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  1. S phase: The stage of the cell cycle during which the cell's DNA is replicated.
  2. S-shaped curve: A pattern of population growth in which the number of individuals in a population at first increases at a rate similar to exponential growth; however, as the number of individuals increases, the growth rate gradually decreases and the population stabilizes at the size that can be supported indefinitely by the environment.
  3. saliva: A fluid that contains enzymes that help to digest carbohydrates and which is released into the mouth by the salivary glands.
  4. salivary gland: A gland in the mouth that produces saliva.
  5. sapwood: Relatively recently formed secondary xylem cells that function to transport water. Compare heartwood.
  6. sarcomere: A unit of a myofibril consisting of an arrangement of actin and myosin filaments that extends between two Z discs.
  7. saturated: Of or referring to a fatty acid that has no double bonds between its carbon atoms. Compare unsaturated.
  8. saturated fat: A fat containing saturated fatty acids. Saturated fats are harmful to human health if eaten in excess because the body converts them into cholesterol.
  9. science: A method of inquiry that provides a rational way to discover truths about the natural world.
  10. scientific method: A series of steps in which a scientist develops a hypothesis, tests its predictions by performing experiments, and then changes or discards the hypothesis if its predictions are not supported by the results of the experiments.
  11. scientific name: The unique two-part name given to each species that consists of, first, a Latin name designating the genus and, second, a Latin name designating that species.
  12. second law of thermodynamics: The law stating that all systems, such as a cell or the universe, tend to become more disordered, and that the creation and maintenance of order in a system requires the transfer of disorder to the environment.
  13. secondary consumer: An organism that eats a primary consumer. Compare primary consumer.
  14. secondary growth: An increase in plant thickness caused by the division of lateral meristem cells.
  15. secondary immune response: The rapid defensive response to a second exposure to a pathogen that is mediated by memory B cells produced during the first exposure to that pathogen. Compare primary immune response.
  16. secondary oocyte: The larger of the two cells resulting from the first meiotic division of the primary oocyte. The secondary oocyte gives rise to the mature ovum through the second meiotic division.
  17. secondary productivity: The rate of new biomass production by consumers per unit of area. Compare net primary productivity.
  18. secondary spermatocyte: The haploid cells that result from the first meiotic division of the primary spermatocyte, and which give rise to mature sperm through the second meiotic division.
  19. secondary structure: The folding of regions in a protein into spirals or sheets.
  20. secondary succession: Ecological succession that occurs as communities recover from disturbance, as when a forest grows back when a field ceases to be used for agriculture. Compare primary succession.
  21. secondhand smoke: Tobacco smoke inhaled by people near the smoker.
  22. secretion: Active transport of materials out of an organ or an organism.
  23. sedimentary cycle: A type of nutrient cycle in which the nutrient does not enter the atmo sphere easily. Compare atmospheric cycle.
  24. seed: A structure produced by a plant in which a plant embryo is encased in a protective covering.
  25. seed dispersal mutualism: A mutualism in which an animal eats a fruit provided by a plant, then later deposits the seeds contained within the fruit far from the parent plant.
  26. selectively permeable: Of or referring to a structure, such as the plasma membrane of a cell, that controls which materials can pass through it.
  27. semicircular canal: One of the three fluid-filled tubes in the vestibular apparatus of the ear that are oriented at right angles to one another, forming a three-dimensional grid. Movement of the fluid within the tubes in forms the animal about head movement and orientation.
  28. sensory input: An action potential that conveys information about stimuli.
  29. sensory neuron: A neuron that carries sensory information to the central nervous system. Compare interneuron, motor neuron.
  30. sepal: A part of a flower that encloses and protects the flower before it opens. Sepals are the outermost of the four whorls of modified leaves that make up a flower. Compare petal, stamen, carpel.
  31. sex chromosome: Either of a pair of chromosomes that determines the sex of an individual. Compare autosome.
  32. sex hormone: One of a class of steroid hormones that regulate the development of sex and sexual behavior in animals.
  33. sex-linked: Of or referring to genes located on a sex chromosome. Genes located on the X chromosome are called X-linked; genes located on the Y chromosome are called Y-linked.
  34. sexual reproduction: Reproduction in which genes from two individuals are recombined. Compare asexual reproduction.
  35. sexual selection: A type of natural selection in which individuals that differ in inherited characteristics differ, as a result of those characteristics, in their ability to get mates.
  36. shared derived feature: A feature unique to a common ancestor that is passed down to all of its descendants, clearly defining them as a group.
  37. shoot system: One of the two basic systems of the plant body, adapted for life in air. Compare root system.
  38. signaling molecule: A molecule produced and released by one cell that affects the activities of another cell (referred to as a target cell). Signaling molecules enable the cells of a multicellular organism to communicate with one another and coordinate their activities.
  39. simple eye: A non-image-forming eye that can distinguish light from dark.
  40. single-lens eye: An eye in which a lens focuses light on an image-forming surface called a retina.
  41. single nucleotide polymorphism (SNP): A single-base-pair difference among the genomes of individuals.
  42. sinoatrial (SA) node: A signaling center in the heart that regulates the cardiac cycle.
  43. skeleton: The structural support tissue of animals.
