Abiogenesis: An early theory that was held that some organisms originated from nonliving material.
Abnormal Hemoglobin: Hemoglobin molecule with a different shape due to an altered amino acid sequence (ultimately caused
Abscisic Acid: A plant hormone that promotes dormancy in perennial plants and causes rapid closure of leaf stomata
Absolute Time: One of the two types of geologic time (relative time being the other), with a definite age date esta
Absorption: The process by which the products of digestion are transferred into the bodys internal environment,
Absorptive Feeders: Animals such as tapeworms that ingest food through the body wall. These animals can be very dangerou
Acetyl Coa: An intermediate compound formed during the breakdown of glucose by adding a two-carbon fragment to a
Acetylcholine: A chemical released at neuromuscular junctions that binds to receptors on the surface of the plasma
Acid: A substance that increases the number of hydrogen ions in a solution.
Acid Rain: The precipitation of sulfuric acid and other acids as rain. The acids form when sulfur dioxide and n
Acoelomates: Animals that do not have a coelom or body cavity; e.g., sponges and flatworms.
Acquired Immunodeficiency Syndrome (Aids): A collection of disorders that develop as a result of infection by the human immunodeficiency virus
Actin: The protein from which microfilaments are composed; forms the contractile filaments of sarcomeres in
Action Potential: A reversal of the electrical potential in the plasma membrane of a neuron that occurs when a nerve c
Active Transport: Transport of molecules against a concentration gradient (from regions of low concentration to region
Adaptation: Tendency of an organism to suit its environment; one of the major points of Charles Darwins theory o
Adaptive Radiation: The development of a variety of species from a single ancestral form; occurs when a new habitat beco
Adenine: One of the four nitrogen-containing bases occurring in nucleotides, the building blocks of the organ
Adenosine Diphosphate (ADP): Lower energy form of ATP, having two (instead of the three in ATP) phosphhate groups attached to the
Adenosine Triphosphate (ATP): A common form in which energy is stored in living systems; consists of a nucleotide (with ribose sug
Adhesion: The ability of molecules of one substance to adhere to a different substance.
Adrenocorticotropic Hormone (ACTH): A hormone produced by the anterior pituitary that stimulates the adrenal cortex to release several h
Adventitious Roots: Roots that develop from the stem following the death of the primary root. Branches from the adventit
Age Structure: The relative proportion of individuals in each age group in a population.
Aggregates: Fairly random associations of animals with little or no internal organization; form in response to a
Albinism: Genetic condition caused by the bodys inability to manufacture pigments; an autosomal recessive trai
Aldosterone: A hormone secreted by the adrenal glands that controls the reabsorption of sodium in the renal tubul
Alleles: Alternate forms of a gene.
Allergens: Antigens that provoke an allergic reaction.
Alpha Decay: Type of radioactive decay in which a radioisotope emits a large but slow-moving particle consisting
Alternation Of Generations: A life cycle in which a multicellular diploid stage is followed by a haploid stage and so on; found
Altitudinal Gradient: As altitude increases, a gradient of cooler, drier conditions occurs.
Alveoli: Tiny, thin-walled, inflatable sacs in the lungs where oxygen and carbon dioxide are exchanged.
Amensalism: A symbiotic relationship in which members of one population inhibit the growth of another population
Amino Acid Sequence: Also known as the primary structure of a protein/polypeptide; the sequence of amino acids in a prote
Amino Acids: The subunits (monomers) from which proteins (polymers) are assembled. Each amino acid consists of an
Amniocentesis: A method of prenatal testing in which amniotic fluid is withdrawn from the uterus through a needle.
Amniote Egg: An egg with compartmentalized sacs (a liquid-filled sac in which the embryo develops, a food sac, an
Amoebocytes: Amoeboid cells in sponges that occur in the matrix between the epidermal and collar cells. They tran
Amphibians: Class of terrestrial vertebrates which lay their eggs (and also mate) in water but live on land as a
Anabolic Reactions: Reactions in cells in which new chemical bonds are formed and new molecules are made; generally requ
Anaerobic: Refers to organisms that are not dependent on oxygen for respiration.
Analogous Structures: Body parts that serve the same function in different organisms, but differ in structure and embryolo
Anaphase: Phase of mitosis in which the chromosomes begin to separate.
Anaphylaxis: A severe allergic reaction in which histamine is released into the circulatory system; occurs upon s
Androecium: Collective term applied to all of the male (stamen) parts of the flower.
Aneuploidy: Variation in chromosome number involving one or a small number of chromosomes; commonly involves the
Angina: Chest pain, especially during physical exertion or emotional stress, that is caused by gradual block
Angiosperms: Flowering plants. First appearing at least 110 million years ago from an unknown gymnosperm ancestor
Animalia: Animal Kingdom. Multicellular eukaryotic group characterized by heterotrophic nutritional mode, usua
Annuals: Plants that grow and reproduce sexually during one year.
Antagonistic Muscles: A pair of muscles that work to produce opposite effects&emdash;one contracts as the other relaxes: f
Anther: The top of a stamens filament; divided into pollen sacs in which the pollen grains form.
Antibiotic Resistance: Tendency of certain bacteria to develop a resistance to commonly over-used antibiotics.
Antibiotics: Substances produced by some microorganisms, plants, and vertebrates that kill or inhibit the growth
Antibodies: Proteins produced by immune system cells that bind to foreign molecules and microorganisms and inact
Antibody-Mediated Immunity: Immune reaction that protects primarily against invading viruses and bacteria through antibodies pro
Anticodon: A sequence of three nucleotides on the transfer RNA molecule that recognizes and pairs with a specif
Antidiuretic Hormone (ADH): A hormone produced by the hypothalamus and released by the pituitary gland that increases the permea
Antigenic Determinant: The site on an antigen to which an antibody binds, forming an antigen-antibody complex.
Antigens: Molecules carried or produced by microorganisms that initiate antibody production; mostly proteins o
Antinutrients: Chemicals produced by plants as a defense mechanism; inhibit the action of digestive enzymes in inse
Anus: The posterior opening of the digestive tract.
Aorta: The artery that carries blood from the left ventricle for distribution throughout the tissues of the
Apical Meristem: A meristem (embryonic tissue) at the tip of a shoot or root that is responsible for increasing the p
Apnea: A disorder in which breathing stops for periods longer than 10 seconds during sleep; can be caused b
Apocrine Glands: Sweat glands that are located primarily in the armpits and groin area; larger than the more widely d
Appendicular Skeleton: The bones of the appendages (wings, legs, and arms or fins) and of the pelvic and pectoral girdles t
Appendix: Blind sac at the end of the large intestine that usually ruptures during final exams; a vestigial or
Archaea: Proposed, but not widely accepted, sixth taxonomic kingdom that would include the archaebacteria.
Archaebacteria: Ancient (over 3.5 billion years old) group of prokaryotes; some biologists want to place this group
Archaeocyathids: An extinct group of animals that were part of Cambrian-aged reef environments, but which were extinc
Archean - Proterozoic Era: The period of time beginning 4.6 billion years ago with the formation of the Earth and ending 570 mi
Aridity: The condition of receiving sparse rainfall; associated with cooler climates because cool air can hol
Arrector Pili: A muscle running from a hair follicle to the dermis. Contraction of the muscle causes the hair to ri
Arteries: Thick-walled vessels that carry blood away from the heart. Singular=artery.
Arterioles: The smallest arteries; usually branch into a capillary bed.
Artificial Selection: The process in which breeders choose the variants to be used to produce succeeding generations.
Ascomycetes: Division of fungi that contains the yeasts and morels; ascomycetes produce an ascus (or sac) in whic
Ascus: Structure produced by sac fungi in which sexual ascospores develop.
Asexual Reproduction: A method of reproduction in which genetically identical offspring are produced from a single parent;
Assortment: A way in which meiosis produces new combinations of genetic information. Paternal and maternal chrom
Aster: Short fibers produced by cells during mitosis and meiosis. These radiate from the centriole (if it i
Asteroid Impacts: Hypothesis that links certain mass extinction events with the impact of a comet or asteroid, most no
Asthma: A respiratory disorder caused by allergies that constrict the bronchioles by inducing spasms in the
Asymmetrical: In animals, a term referring to organisms that lack a general body plan or axis of symmetry that div
Atmosphere: The envelope of gases that surrounds the Earth; consists largely of nitrogen (78%) and oxygen (21%).
Atom: The smallest indivisible particle of matter that can have an independent existence.
Atomic Number: The number of protons in the nucleus of an atom.
Atomic Weight: The sum of the weights of an atoms protons and neutrons, the atomic weight differs between isotopes
Atrioventricular (AV) Node: Tissue in the right ventricle of the heart that receives the impulse from the atria and transmits it
Atrioventricular (AV) Valve: The valve between each auricle and ventricle of the heart.
Auricle: The chamber of the heart that receives blood from the body returned to the heart by the veins. Also
Autonomic System: The portion of the peripheral nervous system that stimulates smooth muscle, cardiac muscle, and glan
Autosomes: The chromosomes other than the sex chromosomes. Each member of an autosome pair (in diploid organism
Autotrophic: Refers to organisms that synthesize their nutrients and obtain their energy from inorganic raw mater
Autotrophs: Organisms that synthesize their own nutrients; include some bacteria that are able to synthesize org
Auxins: A group of hormones involved in controlling plant growth and other functions; once thought responsib
Axial Skeleton: The skull, vertebral column, and rib cage; one of the two components of the skeleton in vertebrates.
Axillary Buds: Buds borne in the axil (where the leaf meets the stem) of a stem.
Axons: Long fibers that carry signals away from the cell body of a neuron.
B Cells: Type of lymphocyte responsible for antibody-mediated immunity; mature in the bone marrow and circula
B Memory Cells: Long-lived B cells that are produced after an initial exposure to an antigen and play an important r
Bacteriophages: Viruses that attack and kill bacterial cells; composed only of DNA and protein.
Bark: The outer layer of the stems of woody plants; composed of an outer layer of dead cells (cork) and an
Barr Body: Inactivated X-chromosome in mammalian females. Although inactivated, the Barr body is replicated pri
Basal Body: A structure at the base of a cilium or flagellum; consists of nine triplet microtubules arranged in
Base: A substance that lowers the hydrogen ion concentration in a solution.
Basidia: Specialized club-shaped structures on the underside of club fungi (Basidiomycetes) within which spor
Basidiomycetes: The club fungi, a major group of fungi that all produce a structure (basidium) on which basidiospore
Basidiospores: The spores formed on the basidia of club fungi (Basidiomycetes).
Benthic Zone: One of the two basic subdivisions of the marine biome; includes the sea floor and bottom-dwelling or
Beta Decay: Type of radioactive decay in which a radioisotope emits a small, negatively-charged and fast-moving
Bicarbonate Ions: A weak base present in saliva that helps to neutralize acids in food.
Big Bang Theory: A model for the evolution of the universe that holds that all matter and energy in the universe wer
Bilateral Symmetry: In animals, refers to those that have a single axis of symmetry.
Biliary System: The bile-producing system consisting of the liver, gallbladder, and associated ducts.
Binary Fission: The method by which bacteria reproduce. The circular DNA molecule is replicated; then the cell split
Binding Sites: Areas on the ribosome within which tRNA-amino acid complexes fit during protein synthesis.
Binomial System Of Nomenclature: A system of taxonomy developed by Linnaeus in the early eighteenth century. Each species of plant an
Biochemical Cycle: The flow of an element through the living tissue and physical environment of an ecosystem; e. g., t
Biochemical Reactions: Specific chemical processes that occur in living things.
Biochemistry: Chemical processes associated with living things.
Biodiversity: Biological diversity; can be measured in terms of genetic, species, or ecosystem diversity.
Biogeography: The study of the distribution of plants and animals across the Earth.
Bioluminescent: Refers to organisms that emit light under certain conditions.
Biomass: The total weight of living tissue in a community.
Biome: A large-scale grouping that includes many communities of a similar nature.
Biosphere: All ecosystems on Earth as well as the Earths crust, waters, and atmosphere on and in which organism
Birds: Taxonomic class of terrestrial vertebrates that are characterized by endothermy and feathers; descen
Birth Rate: The ratio between births and individuals in a specified population at a particular time.
Bladder: A hollow, distensible organ with muscular walls that stores urine and expels it through the urethra
Blastocoel: The fluid-filled cavity at the center of a blastula.
Blastocyst: The developmental stage of the fertilized ovum by the time it is ready to implant; formed from the
Blastula: A ball of cells surrounding a fluid-filled cavity (the blastocoel) that is produced by the repeated
Blending: Term applied to 19th century belief that parental traits blended in their offspring; disproven by Me
Blood Group Or Type: One of the classes into which blood can be separated on the basis of the presence or absence of cert
Body Fossil: The actual remains (however permineralized, compressed or otherwise post-mortem altered) of an organ
Bolus: A mass of chewed food mixed with salivary secretions that is propelled into the espohagus during the
Bony Fish: A term applied collectively to all groups of fish with bony (as opposed to cartilaginous) skeletons.
Bottlenecks: Drastic short-term reductions in population size caused by natural disasters, disease, or predators;
Brachiopods: A phylum of hinge-shelled animals that have left an excellent fossil record; brachiopods live on or
Brachydactly: Human genetic disorder that causes production of an extra digit; an autosomal dominant trait. Someti
Brain: The most anterior, most highly developed portion of the central nervous system.
Brain Stem: The portion of the brain that is continuous with the spinal cord and consists of the medulla oblonga
Bronchi: Tubes that carry air from the trachea to the lungs (sing.: bronchus).
Bronchioles: Small tubes in the lungs that are formed by the branching of the bronchi; terminate in the alveoli.
