spore producing phase of the plant life cycle. PICTURE
Sporozoans Members of the protists that are referred to as slime molds; may include organisms resembling the ancestors of fungi.
stability One of the phases of a population's life cycle. The population's size remains roughly constant, þuctuating around some average density. Also, the ability of a community to persist unchanged.
stabilizing selection A process of natural selection that tends to favor genotypic combinations that produce an intermediate phenotype; selection against the extremes in variation. PICTURE
stalk A leaf's petiole; the slender stem that supports the blade of a leaf and attaches it to a larger stem of the plant.
stamens The male reproductive structures of a þower; usually consist of slender, thread-like filaments topped by anthers. The male reproductive structures in the flower, composed of a filament and anther. PICTURE
stapes One of the three bones that function in hearing.
start codon The codon (AUG) on a messenger RNA molecule where protein synthesis begins.
stem cells Cells in bone marrow that produce lymphocytes by mitotic division.
steroids Compounds with a skeleton of four rings of carbon to which various side groups are attached; one of the three main classes of hormones. PICTURE
sticky ends Term applied to DNA sequences cut with restriction enzymes where the cuts will bond with each other or with another sequence cut with the same enzyme.
stigma Part of the female reproductive structure of the carpel of a þower; the sticky surface at the tip of the style to which pollen grains attach. The receptive surface of the pistil (of the flower) on which pollen is placed by a pollinator. PICTURE 1 | PICTURE 2
stimulus A physical or chemical change in the environment that leads to a response controlled by the nervous system.
stolons Stems that grow along the surface of the ground; a method of plant vegetaive propagation.
stomach The muscular organ between the esophagus and small intestine that stores, mixes, and digests food and controls the passage of food into the small intestine. PICTURE
stomata Pores on the underside of leaves that can be opened or closed to control gas exchange and water loss. Openings in the epidermis (usually of the leaf) that allow gas exchange. PICTURE 1 | PICTURE 2
stomatal apparatus The stomata and guard cells that control the size of the stoma. PICTURE 1 | PICTURE 2
stop codon The codon on a messenger RNA molecule where protein synthesis stops.
stratification The division of water in lakes and ponds into layers with different temperatures and oxygen content. Oxygen content declines with depth, while the uppermost layer is warmest in summer and coolest in winter.
stressed community A community that is disturbed by human activity, such as road building or pollution, and is inadvertently simpliÞed. Some species become superabundant while others disappear.
stroma The matrix surrounding the grana in the inner membrane of chloroplasts. The area between membranes (thylakoids, grana) inside the chloroplast. PICTURE
style Part of the female reproductive structure in the carpel of a þower; formed from the ovary wall. The tip of the style carries the stigma to which pollen grains attach. Part of the pistil that separates the stigma from the ovary. PICTURE 1 | PICTURE 2
subatomic particles The three kinds of particles that make up atoms: protons, neutrons, and electrons.
suberin Waxy, waterproof chemical in some plant cells, notably cork (in stems) and endodermis cells (in roots).
subspecies A taxonomic subdivision of a species; a population of a particular region genetically distinguishable from other such populations and capable of interbreeding with them.
substitution A type of mutation in which one base is substituted for another.
substrate feeders Animals such as earthworms or termites that eat the soil or wood through which they burrow.
sudden infant death syndrome (SIDS) A disorder resulting in the unexpected death during sleep of infants, usually between the ages of two weeks and one year. The causes are not fully understood, but are believed to involve failure of automatic respiratory control.
superior vena cava Blood from the head returns to the heart through this main vein.
suppressor T cells T cells that slow down and stop the immune response of B cells and other T cells. Immune system cells that shut off the antibody production when an infection is under control.
suprachiasmic nucleus (SCN) A region of the hypothalamus that controls internal cycles of endocrine secretion.
symbiosis An interactive association between two or more species living together; may be parasitic, commensal, or mutualistic. The relationship between two organisms.
sympathetic system The subdivision of the autonomic nervous system that dominates in stressful or emergency situations and prepares the body for strenuous physical activity, e.g., causing the heart to beat faster.
synapse The junction between an axon and an adjacent neuron. PICTURE
synapsis The alignment of chromosomes during meiosis I so that each chromosome is beside its homologue.
synaptic cleft The space between the end of a neuron and an adjacent cell.PICTURE
synaptic vesicles Vesicles at the synapse end of an axon that contain the neurotransmitters. PICTURE
synergid Cells in the embryo sac of angiosperms that flank the egg cell. The pollen tube grows through one (usually the smaller) of the synergids. PICTURE
synovial joint The most movable type of joint. The bones are covered by connective tissue, the interior of which is Þlled with synovial þuid, and the ends of the bones are covered with cartilage.
syphilis A sexually transmitted disease caused by a bacterial infection that produces an ulcer on the genitals and can have potentially serious effects if untreated.
systematics The classiÞcation of organisms based on information from observations and experiments; includes the reconstruction of evolutionary relatedness among living organisms. Currently, a system that divides organisms into Þve kingdoms (Monera, Protista, Plantae, Fungi, Animalia) is widely used.
systemic circuit The loop of the circulatory system that carries blood through the body and back to the heart. PICTURE
systole The contraction of the ventricles that opens the semilunar valve and forces blood into the arteries. PICTURE
systolic pressure The peak blood pressure when ventricles contract. PICTURE
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