  44. skeleton: A type of muscle fiber that is specialized for slow, sustained contraction. Compare fast muscle fiber.
  45. small intestine: The portion of the digestive system where digestion of food and most nutrient absorption takes place.
  46. smooth ER: A region of the endoplasmic reticulum that does not have attached ribosomes. Compare rough ER.
  47. smooth muscle: The specialized muscle found in the walls of the digestive system and blood vessels, so called because it lacks the bands associated with other muscle types when viewed under a microscope.
  48. social behavior: Behavior that involves cooperation among members of a group of animals, usually of the same species.
  49. soluble: Of or referring to a chemical compound that will dissolve in water.
  50. solute: A substance dissolved in water.
  51. solution: Any combination of a solute and a solvent.
  52. solvent: A liquid (in biological systems, usually water) into which a solute has dissolved.
  53. spacer DNA: A region of noncoding DNA that separates two genes. Spacer DNA is common in eukaryotes, but not in prokaryotes.
  54. specialist: A species that requires very specific conditions to survive, such as an insect that can eat only one kind of plant, as opposed to being able to eat and survive on many different kinds of plants.
  55. speciation: The process by which one species splits to form two or more species that are reproductively isolated from one another.
  56. species: A group of interbreeding natural populations that is reproductively isolated from other such groups.
  57. specific response: An immune response in which an organism recognizes and responds much more rapidly to a particular parasite or pathogen to which it has had previous exposure. Compare nonspecific response.
  58. sperm: The small, mobile, haploid gamete that is produced by sexually reproducing male eukaryotes. Compare egg.
  59. spermatogenesis: The production of sperm in animals.
  60. spinal cord: A dense collection of neuronal cell bodies and axons that carries information between the brain and the tail end of a vertebrate. The spinal cord of vertebrates forms an intermediate site of information processing that acts as a filter between the brain and the muscles and sensory neurons.
  61. spinal reflex: See reflex arc.
  62. spiracle: An opening to the system of tracheoles that supplies oxygen to insect tissues.
  63. spongy bone: The porous bone that lies within the compact bone of vertebrates. Compare compact bone.
  64. spore: (1) The reproductive cell of a fungus, which is typically encased in a protective coating that shields it from drying or rotting. (2) A structure produced by plants from which a haploid stage develops.
  65. stabilizing selection: A type of natural selection in which individuals with intermediate values of an inherited characteristic have an advantage over other individuals in the population, as when medium-sized individuals produce offspring at a higher rate than small or large individuals. Compare directional selection and disruptive selection.
  66. stamen: The male reproductive part of a flower, consisting of an anther on top of a slender filament. Stamens are the third (from the outside) of the four whorls of modified leaves that make up a flower. Compare sepal, petal, carpel.
  67. stamen: A three-nucleotide sequence on an mRNA molecule (usually the codon AUG) that signals where translation should begin.
  68. stem: A plant organ that provides the plant with structural support and also serves to transport food and water.
  69. stem cell: An undifferentiated cell that can divide to produce one or more specialized cell types.
  70. steroid: A class of lipids formed from cholesterol.
  71. steroid hormone: Any of a class of hydrophobic signaling molecules that can pass through the plasma membrane of a target cell.
  72. stimulus: Anything that can cause a sensory cell to generate a signal.
  73. stomach: The portion of the digestive system that is specialized for mixing and storing food.
  74. stop codon: A three-nucleotide sequence on an mRNA molecule that signals where translation should end.
  75. stoma: (plural: stomata) An air pore, found on the leaf or green stem of a plant, that can open and close, thereby controlling the rates at which CO2 is brought into the plant and O2 and water are lost from the plant.
  76. stroma: The space enclosed by the inner membrane of the chloroplast, in which the thylakoid membranes are situated.
  77. subsidence: Sinking of the land surface when groundwater levels have dropped and hence there is less water underground to support the land above.
  78. substitution: A mutation in which one nitrogen base is replaced by another at a single position in the DNA sequence of a gene. Compare deletion and insertion.
  79. substrate: The specific molecule on which an enzyme acts. Only the substrate will bind to the active site of the enzyme.
  80. succession: A process by which species in a community are replaced over time. For a given location, the order in which species will be replaced over time is fairly predictable.
  81. sugar: An organic compound that has the general chemical formula (CH2O)n.
  82. sustainable: Of or referring to an action or process that can continue indefinitely without using up resources or causing serious damage to ecosystems.
  83. symbiosis: A relationship in which two or more organisms of different species live together in close association.
  84. sympatric speciation: The formation of new species from populations that are not geographically isolated from one another. Compare allopatric speciation.
  85. synapse: The narrow gap that separates a neuron from its target cell.
  86. synaptic cleft: The minute gap between a neuron and its target cell across which neurotransmitters, rather than action potentials, carry the signal.
  87. synovial sac: A fluid-filled sac that forms a cushion in vertebrate joints.
  88. systematist: A scientist who studies evolutionary relationships among organisms and builds evolutionary trees.
  89. systemic circuit: The loop of the circulatory system through which blood travels from the heart to respiring tissues and back to the heart.
  90. systole: The pumping phase of the cardiac cycle. The systolic pressure corresponds to the first and higher of the two values reported in a blood pressure reading. Compare diastole.