Bronchitis: A respiratory disorder characterized by excess mucus production and swelling of the bronchioles; ca
Brown Algae: Multicellular protistans placed in the Division Phaeophyta, includes kelp.
Brush Border: The collection of microvilli forming a border on the intestinal side of the epithelial cells of the
Bryophytes: The nonvascular plants, characterized by life cycles dominated by the gametophyte phase. This group
Bud Sports: Buds that produce fruit that is different from the rest of the fruit on the tree; vegetatively propa
Budding: 1. Asexual production of new organisms; usually found in yeast; 2. the process by which HIV and simi
Buffers: Chemicals that maintain pH values within narrow limits by absorbing or releasing hydrogen ions.
Bulbourethral Glands: Glands that secrete a mucus-like substance that is added to sperm and provides lubrication during in
Bursae: Small sacs lined with synovial membrane and filled with synovial fluid; act as cushions to reduce fr
Calcitonin: A hormone produced by the thyroid that plays a role in regulating calcium levels.
Calcium Carbonate: Chemical that also occurs in limestone and marble.
Calvin Cycle: Series of biochemical, enzyme-mediated reactions during which atmospheric carbon dioxide is reduced
Cambium: A lateral meristem in plants. Types of cambiums include vascular, cork, and intercalary.
Cambrian: Geologic period that begins the Paleozoic Era 570 million years ago. Marked in its beginning by a pr
Campodactyly: A dominant trait in which a muscle is improperly attached to bones in the little finger, causing the
Capillaries: Small, thin-walled blood vessels that allow oxygen to diffuse from the blood into the cells and carb
Capillary Bed: A branching network of capillaries supplied by arterioles and drained by venules.
Capsid: The protein shell of a free virus particle. This definition is from the Glossary at the UCMP site at
Capsule: 1. Structure produced around certain bacteria; 2. Structure produced by the bryophyte sporophyte tha
Carbohydrates: Organic molecules composed of carbon, hydrogen, and oxygen that serve as energy sources and structur
Cardiac Cycle: One heartbeat; consists of atrial contraction and relaxation, ventricular contraction and relaxation
Cardiac Muscle: The type of muscle that is found in the walls of the heart. Cardiac muscle is striated but branched,
Cardiovascular System: The human circulatory system consisting of the heart and the vessels that transport blood to and fro
Carnivores: Term applied to a heterotroph, usually an animal, that eats other animals. Carnivores function as se
Carotenoids: Major group of accessory pigments in plants; includes beta carotene.
Carpals: The bones that make up the wrist joint.
Carpels: The female reproductive structures of a flower; consisting of the ovary, style, and stigma.
Carrageenan: Chemical extracted from red algae that is added to commercial ice creams as an emulsifying agent.
Carrying Capacity: The maximum population size that can be regularly sustained by an environment; the point where the p
Casparian Strip: In plants, an impermeable waxy layer between the cells of the endodermis that stops water and solute
Cast: Type of fossil preservation where the original material of the fossil has decayed and been replaced
Catabolic Reactions: Reactions in cells in which existing chemical bonds are broken and molecules are broken down; genera
Catastrophism: Once-popular belief that events in earth history had occurred in the past a sudden events and by pro
Cell Body: In a neuron, the part that contains the nucleus and most of the cytoplasm and the organelles.
Cell Cycle: The sequence of events from one division of a cell to the next; consists of mitosis (or division) an
Cell Plate: In plants, a membrane-bound space produced during cytokinesis by the vesicles of the Golgi apparatus
Cell Theory: One of the four (or five) unifying concepts in biology. The cell theory states that all living thing
Cell Wall: Structure produced by some cells outside their cell membrane; variously composed of chitin, peptidog
Cell-Mediated Immunity: Immune reaction directed against body cells that have been infected by viruses and bacteria; control
Cells: The smallest structural units of living matter capable of functioning independently.
Cellular Respiration: The transfer of energy from various molecules to produce ATP; occurs in the mitochondria of eukaryot
Cellulose: A polysaccharide that is composed of unbranched chains of glucose; the major structural carbohydrate
Cenozoic Era: The period of geologic time beginning after the end of the Mesozoic Era 65 million years ago and enc
Central Nervous System (CNS): The division of the nervous system that includes the brain and spinal cord.
Centriole: Paired cellular organelle which functions in the organization of the mitotic spindle during cell div
Centromere: A specialized region on each chromatid to which kinetochores and sister chromatids attach.
Cephalization: The concentration of sensory tissues in the anterior part of the body (head).
Cerebellum: That part of the brain concerned with fine motor coordination and body movement, posture, and balanc
Cerebral Cortex: The outer layer of gray matter in the cerebrum; consists mainly of neuronal cell bodies and dendrite
Cerebrum: The part of the forebrain that includes the cerebral cortex; the largest part of the human brain.
Cervix: The lower neck of the uterus that opens into the vagina.
Channels: Transport proteins that act as gates to control the movement of sodium and potassium ions across the
Chemical Equilibrium: The condition when the forward and reverse reaction rates are equal and the concentrations of the pr
Chemiosmosis: The process by which ATP is produced in the inner membrane of a mitochondrion. The electron transpor
Chemotrophs: Organisms (usually bacteria) that derive energy from inorganic reactions; also known as chemosynthet
Chiasma: The site where the exchange of chromosome segments between homologous chromosomes takes place (cross
Chitin: A polysaccharide contained in fungi; also forms part of the hard outer covering of insects.
Chlamydia: A sexually transmitted disease caused by a parasitic bacterium that lives inside cells of the reprod
Chlorofluorocarbons (CFCS): Chemical substances used in refrigerators, air conditioners, and solvents that drift to the upper st
Chlorophyll: The pigment in green plants that absorbs solar energy.
Chlorophyll A: The green photosynthetic pigment common to all photosynthetic organisms.
Chlorophyll B: An accessory chlorophyll found in green algae and plants.
Chlorophyll C: An accessory chlorophyll found in some protistans.
Chlorophyta: The taxonomic division that contains what are commonly called the green algae.
Chloroplasts: Disk-like organelles with a double membrane found in eukaryotic plant cells; contain thylakoids and
Cholecystokinin: A hormone secreted in the duodenum that causes the gallbladder to release bile and the pancreas to s
Chorion: The two-layered structure formed from the trophoblast after implantation; secretes human chorionic g
Chorionic Villi Sampling (CVS): A method of prenatal testing in which fetal cells from the fetal side of the placenta (chorionic vil
Chromatid: Generally refers to a strand of a replicated chromosome; consists of DNA and protein.
Chromatin: A complex of DNA and protein in eukaryotic cells that is dispersed throughout the nucleus during int
Chromosome Theory Of Inheritance: Holds that chromosomes are the cellular components that physically contain genes; proposed in 1903 b
Chromosomes: Structures in the nucleus of a eukaryotic cell that consist of DNA molecules that contain the genes.
Chrysophytes: Protistan division that is referred to as the golden brown algae; includes the diatoms.
Cilia: Hair-like organelles extending from the membrane of many eukaryotic cells; often function in locomot
Circadian Rhythms: Biorhythms that occur on a daily cycle.
Circulatory System: One of eleven major body organ systems in animals; transports oxygen, carbon dioxide, nutrients, and
Classes: Taxonomic subcategories of phyla.
Clavicle: The collar bone.
Cleavage Furrow: A constriction of the cell membrane at the equator of the cell that marks the beginning of cytokines
Climax Community: The stage in community succession where the community has become relatively stable through successfu
Clitoris: A short shaft with a sensitive tip located where the labia minora meet; consists of erectile tissue
Clone: An exact copy of a DNA segment; produced by recombinant DNA technology.
Closed Circulatory System: A system that uses a continuous series of vessels of different sizes to deliver blood to body cells
Closed Community: A community in which populations have similar range boundaries and density peaks; forms a discrete u
Codominance: A type of inheritance in which heterozygotes fully express both alleles.
Codon: A sequence of three nucleotides in messenger RNA that codes for a single amino acid.
Coelom: In animals, a body cavity between the body wall and the digestive system that forms during preadult
Coelomates: Animals that have a coelom or body cavity lined with mesoderm.
Coenzymes: Chemicals required by a number of enzymes for proper functioning; also known as enzyme cofactors.
Cohesion: The force that holds molecules of the same substance together.
Cohesion-Adhesion Theory: Describes the properties of water that help move it through a plant. Cohesion is the ability of wate
Collenchyma: One of the three major cell types in plants; are elongated and have thicker walls than parenchyma ce
Colonial: 1. Level of organization intermediate between unicellular and multicellular - organisms are composed
Commensalism: A symbiotic relationship in which one species benefits and the other is not affected.
Community: All species or populations living in the same area.
Community Age: One of the factors that helps cause the latitudinal diversity gradient. Tropical communities have ha
Community Simplification: The reduction of overall species diversity in a community; generally caused by human activity.
Community Succession: The sequential replacement of species in a community by immigration of new species and by local exti
Compact Bone: The outer dense layer that forms the shaft of the long bones; made up of concentric layers of minera
Companion Cells: Specialized cells in the phloem that load sugars into the sieve elements and help maintain a functio
Competition: One of the biological interactions that can limit population growth; occurs when two species vie wit
Competitive Exclusion: Competition between species that is so intense that one species completely eliminates the second spe
Competitive Release: Occurs when one of two competing species is removed from an area, thereby releasing the remaining sp
Complement System: A chemical defense system that kills microorganisms directly, supplements the inflammatory response,
Complementary Nucleotides: The bonding preferences of nucleotides, Adenine with Thymine, and Cytosine with Guanine. Also referr
Complete Dominance: The type of inheritance in which both heterozygotes and dominant homozygotes have the same phenotype
Complete Flower: Condition in which all flower parts are present. Example: lily.
Compound: A substance formed by two or more elements combined in a fixed ratio.
Compound Leaf: A leaf in which the blade forms small leaflets. Compound leaves that have several small leaflets ori
Compression: Type of fossilization in which the fossil is flattened (compressed)m by the weight of overlying sedi
Conditioned Response: The response to a stimulus that occurs when an animal has learned to associate the stimulus with a c
Cones: Light receptors in primates eyes that operate in bright light; provide color vision and visual acuit
Conifers: Group of gymnosperms that reproduce by cones and have needle-like leaves (in general); includes the
Connective Tissue: Animal tissue composed of cells embedded in a matrix (gel, elastic fibers, liquid, or inorganic mine
Consumers: The higher levels in a food pyramid; consist of primary consumers, which feed on the producers, and
Continuous Variation: Occurs when the phenotypes of traits controlled by a single gene cannot be sorted into two distinct
Contractile Vacuole: Organelle in many eukaryotes that acts as a bilge pump in the active transport of excess water from
Contrast: In relation to microscopes, the ability to distinguish different densities of structures.
Convergent Evolution: The development of similar structures in distantly related organisms as a result of adapting to simi
Convergent Plate Boundary: The boundary between two plates that are moving toward one another.
Coprolites: Fossilized feces.
Cork: The outer layer of the bark in woody plants; composed of dead cells.
Cork Cambium: A layer of lateral meristematic tissue between the cork and the phloem in the bark of woody plants.
Coronary Arteries: Arteries that supply the hearts muscle fibers with nutrients and oxygen.
Corpus Callosum: Tightly bundled nerve fibers that connect the right and left hemispheres of the cerebrum.
Corpus Luteum: A structure formed from the ovulated follicle in the ovary; secretes progesterone and estrogen.
Cortex: 1) The outer part of an organ, e.g., the adrenal cortex, which produces several steroid hormones; 2)
Cortisol: The primary glucocorticoid hormone; released by the adrenal cortex.
Cotyledon: A leaf-like structure that is present in the seeds of flowering plants; appears during seed germinat
Countercurrent Flow: An arrangement by which fish obtain oxygen from the water that flows through their gills. Water flow
Courtship Behavior: Behavioral sequences that precede mating.
Covalent Bond: A chemical bond created by the sharing of electrons between atoms.
Cranium: The braincase; composed of several bones fitted together at immovable joints.
Cretaceous Period: The geologic period between the Jurassic Period (140 milliojn years ago) and the Tertiary Period (be
Cristae: Structures formed by the folding of the inner membrane of a mitochondrion (sing.: crista).
Crossing-Over: During the first meiotic prophase, the process in which part of a chromatid is physically exchanged
Crossopterygians: A type of lobe-finned fish with lungs that were ancestral to amphibians.
Crustaceans: A large taxonomic class of arthropods that includes lobsters, shrimps, and crabs.
Cuticle: A film composed of wax and cutin that occurs on the external surface of plant stems and leaves and
Cyanobacteria: Blue-green bacteria; unicellular or filamentous chains of cells that carry out photosynthesis.
Cycadeoids: A group of gymnosperm seed plants not closely rated to, but superficially similar to, the cycads. Cy
Cycads: Group of gymnosperm seed plants that have large fern-like leaves and reproduce by cones but not flow
Cycle: A recurring sequence of events; e. g., the secretion of certain hormones at regular intervals.
Cyclin: A protein found in the dividing cells of many organisms that acts as a control during cell division
Cystic Fibrosis: An autosomal recessive genetic disorder that causes the production of mucus that clogs the airways o
Cytokinesis: The division of the cytoplasm during cell division.
Cytokinins: A group of hormones that promote cell division and inhibit aging of green tissues in plants.
Cytology: The branch of biology dealing with cell structure.
Cytoplasm: The viscous semiliquid inside the plasma membrane of a cell; contains various macromolecules and org
Cytosine: One of the pyrimidine nitrogenous bases occurring in both DNA and RNA.
Cytoskeleton: A three-dimensional network of microtubules and filaments that provides internal support for the cel
Cytoxic T Cells: T cells that destroy body cells infected by viruses or bacteria; also attack bacteria, fungi, parasi
Dark Reactions: The photosynthetic process in which food (sugar) molecules are formed from carbon dioxide from the a
Death Rate: The ratio between deaths and individuals in a specified population at a particular time.
Decay Series: Most radioisotopes do not decay into a stable daughter element in one single decay, but rather throu
Deciduous: Term applied to trees that lose the leaves and have a dormancy period at least once per year.
Deletion: The loss of a chromosome segment without altering the number of chromosomes.
Dendrites: Short, highly branched fibers that carry signals toward the cell body of a neuron.
Dendrochronology: The process of determining the age of a tree or wood used in structures by counting the number of an
Deoxyribonucleic Acid (DNA): A nucleic acid composed of two polynucleotide strands wound around a central axis to form a double h
Deoxyribose: Five-carbon sugar found in nucleotides of DNA.
Depth Diversity Gradient: The increase in species richness with increasing water depth until about 2000 meters below the surfa
Dermal System: Plant organ system that provides the covering for the plant.
Dermis: One of the two layers of skin; a connective tissue layer under the epidermis containing elastic and
Desert Biome: Characterized by dry conditions and plants and animals that have adapted to those conditions; found
Desmosome: A circular region of membrane cemented to an adjacent membrane by a molecular glue made of polysacch
Deuterostomes: Animals in which the first opening that appears in the embryo becomes the anus while the mouth appea
Devonian: Period of geologic time from 410 - 360 million years before the present. Life on land diversified, w
Diabetes Mellitus: A disorder associated with defects in insulin action. Type I diabetes is characterized by inadequate
Diaphragm: A dome-shaped muscle that separates the thoracic and abdominal cavities.
Diastole: The filling of the ventricle of the heart with blood.
Diatomaceous Earth: Fossilized deposits of diatoms; used for abrasives, polishes and as a filtering agent.
Dicots: One of the two main types of flowering plants; characterized by having two cotyledons, floral organs
Dictyosomes: Organelles in plant cells composed of a series of flattened membrane sacs that sort, chemically modi
Diencephalon: Part of the forebrain; consists of the thalamus and hypothalamus.
Diffusion: The spontaneous movement of particles from an area of higher concentration to an area of lower conce
Digestion: The process of breaking down food into its molecular and chemical components so that these nutrient
Digestive System: One of eleven major body organ systems in animals; converts food from the external environment into
Dihybrid Cross: In genetics, a cross that involves two sets of characteristics.
Dinoflagellates: Single-celled to colonial protistans characterized by two flagella, one girdling the cell and the ot
Dinosaurs: Any of the Mesozoic diapsids (once considered to be reptiles) belonging to the groups designated as
Dioecious: Term applied to plants having separate male and female plants.
Diploid: Cells that contain homologous chromosomes. The number of chromosomes in the cells is the diploid num
Directional Selection: A process of natural selection that tends to favor phenotypes at one extreme of the phenotypic range
Disaccharides: 1. Sugars made up of two monosaccharides held together by a covalent bond; e.g., sucrose and lactose
Discontinuous Variation: Occurs when the phenotypes of traits controlled by a single gene can be sorted into two distinct phe
Disruptive Selection: A process of natural selection that favors individuals at both extremes of a phenotypic range.
Distal Tubule: The section of the renal tubule where tubular secretion occurs.
Divergent Evolution: The divergence of a single interbreeding population or species into two or more descendant species.
Divergent Plate Boundary: The boundary between two tectonic plates that are moving apart.
Diversity: The different types of organisms that occur in a community.
Dna Hybridization: The formation of hybrid DNA molecules that contain a strand of DNA from two different species. The n
Dna Ligase: In recombinant DNA technology, an enzyme that seals together two DNA fragments from different source
Dna Polymerase: In DNA replication, the enzyme that links the complementary nucleotides together to form the newly s
Dominance: The property of one of a pair of alleles that suppresses the expression of the other member of the p
Dominance Hierarchy: A social structure among a group of animals in which one is dominant and the others have subordinate
Dominant: Refers to an allele of a gene that is always expressed in heterozygotes.
Double Fertilization: A characteristic of angiosperms in which a pollen tube carries two sperm cells to the female gametop
Duodenum: The upper part of the small intestine.
Duplication: An extra copy of a chromosome segment without altering the number of chromosomes.
Dystrophin: Protein making up only 0.002% of all protein in skeletal muscle but which appears vital for proper f
Eccrine Glands: Sweat glands that are linked to the sympathetic nervous system and are widely distributed over the b
Ecological Niche: The role an organism occupies and the function it performs in an ecosystem; closely associated with
Ecological Time: A timescale that focuses on community events that occur on the order of tens to hundreds of years.
Ecology: The study of how organisms interact with each other and their physical environment.
Ecosystem: The community living in an area and its physical environment.
Ecotones: Well-defined boundaries typical of closed communities.
Ecotype: A subdivision of a species; a stage in the formation of a species such that reproductive isolation h
Ectoderm: The outer layer of cells in embryonic development; gives rise to the skin, brain, and nervous system
Ectotherms: Animals with a variable body temperature that is determined by the environment. Examples: fish, frog
Effector: In a closed system, the element that initiates an action in response to a signal from a sensor. In h
Ejaculatory Duct: In males, a short duct that connects the vas deferens from each testis to the urethra.
Electron: A subatomic particle with a negative charge. Electrons circle the atoms nucleus in regions of space
Electron Acceptor: A molecule that forms part of the electron transport system that transfers electrons ejected by chlo
Electron Transport: 1) A series of coupled oxidation/reduction reactions where electrons are passed like hot potatoes fr
Electrostatic Attraction: The attraction between atoms of opposite charge that holds the atoms together in ionic bonds.
Element: A substance composed of atoms with the same atomic number; cannot be broken down in ordinary chemica
Elongation: During protein synthesis, the growth of the polypeptide chain through the addition of amino acids; t
Embryo: Term applied to the zygote after the beginning of mitosis that produces a multicellular structure.
Embryo Sac: Alternate term applied to the angiosperm female gametophyte contained within a megaspore.
Emphysema: Lung disease characterized by shortness of breath, often associated with smoking.
Endergonic: Chemical reactions that require energy input to begin.
Endochondral Ossification: The process by which human bones form from cartilage.
Endocrine System: One of eleven major body organ systems in animals; a system of glands that works with the nervous sy
Endocytosis: The incorporation of materials from outside the cell by the formation of vesicles in the plasma memb
Endoderm: The inner layer of cells in embryonic development that gives rise to organs and tissues associated w
Endodermis: A layer of cells surrounding the vascular cylinder of plants.
Endometrium: The inner lining of the uterus.
Endoplasmic Reticulum (ER): A network of membranous tubules in the cytoplasm of a cell; involved in the production of phospholip
Endoskeleton: An internal supporting skeleton with muscles on the outside; in vertebrates, consists of the skull,
Endosperm: A food storage tissue that provides nutrients to the developing embryo in angiosperms; formed from t
Endosymbiosis: Theory that attempts to explain the origin of the DNA-containing mitochondria and chloroplasts in ea
Endothermic: A reaction that gives off energy. The product is in a lower energy state than the reactants.
Endotherms: Animals that have the ability to maintain a constant body temperature over a wide range of environme
Endothermy: The internal control of body temperature; the ability to generate and maintain internal body heat.
Energy: The ability to bring about changes or to do work.
Energy Flow: The movement of energy through a community via feeding relationships.
Energy Of Activation: The minimum amount of energy required for a given reaction to occur; varies from reaction to reactio
Entropy: The degree of disorder in a system. As energy is transferred from one form to another, some is lost
Enzymes: Protein molecules that act as catalysts in biochemical reactions.
Eon: The longest umit of geological time.
Epidermis: 1. The outermost layer of skin consisting of several layers of epithelial cells&emdash;notably, kera
Epididymis: A long, convoluted duct on the testis where sperm are stored.
Epiglottis: A fiap of tissue that closes off the trachea during swallowing.
Epinephrine: A hormone produced by the adrenal medulla and secreted under stress; contributes to the fight or fii
Epistasis: The masking of the effects of one gene by the action of another, example: widows peak masked by the
Epithelial Tissue: Cells in animals that are closely packed in either single or multiple layers, and which cover both i
Epoch: Subdivision of a geological period.
Eras: One of the major divisions of the geologic time scale.
Erythrocytes: Red blood cells; doubly concave, enucleated cells that transport oxygen in the blood.
Esophagus: The muscular tube extending between and connecting the pharynx to the stomach.
Estrogen: A female sex hormone that performs many important functions in reproduction.
Ethylene: A gaseous plant hormone that stimulates fruit ripening and the dropping of leaves.
Eubacteria: The subunit of the Monera that includes the true bacteria such as E. coli. One of the three major gr
Euglenoids: Term applied to a division of protozoans that have one long flagellum, no cell wall, and which may h
Eukaryote: A type of cell found in many organisms including single-celled protists and multicellular fungi, pla
Euphotic Zone: The upper part of the marine biome where light penetrates and photosynthesis occurs; usually extends
Eutrophication: Runaway growth of aquatic plants that occurs when agricultural fertilizers containing phosphorus and
Evaporation: The part of the hydrologic cycle in which liquid water is converted to vapor and enters the atmosphe
Evolution: 1) The change in life over time by adaptation, variation, over-reproduction, and differential surviv
Evolutionary Tree: A diagram showing the evolutionary history of organisms based on differences in amino acid sequences
Excretion: The process of removing the waste products of cellular metabolism from the body.
Excretory System: One of eleven major body systems in animals; regulates the volume and molecular and ionic constituti
Exine: Outer covering of pollen grains, often containing sporopollenin, an acid-resistant polysaccharide th
Exocytosis: The process in which a membrane-enclosed vesicle first fuses with the plasma membrane and then opens
Exon: The DNA bases that code for an amino acid sequence. Exons are separated by introns that code for no
Exoskeleton: A hard, jointed, external covering that encloses the muscles and organs of an organism; typical of m
Exothermic: A reaction where the product is at a higher energy level than the reactants.
Exponential Rate: An extremely rapid increase, e.g., in the rate of population growth.
Expression: In relation to genes, the phenotypic manifestation of a trait. Expression may be age-dependent (e.g.
Extinction: The elimination of all individuals in a group, both by natural (dinosaurs, trilobites) and human-ind
Extracellular Digestion: A form of digestion found in annelids, crustaceans, and chordates including vertebrates; takes place
Extracellular Route: Path taken by water through the root in which water moves through the spaces between cell walls of t
Eyespot: 1. A pigmented photoreceptor in euglenoids. The eyespot senses light and orients the cell for maximu
Families: 1. In taxonomy, term applied to subcategories within orders. 2. Term applied to a group of similar t
Fats: 1. Triglycerides that are solid at room temperature. 2. A legendary pool player from Minnesota?
Fauna: Term referring collectively to all animals in an area. The zoological counterpart of flora.
Feces: Semisolid material containing undigested foods, bacteria, bilirubin, and water that is produced in t
Femur: The upper leg bone.
Fermentation: The synthesis of ATP in the absence of oxygen through glycolysis.
Fertilization: The fusion of two gametes (sperm and ovum) to produce a zygote that develops into a new individual w
Fibroblast: A term applied to a cell of connective tissue that is separated from similar cells by some degree of
Fibrous Root: A root system found in monocots in which branches develop from the adventitious roots, forming a sys
Filaments: Slender, thread-like stalks that make up the stamens of a fiower; topped by the anthers.
Filter Feeders: Organisms such as sponges that feed by removing food from water that filters through their body.
Filtration: The removal of water and solutes from the blood; occurs in the glomerulus of the nephron.
First Law Of Thermodynamics (Conservation): Energy is neither created nor destroyed, it changes from one form to another.
Fitness: A measure of an individuals ability to survive and reproduce; the chance that an individual will lea
Flame Cell: A specialized cell at the blind end of a nephridium that filters body fiuids.
Flora: Term collectively applied to all of the plants in an area. The botanical counterpart of fauna.
Flowers: The reproductive structures in angiosperm sporophytes where gametophytes are generated.
Fluid Feeders: Animals such as aphids, ticks, and mosquitoes that pierce the body of a host plant or animal and obt
Fluid-Mosaic: Widely accepted model of the plasma membrane in which proteins (the mosaic) are embedded in lipids (
Follicle-Stimulating Hormone (FSH): A hormone secreted by the anterior pituitary that promotes gamete formation in both males and female
Follicles (Ovary): Structures in the ovary consisting of a developing egg surrounded by a layer of follicle cells.
Follicles (Thyroid): Spherical structures that make up the thyroid gland; contain a gel-like colloid surrounded by a sing
Fontanels: Membranous areas in the human cranial bones that do not form bony structures until the child is 14 t
Food Chain: The simplest representation of energy fiow in a community. At the base is energy stored in plants, w
Food Pyramid: A way of depicting energy fiow in an ecosystem; shows producers (mostly plants or other phototrophs)
Food Web: A complex network of feeding interrelations among species in a natural ecosystem; more accurate and
Foraminifera: Single-celled protists that secrete a shell or test. Accumulations of the shells of dead foraminifer
Forebrain: The part of the brain that consists of the diencephalon and cerebrum.
Fossil: 1. The remains or traces of prehistoric life preserved in rocks of the Earths crust. 2. Any evidence
Fossil Fuels: Fuels that are formed in the Earth from plant or animal remains; e.g., coal, petroleum, and natural
Fossil Record: 1. The observed remains of once-living organisms taken as a whole. 2. the album Meet the Beatles.
Founder Effect: The difference in gene pools between an original population and a new population founded by one or a
Fovea: The area of the eye in which the cones are concentrated.
Freshwater Biome: The aquatic biome consisting of water containing fewer salts than the waters in the marine biome; di
Frontal Lobe: The lobe of the cerebral cortex that is responsible for motor activity, speech, and thought processe
Fruit: A ripened ovary wall produced from a flower.
Fucoxanthin: Brown accessory pigment found in and characteristic of the brown algae.
Gaia: A hypothetical superorganism composed of the Earths four spheres: the biosphere, hydrosphere, lithos
Gametes: Haploid reproductive cells (ovum and sperm).
Gametophyte: The haploid stage of a plant exhibiting alternation of generations, generates gametes by the process
Ganglia: Clusters of neurons that receive and process signals; found in fiatworms and earthworms.
Gap Junctions: Junctions between the plasma membranes of animal cells that allow communication between the cytoplas
Gastric Pits: The folds and grooves into which the stomach lining is arranged.
Gastrin: A hormone produced by the pyloric gland area of the stomach that stimulates the secretion of gastric
Gastroesophageal Sphincter: A ring of muscle at the junction of the esophagus and the stomach that remains closed except during
Gene Pool: The sum of all the genetic information carried by members of a population. Note: there is no diving
Gene Therapy: The insertion of normal or genetically altered genes into cells through the use of recombinant DNA t
Genera: Taxonomic subcategories within families (sing.: genus), composed of one or more species.
Genes: Specific segments of DNA that control cell structure and function; the functional units of inheritan
Genetic Code: The linear series of nucleotides, read as triplets, that specifies the sequence of amino acids in pr
Genetic Divergence: The separation of a populations gene pool from the gene pools of other populations due to mutation,
Genetic Drift: Random changes in the frequency of alleles from generation to generation; especially in small popula
Genetic Maps: Diagrams showing the order of and distance between genes; constructed using crossover information.
Genetics: The study of the structure and function of genes and the transmission of genes from parents to offsp
Genital Herpes: A sexually transmitted disease caused by the herpes virus; results in sores on the mucus membranes o
Genome: 1. The set of genes carried by an individual. 2. The set of genes shared by members of a reproductiv
Genotype: The genetic (alleleic) makeup of an organism with regard to an observed trait.
Geographic Isolation: Separation of populations of a species by geographic means (distance, mountains, rivers, oceans, etc
Geographic Range: The total area occupied by a population.
Geological Time: The span of time that has passed since the formation of the Earth and its physical structures; also,
Geotropism: Plants response to gravity: roots grow downward, showing positive geotropism, while shoots grow upwa
Germ Cells: Collective term for cells in the reproductive organs of multicellular organisms that divide by meios
Gestation: Period of time between fertilization and birth of an animal. Commonly called pregnancy.
Gibberellins: A group of hormones that stimulate cell division and elongation in plants. Gibberellic acid (GA), th
Gill Slits: Opening or clefts between the gill arches in fish. Water taken in by the mouth passes through the gi
Ginkgos: Group of seed plants today restricted to a single genus (Ginkgo biloba); ginkgos were more diverse d
Glial Cells: Nonconducting cells that serve as support cells in the nervous system and help to protect neurons.
Glomerulus: A tangle of capillaries that makes up part of the nephron; the site of filtration.
Glucagon: A hormone released by the pancreas that stimulates the breakdown of glycogen and the release of gluc
Glucocorticoids: A group of steroid hormones produced by the adrenal cortex that are important in regulating the meta
Glucose: A six-carbon single sugar; the most common energy source.
Glycogen: Polysaccharide consisting of numerous monosaccharide glucoses linked together. The animal equivalent
Glycolipids: Polysaccharides formed of sugars linked to lipids, a part of the cell membrane.
Glycolysis: The universal cellular metabolic process in the cells cytoplasm where 6-carbon glucose is split into
Glycoproteins: Polysaccharides formed of sugars linked to proteins. On the outer surface of a membrane, they act as
Gnetales: Group of seed plants restricted to three genera today (Gnetum, Ephedra, and Welwitschia); the possib
Golden Brown Algae: Common name applied to the protistan division Chrysophyta.
Golgi Complex: Organelles in animal cells composed of a series of fiattened sacs that sort, chemically modify, and
Gonadotropin-Releasing Hormone (GNRH): A hormone produced by the hypothalamus that controls the secretion of luteinizing hormone.
Gonadotropins: Hormones produced by the anterior pituitary that affect the testis and ovary; include follicle-stimu
Gonads: The male and female sex organs.
Gondwana: Name applied to the ancient (Paleozoic-early Mesozoic) southern hemisphere supercontinent that rifte
Gonorrhea: A sexually transmitted disease that is caused by a bacterium that infiames and damages epithelial ce
Grana: A series of stacked thylakoid disks containing chlorophyll; found in the inner membrane of chloropla
Grasslands Biome: Occurs in temperate and tropical regions with reduced rainfall or prolonged dry seasons; characteriz
Green Algae: Common name for algae placed in the division Chlorophyta.
Greenhouse Effect: The heating that occurs when gases such as carbon dioxide trap heat escaping from the Earth and radi
Ground System: Plant tissue system, composed mainly of parenchyma cells with some collenchyma and sclerenchyma cell
Growth Hormone (GH): A peptide hormone produced by the anterior pituitary that is essential for growth.
Growth Rings: Features of woody stems produced by plants growing in areas with seasonal (as opposed to year-long)
Guanine: One of the nitrogenous bases in nucleic acids, guanine is one of the two purine bases.
Guard Cells: Specialized epidermal cells that flank stomates and whose opening and closing regulates gas exchange
Gymnosperms: Flowerless, seed-bearing land plants; the first seed plants; living groups include the pines, ginkgo
Gynoecium: Collective term for all of the carpels (or pistils) in a flower. Some flowers have many pistils that
Habitat Disruption: A disturbance of the physical environment in which a population lives.
Hair Bulb: The base of a hair; contains cells that divide mitotically to produce columns of hair cells.
Hair Root: The portion of a hair that extends from the skins surface to the hair bulb.
Hair Shaft: The portion of a hair that extends above the skins surface.
Half-Life: The time required for one-half of an original unstable radioactive element to be converted to a more
Halophiles: A group of archaebacteria that are able to tolerate high salt concentrations.
Haploid: Cells that contain only one member of each homologous pair of chromosomes (haploid number = n). At f
Hardwoods: Term applied to dicot trees, as opposed to softwoods, a term applied to gymnosperms.
Haversian Canal: The central opening of compact bone; contains nerves and blood vessels.
Heart: The multicellular, chambered, muscular structure that pumps blood through the circulatory system by
Heartwood: Inner rings of xylem that have become clogged with metabolic by-products and no longer transport wat
Helper T Cells: A type of lymphocyte that stimulates the production of antibodies by activating B cells when an anti
Hemizygous: Having one or more genes that have no allele counterparts. Usually applied to genes on the males X c
Hemoglobin: A red pigment in red blood cells that can bind with oxygen and is largely responsible for the bloods
Hemophilia: A human sex-linked recessive genetic disorder that results in the absence of certain blood-clotting
Hepatitis B: A potentially serious viral disease that affects the liver; can be transmitted through sexual contac
Herbaceous: Term applied to a nonwoody stem/plant with minimal secondary growth.
Herbivores: Term pertaining to a heterotroph, usually an animal, that eats plants or algae. Herbivores function
Heterogametic Sex: The sex with two different chromosomes, such as males in humans and Drosophila.
Heterotrophic: Refers to organisms, such as animals, that depend on preformed organic molecules from the environmen
Heterotrophs: Organisms that obtain their nutrition by breaking down organic molecules in foods; include animals a
Heterozygous: Having two different alleles (one dominant, one recessive) of a gene pair.
Histamine: A chemical released during the infiammatory response that increases capillary blood fiow in the affe
Histone Proteins: Proteins associated with DNA in eukaryote chromosomes.
Homeobox Genes: Pattern genes that establish the body plan and position of organs in response to gradients of regula
Homeostasis: The ability to maintain a relatively constant internal environment.
Hominid: Primate group that includes humans and all fossil forms leading to man only.
Hominoid: Primate group that includes common ancestors of humans and apes.
Homologous Structures: Body parts in different organisms that have similar bones and similar arrangements of muscles, blood
Homologues: A pair of chromosomes in which one member of the pair is obtained from the organisms maternal parent
Homozygous: Having identical alleles for a given gene.
Hormones: Chemical substances that are produced in the endocrine glands and travel in the blood to target orga
Human Chorionic Gonadotropin (HCG): A peptide hormone secreted by the chorion that prolongs the life of the corpus luteum and prevents t
Human Genome Project: Federally funded project to determine the DNA base sequence of every gene in the human genome.
Human Immunodeficiency Virus (HIV): The retrovirus that attacks T-cells in the human immune system, destroying the bodys defenses and al
Huntington Disease: A progressive and fatal disorder of the nervous system that develops between the ages of 30 and 50 y
Hydrogen Bond: A weak bond between two atoms (one of which is hydrogen) with partial but opposite electrical charge
Hydrophilic: Water-loving. Term applied to polar molecules that can form a hydrogen bond with water.
Hydrophobic: Water-fearing.Term applied to nonpolar molecules that cannot bond with water.
Hydrophytic Leaves: The leaves of plants that grow in water or under conditions of abundant moisture.
Hydrosphere: The part of the physical environment that consists of all the liquid and solid water at or near the
Hydrostatic Skeleton: Fluid-filled closed chambers that give support and shape to the body in organisms such as jellyfish
Hypertension: High blood pressure; blood pressure consistently above 140/90.
Hypertonic: A solution having a high concentration of solute.
Hyphae: The multinucleate or multicellular filaments that make up the mycelium (body) of a fungus (sing.: hy
Hypothalamus: A region in the brain beneath the thalamus; consists of many aggregations of nerve cells and control
Hypothesis: An idea that can be experimentally tested; an idea with the lowest level of confidence.
Hypotonic: A solution having a low concentration of solute.
Ice Age: Interval of geologic time between 2 million and 10,000 years ago during which the northern hemispher
Ileum: The third and last section of the small intestine.
Immovable Joint: A joint in which the bones interlock and are held together by fibers or bony processes that prevent
Immune System: One of the eleven major body organ systems in vertebrates; defends the internal environment against
Immunoglobulins: The five classes of protein to which antibodies belong (IgD, IgM, IgG, IgA, IgE).
Implantation: The process in which the blastocyst embeds in the endometrium.
Incomplete Dominance: A type of inheritance in which the heterozygote has a phenotype intermediate to those of the homozyg
Incomplete Flower: Condition in which one or more typical flower parts are absent. Example: grass flowers such as corn
Incus: One of the three bones comprising the middle ear of mammals.
Inflammation: A reaction to the invasion of microorganisms through the skin or through the epithelial layers of th
Inflammatory Response: The bodys reaction to invading infectious microorganisms; includes an increase in blood fiow to the
Ingestive Feeders: Animals that ingest food through a mouth.
Inheritance Of Acquired Characteristics: Lamarcks view that features acquired during an organisms lifetime would be passed on to succeeding g
Initiation: The first step in translation; occurs when a messenger RNA molecule, a ribosomal subunit, and a tran
Initiation Codon (AUG): Three-base sequence on the messenger RNA that codes for the amino acid methionine; the start command
Insertion: A type of mutation in which a new DNA base is inserted into an existing sequence of DNA bases. This
Insulin: A hormone secreted by the pancreas that stimulates the uptake of glucose by body cells. Insulin work
Integration: The process of combining incoming information; one of the functions of the nervous system.
Integument: Something that covers or encloses, e.g., the skin.
Integumentary System: The skin and its derivatives (hair, nails, feathers, horns, antlers, and glands), which in multicell
Interferons: Proteins released by cells in response to viral infection; activate the synthesis and secretion of a
Internal Environment: In multicellular organisms, the aqueous environment that is outside the cells but inside the body.
Interneurons: Neurons that process signals from one or more sensory neurons and relay signals to motor neurons. Ak
Internodes: The stem regions between nodes in plants.
Interphase: The period between cell divisions when growth and replacement occur in preparation for the next divi
Interstitial: Being situated within a particular organ or tissue.
Interstitial Fluid: Fluid surrounding the cells in body tissues; provides a path through which nutrients, gases, and was
Intracellular Digestion: A form of digestion in which food is taken into cells by phagocytosis; found in sponges and most pro
Intracellular Parasites: Viruses that enter a host cell and take over the hosts cellular machinery to produce new viral parti
Intracellular Route: Path taken by water through the cells of the root between the epidermis and the xylem, moving throug
Intron: In eukaryotes, bases of a gene transcribed but later excised from the mRNA prior to exporting from t
Inversion: A reversal in the order of genes on a chromosome segment.
Ion: An atom that has lost or gained electrons from its outer shell and therefore has a positive or negat
Ionic Bond: A chemical bond in which atoms of opposite charge are held together by electrostatic attraction.
Isotonic: Term applied to two solutions with equal solute concentrations.
Isotopes: Atoms with the same atomic number but different numbers of neutrons; indicated by adding the mass nu
Jejunum: The second portion of the small intestine. Also, a popular month for weddings!
Jurassic Period: Middle period of the Mesozoic Era, between 185-135 million years ago. Characterized by the (possible
Karyotype: The chromosomal characteristics of a cell; also, a representation of the chromosomes aligned in pair
Keratin: A fibrous protein that fills mature keratinocytes near the skins surface.
Keratinocytes: The basic cell type of the epidermis; produced by basal cells in the inner layer of the epidermis.
Kidney Stones: Crystallized deposits of excess wastes such as uric acid, calcium, and magnesium that may form in th
Kilocalorie: The energy needed to heat 1000 grams of water from 14.5 to 15.5 degrees C.
Kinetochores: Structures at the centromeres of the chromosomes to which the fibers of the mitotic spindle connect.
Kingdoms: Five broad taxonomic categories (Monera, Protista, Plantae, Fungi, Animalia) into which organisms ar
Klinefelter Syndrome: In humans, a genetically determined condition in which the individual has two X and one Y chromosome
Krebs Cycle: Biochemical cycle in cellular aerobic metabolism where acetyl CoA is combined with oxaloacetate to f
L-Dopa: A chemical related to dopamine that is used in the treatment of Parkinsons disease.
Labia Majora: The outer folds of skin that cover and protect the genital region in women.
Labia Minora: Thin membranous folds of skin outside the vaginal opening.
Lactose Intolerance: A genetic trait characterized by the absence of the enzyme lactase, which breaks down lactose, the m
Langerhans Cells: Epidermal cells that participate in the infiammatory response by engulfing microorganisms and releas
Large Intestine: Consists of the cecum, appendix, colon, and rectum; absorbs some nutrients, but mainly prepares fece
Larva: A stage in the development of many insects and other organisms including sea urchins and sponges. In
Larynx: A hollow structure at the beginning of the trachea. The vocal cords extend across the opening of the
Lateral Roots: Roots extending away from the main (or taproot) root.
Latitudinal Diversity Gradient: The decrease in species richness that occurs as one moves away from the equator.
Latitudinal Gradient: As latitude increases, a gradient of cooler, drier conditions occurs.
Laurasia: The northern part of the supercontinent of Pangaea, composed of the present-day North America, Europ
Laurentia: Name applied to the core of North America in the times from the breakup of the precambrian supercont
Law Of The Minimum: Holds that population growth is limited by the resource in shortest supply.
Leaf Primordia: Young leaves, recently formed by the shoot apical meristem, located at the tip of a shoot.
Leaf Veins: Vascular tissue in leaves, arranged in a net-like network (reticulate vennation) in dicots, and runn
Leaves: The site of photosynthesis; one of the three major organs in plants.
Leukocytes: White blood cells; primarily engaged in fighting infection.
Lichens: Autotrophic organisms composed of a fungus (sac or club fungus) and a photosynthetic unicellular org
Life History: The age at sexual maturity, age at death, and age at other events in an individuals lifetime that in
Ligaments: Dense parallel bundles of connective tissue that strengthen joints and hold the bones in place.
Light Reactions: The photosynthetic process in which solar energy is harvested and transferred into the chemical bond
Lignin: A polymer in the secondary cell wall of woody plant cells that helps to strengthen and stiffen the w
Linkage: The condition in which the inheritance of a specific chromosome is coupled with that of a given gene
Lipases: Enzymes secreted by the pancreas that are active in the digestion of fats.
Lipids: One of the four classes of organic macromolecules. Lipids function in the long-term storage of bioch
Lithosphere: The solid outer layer of the Earth; includes both the land area and the land beneath the oceans and
Lobe-Finned: Fish with muscular fins containing large jointed bones that attach to the body; one of the two main
Logistic Growth Model: A model of population growth in which the population initially grows at an exponential rate until it
Long-Day Plants: Plants that fiower in the summer when nights are short and days are long; e.g., spinach and wheat.
Loop Of Henle: A U-shaped loop between the proximal and distal tubules in the kidney.
Lungfish: A type of lobe-finned fish that breathe by a modified swim bladder (or lung) as well as by gills.
Lungs: Sac-like structures of varying complexity where blood and air exchange oxygen and carbon dioxide; co
Luteal Phase: The second half of the ovarian cycle when the corpus luteum is formed; occurs after ovulation.
Luteinizing Hormone (LH): A hormone secreted by the anterior pituitary gland that stimulates the secretion of testosterone in
Lymph: Interstitial fiuid in the lymphatic system.
Lymph Hearts: Contractile enlargements of vessels that pump lymph back into the veins; found in fish, amphibians,
Lymphatic Circulation: A secondary circulatory system that collects fiuids from between the cells and returns it to the mai
Lymphatic System: A network of glands and vessels that drain interstitial fiuid from body tissues and return it to the
Lymphocytes: White blood cells that arise in the bone marrow and mediate the immune response; include T cells and
Lyon Hypothesis: Idea proposed by Mary Lyon that mammalian females inactivate one or the other X-chromosome during ea
Lysosomes: Membrane-enclosed organelles containing digestive enzymes. The lysosomes fuse with food vacuoles and
Macroevolution: The combination of events associated with the origin, diversification, extinction, and interactions
Macromolecules: Large molecules made up of many small organic molecules that are often referred to as monomers; e.g.
Macronucleus: In ciliates, the large nucleus that carries up to several hundred copies of the genome and controls
Macronutrients: 1. Elements needed by plants in relatively large (primary) or smaller (secondary) quantities. 2. Foo
Macrophages: A type of white blood cell derived from monocytes that engulf invading antigenic molecules, viruses,
Malleus: One of the bones comprising the middle ear of mammals.
Malpighian Tubules: The excretory organs of insects; a set of long tubules that open into the gut.
Mammal-Like Reptiles: Group of Permian-Triassic reptiles having some possible mammalian features, notably a more prominent
Mantle: In mollusks, a membranous or muscular structure that surrounds the visceral mass and secretes a shel
Marine Biome: The aquatic biome consisting of waters containing 3.5% salt on average; includes the oceans and cove
Marsupials: Pouched mammals. The young develop internally, but are born while in an embryonic state and remain i
Mass Extinction: A time during which extinction rates are generally accelerated so that more than 50% of all species
Mast Cells: Cells that synthesize and release histamine, as during an allergic response; found most often in con
Matter: Anything that has mass and occupies space.
Matter Cycling: The fiow of matter through various organisms and the physical environment of an ecosystem.
Maximum Sustainable Yield (MSy): The maximum number of a food or game population that can be harvested without harming the population
Medulla: 1. A term referring to the central portion of certain organs; e.g., the medulla oblongata of the bra
Medulla Oblongata: The region of the brain that, with the pons, makes up the hindbrain; controls heart rate, constricti
Medusa: The motile bell-shaped form of body plan in cnidarians; e.g., jellyfish.
Megakarocytes: Cells found in the bone marrow that produce platelets.
Megaspore Mother Cell: Cells that undergo meiosis to produce megaspores.
Megaspores: Four haploid cells produced by meiosis in the ovule of a fiower. Usually, three of these cells degen
Meiosis: Cell division in which the chromosomes replicate, followed by two nuclear divisions. Each of the res
Meissners Corpuscles: Sensory receptors concentrated in the epidermis of the fingers and lips that make these areas very s
Melanin: A pigment that gives the skin color and protects the underlying layers against damage by ultraviolet
Melanocytes: The cells in the inner layer of the epidermis that produce melanin.
Membrane-Attack Complex (MAC): A large cylindrical multiprotein complex formed by the complement system; kills invading microorgani
Menstrual Cycle: The recurring secretion of hormones and associated uterine tissue changes; typically 28 days in leng
Menstruation: The process in which the uterine endometrium breaks down and sheds cells, resulting in bleeding; occ
Meristematic Tissue: Embryonic tissue located at the tips of stems and roots and occasionally along their entire length;
Mesentary: Epithelial cells supporting the digestive organs.
Mesoderm: The middle layer of cells in embryonic development; gives rise to muscles, bones, and structures ass
Mesoglea: A gel-like matrix that occurs between the outer and inner epithelial layers in cnidarians.
Mesophyll: Layer of leaf tissue between the epidermis layers; literally meaning middle of the leaf.
Mesophytic Leaves: The leaves of plants that grow under moderately humid conditions with abundant soil and water.
Mesozoic Era: The period of geologic time beginning 245 million years ago and ending 65 million years ago; the age
Messenger Rna (MRNA): Blueprint for protein synthesis that is transcribed from one strand of the DNA (gene) and which is t
Metabolic Pathway: A series of individual chemical reactions in a living system that combine to perform one or more imp
Metabolism: The sum of all chemical reactions (energy exchanges) in cells.
Metamorphosis: The process of changing from one form to another; e.g., in insects, from the larval stage to the pup
Metaphase: The stage of eukaryotic cell division (mitosis or meiosis) in which the chromosomes line up at the e
Metastasis: The process in which cancer cells break away from the original tumor mass and establish new tumor si
Methanogens: A group of archaebacteria that produce methane as a by product of their metabolism.
Methionine: The amino acid coded for by the initiation codon; all polypeptides begin with methionine, although p
Micelles: Structures formed when bile salts surround digested fats in order to enable the water-insoluble fats
Microevolution: A small-scale evolutionary event such as the formation of a species from a preexisting one or the di
Microfilaments: Rods composed of actin that are found in the cytoskeleton and are involved in cell division and move
Microgametophyte: Stage of the plant life cycle that develops from or within a microspore. The microgametophyte produc
Micronucleus: In the protistan group known as the ciliates, the small nucleus containing a single copy of the geno
Micronutrients: Elements that are required by plants in very small quantities, but are toxic in large quantities: ir
Micropyle: The end of the embryo sac where the egg cell and synergids are located.
Microsporangia: Structures of the sporophyte in which microspores are produced by meiosis. In flowering plants the m
Microspore Mother Cell: Cells in the microsporangium that undergo meiosis to produce microspores. In flowering plants the mi
Microspores: Four haploid cells produced by the meiotic division in the pollen sacs of fiowers or microsporangia
Microtubules: Filaments about 25 nanometers in diameter found in cilia, fiagella, and the cytoskeleton.
Microvilli: Hair-like projections on the surface of the epithelial cells of the villi in the small intestine; in
Midbrain: A network of neurons that connects with the forebrain and relays sensory signals to other integratin
Middle Lamella: A layer composed of pectin that cements two adjoining plant cells together.
Migration: Movement of organisms either permanently (as in the migration of humans to the Americas) or temporar
Mineralocorticoids: A group of steroid hormones produced by the adrenal cortex that are important in maintaining electro
Minerals: Trace elements required for normal metabolism, as components of cells and tissues, and in nerve cond
Minimum Viable Population (MVP): The smallest population size that can avoid extinction due to breeding problems or random environmen
Mitochondria: Self-replicating membrane-bound cytoplasmic organelles in most eukaryotic cells that complete the br
Mitosis: The division of the cells nucleus and nuclear material of a cell; consists of four stages: prophase,
Mitotic Spindle: A network of microtubules formed during prophase. Some microtubules attach to the centromeres of the
Mold: Type of fossil preservation where the original material of the fossil has decayed but has left an im
Mole: Avogadros number (6.02 X 1023 atoms) of a substance.
Molecular Biology: Field of biology that studies the molecular level of organization.
Molecules: Units of two or more atoms held together by chemical bonds. The combination of atoms by chemical bon
Monera: Prokaryotic kingdom that includes (in the most widely accepted classification system) archaebacteria
Monocots: One of the two major types of fiowering plants; characterized by having a single cotyledon, fioral o
Monoculture: The growth of only one species in a given area; such as a cornfield or other agricultural field.
Monocytes: White blood cells that clean up dead viruses, bacteria, and fungi and dispose of dead cells and debr
Monohybrid Cross: In genetics, a cross that involves only one characteristic.
Monomer: An organic chemical unit linked to other units (usually by a covalent bond formed by the removal of
Monophyletic Group: A group of organisms descended from a common ancestor. For example: your immediate family may be con
Monosaccharides: Simple carbohydrates, usually with a five- or six-carbon skeleton; e.g., glucose and fructose. A car
Monotremes: Egg-laying mammals; e.g., the spiny anteater and the duck-billed platypus.
Morph: A distinct phenotypic variant within a population.
Morphological Convergence: The evolution of basically dissimilar structures to serve a common function. For example: the wings
Morula: The solid-ball stage of the pre-emplantation embryo.
Mosaic Evolution: A pattern of evolution where all features of an organism do not evolve at the same rate. Some charac
Motor (Efferent) Pathways: The portion of the peripheral nervous system that carries signals from the central nervous system to
Motor Neurons: Neurons that receive signals from interneurons and transfer the signals to effector cells that produ
Motor Output: A response to the stimuli received by the nervous system. A signal is transmitted to organs that can
Motor Units: Consist of a motor neuron with a group of muscle fibers; form the units into which skeletal muscles
Mouth: The oral cavity; the entrance to the digestive system where food is broken into pieces by the teeth
Mucus: A thick, lubricating fluid produced by the mucous membranes that line the respiratory, digestive, ur
Multicellular: Organisms composed of multiple cells and exhibiting some division of labor and specialization of cel
Multinucleate: Cells having more than one nucleus per cell.
Muscle Fibers: Long, multinucleated cells found in skeletal muscles; made up of myofibrils. One of the four major g
Muscular System: One of eleven major body organ systems in animals; allows movement and locomotion, powers the circul
Mutation: Any heritable change in the nucleotide sequence of DNA; can involve substitutions, insertions, or de
Mutation Rate: The average occurrence of mutations in a species per a given unit of time.
Mutualism: A form of symbiosis in which both species benefit. A type of symbiosis where both organisms benefit.
Mycelium: The mass of interwoven filaments of hyphae in a fungus.
Mycorrhiza: Occurs when a fungus (basidiomycete or zygomycete) weaves around or into a plants roots and forms a
Myelin Sheath: Layers of specialized glial cells, called Schwann cells, that coat the axons of many neurons.
Myofibrils: Striated contractile microfilaments in skeletal muscle cells.
Myosin: Thick protein filaments in the center sections of sarcomeres.
Nares: Nostrils; the openings in the nose through which air enters.
Nastic Movement: A plants response to a stimulus in which the direction of the response is independent of the directi
Natural Selection: The process of differential survival and reproduction of fitter genotypes; can be stabilizing, direc
Nectaries: Nectar-secreting organs in fiowering plants that serve as insect feeding stations and thus attract i
Negative Feedback: The stopping of the synthesis of an enzyme by the accumulation of the products of the enzyme-mediate
Negative Feedback Control: Occurs when information produced by the feedback reverses the direction of the response; regulates t
Negative Feedback Loop: A biochemical pathway where the products of the reaction inhibit production of the enzyme that contr
Nektonic Organisms: Swimmers; one of the two main types of organisms in the pelagic zone of the marine biome.
Nephridium: The excretory organ in fiatworms and other invertebrates; a blind-ended tubule that expels waste thr
Nephron: A tubular structure that is the filtering unit of the kidney; consists of a glomerulus and renal tub
Nerve Cord: A dorsal tubular cord of nervous tissue above the notochord of a chordate.
Nerve Net: An interconnected mesh of neurons that sends signals in all directions; found in radially symmetrica
Nerves: Bundles of neuronal processes enclosed in connective tissue that carry signals to and from the centr
Nervous System: One of eleven major body organ systems in animals; coordinates and controls actions of internal orga
Net Primary Productivity (NPP): The rate at which producer (usually plants) biomass is created in a community.
Net Secondary Productivity (NSP): The rate at which consumer and decomposer biomass is produced in a community.
Neural Tube: A tube of ectoderm in the embryo that will form the spinal cord.
Neuromuscular Junction: The point where a motor neuron attaches to a muscle cell.
Neurons: Highly specialized cells that generate and transmit bioelectric impulses from one part of the body t
Neurotoxin: Chemical that paralyzes nerves. Neurotoxins are produced by a variety of organisms, most notably som
Neurotransmitters: Chemicals released from the tip of an axon into the synaptic cleft when a nerve impulse arrives; may
Neutron: An uncharged subatomic particle in the nucleus of an atom. The large (mass approximately equal to 1
Niche: The biological role played by a species.
Niche Overlap: The extent to which two species require similar resources; specifies the strength of the competition
Nicotine Adenine Dinucleotide Phosphate (NADP): A substance to which electrons are transferred from photosystem I during photosynthesis; the additio
Node: The stem region of a plant where one or more leaves attach. Where leaves are attached to stems.
Node Of Ranvier: A gap between two of the Schwann cells that make up an axons myelin sheath; serves as a point for ge
Nondisjunction: The failure of chromosomes to separate properly during cell division. The unequal segregation of chr
Nonvascular Plants: Plants lacking lignified vascular tissue (xylem), vascularized leaves, and having a free-living, pho
Norepinephrine: A hormone produced in the adrenal medulla and secreted under stress; contributes to the fight or fii
Notochord: In chordates, a cellular rod that runs the length of the body and provides dorsal support. Also, a s
Nuclear Area: In prokaryotic cells, a region containing the cells genetic information. Unlike the nucleus in eukar
Nuclear Pores: Openings in the membrane of a cells nuclear envelope that allow the exchange of materials between th
Nucleic Acids: Polymers composed of nucleotides; e.g., DNA and RNA.
Nucleoid: The area of the prokaryotic cytoplasm where the chromatin is localized.
Nucleolus: A round or oval body in the nucleus of a eukaryotic cell; consists of DNA and RNA and produces ribos
Nucleosomes: Spherical bodies formed by coils of chromatin. The nucleosomes in turn are coiled to form the fibers
Nucleotide Sequences: The genetic code encrypted in the sequence of bases along a nucleic acid.
Nucleotides: The subunits of nucleic acids; composed of a phosphate, a sugar, and a nitrogen-containing base. The
Nucleus (Atom): An atoms core; contains protons and one or more neutrons (except hydrogen, which has no neutrons).
Nucleus (Cell): The largest, most prominent organelle in eukaryotic cells; a round or oval body that is surrounded b
Nyctinasty: A nastic movement in a plant that is caused by light and dark.
Occipital Lobe: The lobe of the cerebral cortex located at the rear of the head; is responsible for receiving and pr
Oils: Triglycerides that are liquid at room temperature.
Oncogenes: Genes that can activate cell division in cells that normally do not divide or do so only slowly. A g
One Gene, One Enzyme Hypothesis: Holds that a single gene controls the production, specificity, and activity of each enzyme in a meta
Oocyte: A cell that will/is undergo/ing development into a female gamete.
Oogenesis: The production of ova. The development of a diploid cell into a haploid ovum or egg cell.
Open Circulatory System: A system in which the circulating fluid is not enclosed in vessels at all times; found in insects, c
Open Community: A community in which the populations have different density peaks and range boundaries and are distr
Opposable: The capability of being placed against the remaining digits of a hand or foot; e.g., the ability of
Opsins: Molecules in cone cells that bind to pigments, creating a complex that is sensitive to light of a gi
Orders: Taxonomic subcategories of classes.
Ordovician Extinction: Paleozoic-aged mass extinction possibly related to glaciation in the southern-hemisphere supercontin
Ordovician Period: Geologic period of the Paleozoic Era after the Cambrian Period between 500 and 435 million years ago
Organ Systems: Groups of organs that perform related functions.
Organelles: Cell components that carry out individual functions; e.g., the cell nucleus and the endoplasmic reti
Organism: An individual, composed of organ systems (if multicellular). Multiple organisms make up a population
Organs: Differentiated structures consisting of tissues and performing some specific function in an organism
Orgasm: Rhythmic muscular contractions of the genitals (sex organs) combined with waves of intense pleasurab
Osmoconformers: Marine organisms that have no system of osmoregulation and must change the composition of their body
Osmoregulation: The regulation of the movement of water by osmosis into and out of cells; the maintenance of water b
Osmoregulators: Marine vertebrates whose body fiuids have about one-third the solute concentration of seawater; must
Osmosis: Diffusion of water molecules across a membrane in response to differences in solute concentration. W
Osmotic Pressure: Pressure generated by water moving by osmosis into or out of a cell.
Ossification: The process by which embryonic cartilage is replaced with bone.
Osteoarthritis: A degenerative condition associated with the wearing away of the protective cap of cartilage at the
Osteoblasts: Bone-forming cells.
Osteoclasts: Cells that remove material to form the central cavity in a long bone.
Osteocytes: Bone cells that lay down new bone; found in the concentric layers of compact bone. Bone cell, a type
Osteoporosis: A disorder in which the mineral portion of bone is lost, making the bone weak and brittle; occurs mo
Out Of Africa Hypothesis: Holds that modern human populations (Homo sapiens) are all derived from a single speciation event th
Ovaries: 1) In animals, the female gonads, which produce eggs (ova) and female sex hormones. 2) In fiowers, p
Overkill: The shooting, trapping, or poisoning of certain populations, usually for sport or economic reasons.
Oviducts: Tubes that connect the ovaries and the uterus; transport sperm to the ova, transport the fertilized
Ovulation: The release of the oocyte onto the surface of the ovary; occurs at the midpoint of the ovarian cycle
Ovule: In seed plants, a protective structure in which the female gametophyte develops, fertilization occur
Ovum: The female gamete, egg.
Oxidation: The loss of electrons from the outer shell of an atom; often accompanied by the transfer of a proton
Oxytocin: A peptide hormone secreted by the posterior pituitary that stimulates the contraction of the uterus
Ozone: A triatomic (O3) form of oxygen that is formed in the stratosphere when sunlight strikes oxygen atom
Pacinian Corpuscles: Sensory receptors located deep in the epidermis that detect pressure and vibration.
Paleontology: The study of ancient life by collection and analysis of fossils.
Paleozoic Era: The period of time beginning 570 million years ago ending 245 million years ago; falls between the P
Palindrome: A sequence that reads the same in either direction; in genetics, refers to an enzyme recognition seq
Palisade: Layer of mesophyll cells in leaves that are closely placed together under the epidermal layer of the
Palynology: The study of palynomorphs and other acid-resistant microfossils usually produced by plants, protists
Palynomorph: Generic term for any object a palynologist studies.
Pancreas: A gland in the abdominal cavity that secretes digestive enzymes into the small intestine and also se
Pancreatic Islets: Clusters of endocrine cells in the pancreas that secrete insulin and glucagon; also known as islets
Pangaea: The name proposed by German meteorologist Alfred Wegener for a supercontinent that existed at the en
Parallel Evolution: The development of similar characteristics in organisms that are not closely related (not part of a
Parasites: Organisms that live in, with, or on another organism. The parasites benefit from the association wit
Parasitism: A form of symbiosis in which the population of one species benefits at the expense of the population
Parasympathetic System: The subdivision of the autonomic nervous system that reverses the effects of the sympathetic nervous
Parenchyma: One of the three major cell types in plants. Parenchyma cells have thin, usually multisided walls, a
Parietal Lobe: The lobe of the cerebral cortex that lies at the top of the brain; processes information about touch
Passive Transport: Diffusion across a plasma membrane in which the cell expends no energy.
Pectin: A substance in the middle lamella that cements adjoining plant cells together.
Pectoral Girdle: In humans, the bony arch by which the arms are attached to the rest of the skeleton; composed of the
Pedigree Analysis: A type of genetic analysis in which a trait is traced through several generations of a family to det
Pelagic Zone: One of the two basic subdivisions of the marine biome; consists of the water above the sea fioor and
Pelvic Girdle: In humans, the bony arch by which the legs are attached to the rest of the skeleton; composed of the
Pelvis: The hollow cavity formed by the two hipbones.
Penicillin: The first of the so-called wonder drugs; discovered by Sir Alexander Fleming.
Pepsin: An enzyme produced from pepsinogen that initiates protein digestion by breaking down protein into la
Pepsinogen: An inactive form of pepsin; synthesized and stored in cells lining the gastric pits of the stomach.
Peptic Ulcer: Damage to the epithelial layer of the stomach lining; generally caused by bacterial infection.
Peptide Bond: A covalent bond that links two amino acids together to form a polypeptide chain. A covalent bond bet
Peptides: Short chains of amino acids.
Perennials: Plants that persist in the environment for more than one year (as in the case of annuals).
Perichondrium: A layer of connective tissue that forms around the cartilage during bone formation. Cells in the per
Period: The fundamental unit in the hierarchy of time units; a part of geologic time during which a particul
Periosteum: A fibrous membrane that covers bones and serves as the site of attachment for skeletal muscles; cont
Peripheral Nervous System: The division of the nervous system that connects the central nervous system to other parts of the bo
Peristalsis: Involuntary contractions of the smooth muscles in the walls of the esophagus, stomach, and intestine
Permian Period: The last geologic time period of the Paleozoic Era, noted for the greatest mass extinction in earth
Peroxisomes: Membrane-bound vesicles in eukaryotic cells that contain oxidative enzymes.
Pesticides: Chemicals that are applied to agricultural crops or domesticated plants and which kill or inhibit gr
Petals: Usually brightly colored elements of a fiower that may produce fragrant oils; nonreproductive struct
Petiole: The generally non-leafy part of the leaf that attaches the leaf blade to the stem; celery and rhubar
Petrifaction: Mode of fossilization where f organic matter is replaced with silica.
PH: The negative logarithm of the H+ ion concentration. The pH is a measure of the acidity or basic char
Phagocytes: White blood cells that can engulf (by phagocytosis) and destroy microorganisms including viruses and
Phagocytosis: A form of endocytosis in which white blood cells surround and engulf invading bacteria or viruses.
Pharynx: The passageway between the mouth and the esophagus and trachea. Food passes from the pharynx to the
Phenotype: The observed properties or outward appearance of a trait. The physical expression of the alleles pos
Pheromones: Chemical signals that travel between organisms rather than between cells within an organism; serve a
Phloem: Tissue in the vascular system of plants that moves dissolved sugars and other products of photosynth
Phosphate Group: A chemical group composed of a central phosphorous bonded to three or four oxygens. The net charge o
Phosphoglyceraldehyde (PGAL): A substance formed from PGA during the dark reaction of photosynthesis. Some PGAL leaves the cycle
Phosphoglycerate (PGA): A three-carbon molecule formed when carbon dioxide is added to ribulose biphosphate (RuBP) during th
Phospholipids: Asymmetrical lipid molecules with a hydrophilic head and a hydrophobic tail. Lipids with a phosphate
Phosphorylation: The chemical attachment of phosphorous to a molecule, usually associated with the storage of energy
Photic Zone: The layer of the ocean that is penetrated by sunlight; extends to a depth of about 200 meters.
Photoperiodism: The ability of certain plants to sense the relative amounts of light and dark in a 24-hour period; c
Photosynthesis: The process by which plant cells use solar energy to produce ATP. The conversion of unusable sunligh
Photosystems: Clusters of several hundred molecules of chlorophyll in a thylakoid in which photosynthesis takes pl
Phototrophs: Organisms that use sunlight to synthesize organic nutrients as their energy source; e.g., cyanobacte
Phototropism: The reaction of plants to light in which the plants bend toward the light. Plant response to light b
Phycocyanin: An accessory pigment found in cyanobacteria and the chloroplasts of red algae.
Phycoerythrin: An accessory pigment found in cyanobacteria and the chloroplasts of red algae.
Phylogenetic: Pertaining to a phylogeny.
Phylogeny: 1) the study of evolutionary relationships within a monophyletic group. 2) evolutionary hypotheses r
Phylum: The broadest taxonomic category within kingdoms (pl.: phyla).
Phytochrome: A pigment in plant leaves that detects day length and generates a response; partly responsible for p
Phytoplankton: A fioating layer of photosynthetic organisms, including algae, that are an important source of atmos
Pilus: Projection from surface of a bacterial cell (F+) that can donate genetic material to another (F-).
Pineal Gland: A small gland located between the cerebral hemispheres of the brain that secretes melatonin.
Pioneer Community: The initial community of colonizing species.
Pistil: Female reproductive structures in flowers, consisting of the stigma, style, and ovary. Also known as
Pith: Central area in plant stems, largely composed of parenchyma tissue modified for storage.
Pituitary Gland: A small gland located at the base of the brain; consists of an anterior and a posterior lobe and pro
Placenta: An organ produced from interlocking maternal and embryonic tissue in placental mammals; supplies nut
Placental Mammals: One of three groups of mammals that carry their young in the mothers body for long periods during wh
Planaria: Small free-living fiatworms (Phylum Platyhelminthes) with bilateral symmetry and cephalization. The
Planktonic Organisms: Floaters; one of the two main types of organisms in the pelagic zone of the marine biome.
Plantae: The plant kingdom; nonmobile, autotrophic, multicellular eukaryotes. Kingdom of the plants, autotrop
Plasma: The liquid portion of the blood. Along with the extracellular fiuid, it makes up the internal enviro
Plasma Cells: Cells produced from B cells that synthesize and release antibodies.
Plasmids: Self-replicating, circular DNA molecules found in bacterial cells; often used as vectors in recombin
Plasmodesmata: Junctions in plants that penetrate cell walls and plasma membranes, allowing direct communication be
Plasmolysis: Osmotic condition in which a cell loses water to its outside environment.
Plastids: Membrane-bound organelles in plant cells that function in storage (of food or pigments) or food prod
Plate Tectonics: The movement of the plates that make up the surface of the Earth. The revolutionary paradigm in geol
Platelets: In vertebrates, cell fragments that bud off from the megakaryocytes in the bone marrow; carry chemic
Pleiotropic: A term describing a genotype with multiple phenotypic effects. For example: sickle-cell anemia produ
Pleistocene: The first geologic epoch of the Quaternary Period of the Cenozoic Era that ended 10,000 years ago wi
Pleura: A thin sheet of epithelium that covers the inside of the thoracic cavity and the outer surface of th
Pleural Cavity: The space between the sheets of pleura (one covering the inside of the thoracic cavity, the other co
Polar Covalent Bond: A covalent bond in which atoms share electrons in an unequal fashion. The resulting molecule has reg
Pollen Grains: The containers for male gametophytes of seed plants produced in a microsporangium by meiosis. Micros
Pollen Tube: Structure produced by the tube nucleus in the pollen grain through which the sperm nucleus (or nucle
Pollination: The transfer of pollen from the anthers to the stigma by a pollinating agent such as wind, insects,
Polygenic Inheritance: Occurs when a trait is controlled by several gene pairs; usually results in continuous variation.
Polymer: Organic molecule composed of smaller units known as monomers. A large molecule composed of smaller s
Polymerase Chain Reaction (PCR): A method of amplifying or copying DNA fragments that is faster than cloning. The fragments are combi
Polynucleotides: Long chains of nucleotides formed by chemical links between the sugar and phosphate groups.
Polyp: The sessile form of life history in cnidarians; e.g., the freshwater hydra.
Polyploidy: Abnormal variation in the number of chromosome sets. The condition when a cell or organism has more
Polysaccharides: Long chains of monosaccharide units bonded together; e.g., glycogen, starch, and cellulose.
Pons: The region that, with the medulla oblongata, makes up the hindbrain, which controls heart rate, cons
Population: A group of individuals of the same species living in the same area at the same time and sharing a co
Population Dynamics: The study of the factors that affect the growth, stability, and decline of populations, as well as t
Portal System: An arrangement in which capillaries drain into a vein that opens into another capillary network.
Positive Feedback: Biochemical control where the accumulation of the product stimulates production of an enzyme respons
Positive Feedback Control: Occurs when information produced by the feedback increases and accelerates the response.
Precambrian: Informal term describing 7/8 of geologic time from the beginning of the earth to the beginning of th
Precipitation: The part of the hydrologic cycle in which the water vapor in the atmosphere falls to Earth as rain o
Predation: One of the biological interactions that can limit population growth; occurs when organisms kill and
Predatory Release: Occurs when a predator species is removed from a prey species such as by great reduction in the pred
Prehensile Movement: The ability to seize or grasp.
Prenatal Testing: Testing to detect the presence of a genetic disorder in an embryo or fetus; commonly done by amnioce
Presymptomatic Screening: Testing to detect genetic disorders that only become apparent later in life. The tests are done befo
Prey Switching: The tendency of predators to switch to a more readily available prey when one prey species becomes r
Primary Body: Those parts of a plant produced by the shoot and root apical meristems.
Primary Cell Wall: The cell wall outside the plasma membrane that surrounds plant cells; composed of the polysaccharide
Primary Compounds: Chemicals made by plants and needed for the plants own metabolism.
Primary Growth: Cells produced by an apical meristem. The growth a plant by the actions of apical meristems on the s
Primary Macronutrients: Elements that plants require in relatively large quantities: nitrogen, phosphorus, and potassium.
Primary Meristems: The apical meristems on the shoot and root apices in plants that produce plant primary tissues.
Primary Root: The first root formed by a plant.
Primary Structure: The sequence of amino acids in a protein.
Primates: The taxonomic order of mammals that includes prosimians (lemurs and tarsiers), monkeys, apes, and hu
Principle Of Independent Assortment: Mendels second law; holds that during gamete formation, alleles in one gene pair segregate into game
Principle Of Segregation: Mendels first law; holds that each pair of factors of heredity separate during gamete formation so t
Prions: Infectious agents composed only of one or more protein molecules without any accompanying genetic in
Producers: The first level in a food pyramid; consist of organisms that generate the food used by all other org
Progesterone: One of the two female reproductive hormones secreted by the ovaries.
Prokaryote: Type of cell that lacks a membrane-bound nucleus and has no membrane organelles; a bacterium. Prokar
Prolactin: A hormone produced by the anterior pituitary; secreted at the end of pregnancy when it activates mil
Promoter: The specific nucleotide sequence in DNA that marks the beginning of a gene.
Prophase: 1) The first stage of mitosis during which chromosomes condense, the nuclear envelope disappears, an
Prostaglandins: A class of fatty acids that has many of the properties of hormones; synthesized and secreted by many
Prostate Gland: A gland that is located near and empties into the urethra; produces a secretion that enhances sperm
Proteinoids: Polymers of amino acids formed spontaneously from inorganic molecules; have enzyme-like properties a
Proteins: Polymers made up of amino acids that perform a wide variety of cellular functions. One of the classe
Prothallus: In ferns, a small heart-shaped bisexual gametophyte.
Protista: The taxonomic Kingdom from which the other three eukaryotic kingdoms (Fungi, Animalia and Plantae) a
Protists: Single-celled organisms; a type of eukaryote. Protista
Proton: A subatomic particle in the nucleus of an atom that carries a positive charge. The positively charge
Protostomes: Animals in which the first opening that appears in the embryo becomes the mouth; e.g., mollusks, ann
Protozoa: Single-celled protists grouped by their method of locomotion. This group includes Paramecium, Amoeba
Proximal Tubule: The winding section of the renal tubule where most reabsorption of water, sodium, amino acids, and s
Pseudocoelom: In nematodes, a closed fiuid-containing cavity that acts as a hydrostatic skeleton to maintain body
Pseudocoelomates: Animals that have a body cavity that is in direct contact with the outer muscular layer of the body
Pseudopodia: Temporary cytoplasmic extensions from a cell that enables it to move (sing.: pseudopodium).
Pulmonary Artery: The artery that carries blood from the right ventricle of the vertebrate heart to the lungs. Artery
Pulmonary Circuit: The loop of the circulatory system that carries blood to and from the lungs.
Pulmonary Vein: The vein that carries oxygenated blood from the lungs to the left atrium of the heart. Veins carryin
Punctuated Equilibrium: A model that holds that the evolutionary process is characterized by long periods with little or no
Purine: One of the groups of nitrogenous bases that are part of a nucleotide. Purines are adenine and guanin
Pyloric Sphincter: The ring of muscle at the junction of the stomach and small intestine that regulates the movement of
Pyrimidine: One of the groups of nitrogenous bases that are part of a nucleotide. Pyrimidines are single ringed,
Quantum Models Of Speciation: Models of evolution that hold that speciation sometimes occurs rapidly as well as over long periods,
Quaternary Period: The most recent geologic period of the Cenozoic Era, the Quaternary began 2 million years ago with t
Quaternary Structure: In some proteins, a fourth structural level created by interactions with other proteins. Aspect of p
Race: A subdivision of a species that is capable of interbreeding with other members of the species.
Radially Symmetrical: In animals, refers to organisms with their body parts arranged around a central axis. Such animals t
Radiation: Energy emitted from the unstable nuclei of radioactive isotopes.
Radioactive Decay: The spontaneous decay of an atom to an atom of a different element by emission of a particle from it
Radioisotope: Term applied to a radioactive isotope, such as carbon-14 or uranium 238. Radioisotope nuclei are uns
Radiometric Time: Type of absolute time determined by the relative porportions of radioisotopes to stable daughter iso
Ray-Finned: Taxonomic group of fish, such as trout, tuna, salmon, and bass, that have thin, bony supports holdin
Reabsorption: The return to the blood of most of the water, sodium, amino acids, and sugar that were removed durin
Receptacle: The base that attaches a fiower to the stem.
Receptor: Protein on or protruding from the cell surface to which select chemicals can bind. The opiate recept
Recessive: Refers to an allele of a gene that is expressed when the dominant allele is not present. An allele e
Recombinant Dna Molecules: New combinations of DNA fragments formed by cutting DNA segments from two sources with restriction e
Recombinant Dna Technology: A series of techniques in which DNA fragments are linked to self-replicating forms of DNA to create
Recombination: A way in which meiosis produces new combinations of genetic information. During synapsis, chromatids
Red Algae: Common name for the algae placed in the division Rhodophyta.
Red Blood Cell: Component of the blood that transports oxygen with the hemoglobin molecule. See also erythrocyte
Red Tides: Phenomenon associated with population explosions (blooms) of certain types of dinoflagellates; red s
Reduction: The gain of an electron or a hydrogen atom. The gain of electrons or hydrogens in a chemical reactio
Reductional Division: The first division in meiosis; results in each daughter cell receiving one member of each pair of ch
Reflex: A response to a stimulus that occurs without conscious effort; one of the simplest forms of behavior
Reflex Arc: Pathway of neurons, effector(s) and sensory receptors that participate in a reflex.
Region Of Division: The area of cell division in the tip of a plant root.
Region Of Elongation: The area in the tip of a plant root where cells grow by elongating, thereby increasing the length of
Region Of Maturation: The area where primary tissues and root hairs develop in the tip of a plant root.
Relative Time: Type of geologic time (absolute time being the other) that places events in a sequence relative to e
Renal Tubule: The portion of the nephron where urine is produced.
Renin: An enzyme secreted by the kidneys that converts angiotensinogen into angiotensin II.
Replication: Process by which DNA is duplicated prior to cell division.
Reproductive Isolating Mechanism: Biological or behavioral characteristics that reduce or prevent interbreeding with other populations
Reproductive System: One of eleven major body organ systems in animals; is responsible for reproduction and thus the surv
Reptiles: Taxonomic class of vertebrates characterized by scales and amniotic eggs; the first truly terrestria
Resolution: In relation to microscopes, the ability to view adjacent objects as distinct structures.
Resource Partitioning: The division of resources such that a few dominant species exploit most of the available resources w
Respiration: 1) breathing as part of gas exchange; or 2) cellular metabolism.
Respiratory Surface: A thin, moist, epithelial surface that oxygen can cross to move into the body and carbon dioxide can
Respiratory System: One of eleven major body organ systems in animals; moves oxygen from the external environment into t
Resting Potential: The difference in electrical charge across the plasma membrane of a neuron.
Restriction Enzymes: A series of enzymes that attach to DNA molecules at specific nucleotide sequences and cut both stran
Restriction Fragment Length Polymorphism (RFLP): A heritable difference in DNA fragment length and fragment number; passed from generation to generat
Retina: The inner, light-sensitive layer of the eye; includes the rods and cones.
Retroviruses: Viruses that contain a single strand of RNA as their genetic material and reproduce by copying the R
Reverse Transcriptase: An enzyme used in the replication of retroviruses; aids in copying the retroviruss RNA into a comple
Reverse Transcription: Process of transcribing a single-stranded DNA from a single-stranded RNA (the reverse of transcripti
Rheumatoid Arthritis: A crippling form of arthritis that begins with infiammation and thickening of the synovial membrane,
Rhizoids: Filamentous structures in the plants group known as bryophytes that attach to a substrate and absorb
Rhizome: In ferns, a horizontal stem with upright leaves containing vascular tissue.
Rhodopsin: A visual pigment contained in the rods of the retina in the eye..
Ribonucleic Acid (RNA): Nucleic acid containing ribose sugar and the base Uracil; RNA functions in protein synthesis. The si
Ribosomal Subunits: Two units that combine with mRNA to form the ribosomal-mRNA complex at which protein synthesis occur
Ribosomes: Small organelles made of rRNA and protein in the cytoplasm of prokaryotic and eukaryotic cells; aid
Rna Polymerase: During transcription, an enzyme that attaches to the promoter region of the DNA template, joins nucl
Rna Transcript: Term applied to RNA transcribed in the nucleus.
Rodinia: Name applied to the precambrian supercontinent.
Rods: Light receptors in primates eyes that provide vision in dim light.
Root Cap: Structure that covers and protects the apical meristem in plant roots. Cells forming a protective se
Root Hairs: Extensions of the root epidermis that increase the roots ability to absorb water.
Root System: Plant organ systems that anchors the plant in place, stores excess sugars, and absorbs water and min
Root-Leaf-Vascular System Axis: Refers to the arrangement in vascular plants in which the roots anchor the plant and absorb water an
Roots: Organs, usually occurring underground, that absorb nutrients and water and anchor the plant; one of
Rubp: Ribulose biphosphate; the 5-carbon chemical that combines with carbon dioxide at the beginning of th
S Phase: That period of interphase when new DNA is synthesized as part of replication of the chromatin.
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
Sapwood: Layers of secondary xylem that are still functional in older woody plants; visible as the outer ligh
Sarcomeres: The functional units of skeletal muscle; consist of filaments of myosin and actin.
Saturated Fat: A fat with single covalent bonds between the carbons of its fatty acids.
Schwann Cells: Specialized glial cells that form the myelin sheath that coats many axons. Cells surrounding the axo
Scientific Method: Systematic apporach of observation, hypothesis formation, hypothesis testing and hypothesis evaluati
Sclereids: Plant cells with thick secondary walls that provide the gritty textures in pears.
Sclerenchyma: One of the three major cell types in plants; have thickened, rigid, secondary walls that are hardene
Scrotum: In mammals, a pouch of skin located outside the body cavity into which the testes descend; provides
Second Law Of Thermodynamics (Entropy): The energy available after a chemical reaction is less than that at the beginning of a reaction; ene
Second Messenger: The mechanism by which nonsteroid hormones work on target cells. A hormone binds to receptors on the
Secondary (Lateral) Meristems: Plant meristems that produce secondary growth from a cambium.
Secondary Cell Wall: In woody plants, a second wall inside the primary cell wall; contains alternating layers of cellulos
Secondary Compounds: Plant products that are not important in metabolism but serve other purposes, such as attracting ani
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 l
Secondary Immunity: Resistance to an antigen the second time it appears. Because of the presence of B and T memory cells
Secondary Macronutrients: Elements that plants require in relatively small quantities: calcium, magnesium, and sulfur.
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;
Secondary Xylem: Xylem produced by the vascular cambium in a woody plant stem or root; wood.
Secretin: A hormone produced in the duodenum that stimulates alkaline secretions by the pancreas and inhibits
Secretion: The release of a substance in response to the presence of food or specific neural or hormonal stimul
Sediment: Loose aggregate of solids derived from preexisting rocks, or solids precipitated from solution by in
Sedimentary Rock: Any rock composed of sediment, i.e., solid particles and dissolved minerals. Examples include rocks
Seed: Structure produced by some plants in which the next generation sporophyte is surrounded by gametophy
Seed Coat: The tough outer layer of the seed, derived from the outer layers of the ovule.
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 chrom
Selective Breeding: The selection of individuals with desirable traits for use in breeding. Over many generations, the p
Selectively Permeable: Term describing a barrier that allows some chemicals to pass but not others. The cell membrane is su
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 fo
Semilunar Valve: A valve between each ventricle of the heart and the artery connected to that ventricle.
Seminal Vesicles: Glands that contribute fructose to sperm. The fructose serves as an energy source. The structures th
Seminiferous Tubules: Tubules on the interior of the testes where sperm are produced.
Sensor: In a closed system, the element that detects change and signals the effector to initiate a response.
Sensory (Afferent) Pathways: The portion of the peripheral nervous system that carries information from the organs and tissues of
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 pressur
Sensory Neurons: Neurons that carry signals from receptors and transmit information about the environment to processi
Sepals: Modified leaves that protect a flowers inner petals and reproductive structures. Small, leaf-like st
Separation: Splitting of the cytoplasm by cytokinesis (= cytokinesis).
Severe Combined Immunodeficiency (SCID): A genetic disorder in which affiicted individuals have no functional immune system and are prone to
Sex Chromosomes: The chromosomes that determine the sex of an organism. In humans, females have two X chromosomes, an
Sex Hormones: A group of steroid hormones produced by the adrenal cortex. Hormones that are produced in the gonads
Sex Linkage: The condition in which the inheritance of a sex chromosome is coupled with that of a given gene; e.g
Sexual Reproduction: A system of reproduction in which two haploid sex cells (gametes) fuse to produce a diploid zygote.
Shoot: The plant stem; provides support for the leaves and fiowers; one of the three major plant organs; al
Short-Day Plants: Plants that fiower during early spring or fall when nights are relatively long and days are short; e
Sickle Cell Anemia: Human autosomal recessive disease that causes production of abnormal red blood cells that collapse (
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
Sieve Plates: Pores in the end walls of sieve elements that connect the sieve elements together. The end walls of
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 millio
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 a
Sinoatrial (SA) Node: A region of modified muscle cells in the right atrium that sends timed impulses to the hearts other
Sister Chromatids: Chromatids joined by a common centromere and carrying identical genetic information (unless crossing
Skeletal Muscle: Muscle that is generally attached to the skeleton and causes body parts to move; consists of muscle
Skeletal System: One of eleven major body organ systems in animals; supports the body, protects internal organs, and,
Skin: One of eleven major body organ systems in animals; the outermost layer protecting multicellular anim
Sleep Movement: In legumes, the movement of the leaves in response to daily rhythms of dark and light. The leaves ar
Sliding Filament Model: Model of muscular contraction in which the actin filaments in the sarcomere slide past the myosin fi
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
Smog: A local alteration in the atmosphere caused by human activity; mainly an urban problem that is often
Smooth Muscle: Muscle that lacks striations; found around circulatory system vessels and in the walls of such organ
Social Behavior: Behavior that takes place in a social context and results from the interaction between and among ind
Societies: The most highly organized type of social organization; consist of individuals that show varying degr
Sodium-Potassium Pump: The mechanism that uses ATP energy to reset the sodium and potassium ions after transmission of a ne
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 organisms body.
Somatic Cell: A cell that is not or will not become a gamete; the cells of the body.
Somatic Nervous System: The portion of the peripheral nervous system consisting of the motor neuron pathways that innervate
Somatic Senses: All senses except vision, hearing, taste, and smell; include pain, temperature, and pressure.
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 suc
Special Senses: Vision, hearing, taste, and smell.
Species: One or more populations of interbreeding or potentially interbreeding organisms that are reproductiv
Species Diversity: The number of living species on Earth.
Species Packing: The phenomenon in which present-day communities generally contain more species than earlier communit
Species Richness: The number of species present in a community.
Sperm: The male gamete.
Spermatogenesis: The development of sperm cells from spermatocytes to mature sperm, including meiosis.
Spicules: Needle-shaped skeletal elements in sponges that occur in the matrix between the epidermal and collar
Spinal Cord: A cylinder of nerve tissue extending from the brain stem; receives sensory information and sends out
Spindle Apparatus: Microtubule construction that aligns and segregates chromosomes during eukaryotic cell division.
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 s
Spongy Mesophyll: Parenchyma cells found in plant leaves that are irregularly shaped and have large intracellular spac
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 envi
Sporophyte: The diploid stage of a plant exhibiting alternation of generations. The diploid, spore producing pha
Sporozoans: Members of the protists that are referred to as slime molds; may include organisms resembling the an
Stability: One of the phases of a populations life cycle. The populations size remains roughly constant, fiuctu
Stabilizing Selection: A process of natural selection that tends to favor genotypic combinations that produce an intermedia
Stalk: A leafs petiole; the slender stem that supports the blade of a leaf and attaches it to a larger stem
Stamens: The male reproductive structures of a fiower; usually consist of slender, thread-like filaments topp
Stapes: One of the three bones that function in hearing.
Start Codon: The codon (AUG) on a messenger RNA molecule where protein synthesis begins.
Steinkerns: Internal casts of a fossil. Steinkerns may reveal internal anatomy of an organism, such as muscle at
Stem Cells: Cells in bone marrow that produce lymphocytes by mitotic division.
Sternum: The breastbone.
Steroids: Compounds with a skeleton of four rings of carbon to which various side groups are attached; one of
Sticky Ends: Term applied to DNA sequences cut with restriction enzymes where the cuts will bond with each other
Stigma: Part of the female reproductive structure of the carpel of a fiower; the sticky surface at the tip o
Stimulus: A physical or chemical change in the environment that leads to a response controlled by the nervous
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 an
Stomata: Pores on the underside of leaves that can be opened or closed to control gas exchange and water loss
Stomatal Apparatus: The stomata and guard cells that control the size of the stoma.
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.
Stressed Community: A community that is disturbed by human activity, such as road building or pollution, and is inadvert
Stroma: The matrix surrounding the grana in the inner membrane of chloroplasts. The area between membranes (
Stromatolite: A sedimentological and biological fossil representinmg colonies of bacteria altenating with layers o
Style: Part of the female reproductive structure in the carpel of a fiower; formed from the ovary wall. The
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 root
Subspecies: A taxonomic subdivision of a species; a population of a particular region genetically distinguishabl
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 tw
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 cell
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,
Sympathetic System: The subdivision of the autonomic nervous system that dominates in stressful or emergency situations
Synapse: The junction between an axon and an adjacent neuron.
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.
Synergid: Cells in the embryo sac of angiosperms that flank the egg cell. The pollen tube grows through one (u
Synovial Joint: The most movable type of joint. The bones are covered by connective tissue, the interior of which is
Syphilis: A sexually transmitted disease caused by a bacterial infection that produces an ulcer on the genital
Systematics: The classification of organisms based on information from observations and experiments; includes the
Systemic Circuit: The loop of the circulatory system that carries blood through the body and back to the heart.
Systole: The contraction of the ventricles that opens the semilunar valve and forces blood into the arteries.
Systolic Pressure: The peak blood pressure when ventricles contract.
T Cells: The type of lymphocyte responsible for cell-mediated immunity; also protects against infection by pa
Taiga Biome: The region of coniferous forest extending across much of northern Europe, Asia, and North America; c
Tap Root: A primary root that grows vertically downward and gives off small lateral roots; occurs in dicots. R
Target Cell: A cell that a particular hormone effects by its direct action (either passing through the membrane o
Tarsals: The bones that make up the ankle joint.
Taxis: The behavior when an animal turns and moves toward or away from an external stimulus (pl.: taxes).
Taxon: Term applied group of organisms comprising a given taxonomic category
Taxonomy: A systematic method of classifying plants and animals. Classification of organisms based on degrees
Tectonic Plates: Segments of the lithosphere that comprise the surface of the Earth much the way a turtle shell is co
Telophase: The final stage of mitosis in which the chromosomes migrate to opposite poles, a new nuclear envelop
Temperate Forest Biome: Extends across regions of the northern hemisphere with abundant rainfall and long growing seasons. D
Template Strand: The strand of DNA that is transcribed to make RNA.
Temporal Lobe: The lobe of the cerebral cortex that is responsible for processing auditory signals.
Tendons: Bundles of connective tissue that link muscle to bone. Fibrous connective tissue that connects muscl
Terminal Buds: Buds located at the end of a plant shoot.
Termination: The end of translation; occurs when the ribosome reaches the stop codon on the messenger RNA molecul
Termination Codon: One of three three-base sequences that initiate termination of the protein synthesis process. See st
Tertiary Structure: The folding of a proteins secondary structure into a functional three-dimensional configuration. Sha
Test Cross: Genetic crossing of an organism with known genotype (one that exhibits the recessive phenotype) with
Testes: The male gonad; produce spermatozoa and male sex hormones. Male gonads in mammals. Singular, testis.
Testosterone: Male sex hormone that stimulates sperm formation, promotes the development of the male duct system i
Tetrad: The four chromatids in each cluster during synapsis; formed by the two sister chromatids in each of
Thalamus: The brain region that serves as a switching center for sensory signals passing from the brain stem t
Thecodonts: An informal term for a variety of Permian and Triassic reptiles that had teeth set in individual soc
Theory: A hypothesis that has withstood extensive testing by a variety of methods, and in which a higher deg
Thermacidophiles: A group of archaebacteria that are able to tolerate high temperatures and acidic pH.
Thermiogenesis: The generation of heat by raising the bodys metabolic rate; controlled by the hypothalamus.
Thermoregulation: The regulation of body temperature.
Thigmotropism: Plants response to contact with a solid object; e.g., tendrils twining around a pole. Plant response
Thoracic Cavity: The chest cavity in which the heart and lungs are located.
Thorax: In many arthropods, one of three regions formed by the fusion of the segments (others are the head a
Thorns: Stems modified to protect the plant.
Thoroughfare Channels: Shortcuts within the capillary network that allow blood to bypass a capillary bed.
Thylakoids: The specialized membrane structures in which photosynthesis takes place. Internal membranes in the c
Thymine: One of the pyrimidine bases in DNA, thymine is replaced by uracil in RNA.
Thyroid-Stimulating Hormone: A hormone produced by the anterior pituitary that stimulates the production and release of thyroid h
Tight Junctions: Junctions between the plasma membranes of adjacent cells in animals that form a barrier, preventing
Tissues: Groups of similar cells organized to carry out one or more specific functions. Groups of cells perfo
Toxins: Term applied to poisons in living systems.
Trace Fossil: Any indication of prehistoric organic activity, such as tracks, trails, burrows, or nests.
Trachea: In insects and spiders, a series of tubes that carry air directly to cells for gas exchange; in huma
Tracheids: Long, tapered cells with pitted walls that form a system of tubes in the xylem and carry water and s
Transcription: The synthesis of RNA from a DNA template. The making of RNA from one strand of the DNA molecule.
Transfer Rnas (TRNAS): Small, single-stranded RNA molecules that bind to amino acids and deliver them to the proper codon o
Transformation: In Griffiths experiments with strains of pneumonia bacterium, the process by which hereditary inform
Transforming Factor: Griffiths name for the unknown material leading to transformation; later found to be DNA.
Transition Reaction: Biochemical process of converting 3-carbon pyruvate into 2-carbon acetyl and attaching it to coenzym
Translation: The synthesis of protein on a template of messenger RNA; consists of three steps: initiation, elonga
Translocation: 1) The movement of a segment from one chromosome to another without altering the number of chromosom
Transpiration: The loss of water molecules from the leaves of a plant; creates an osmotic gradient; producing tensi
Triassic Period: The first period of the Mesozoic Era between 225 and 185 million years ago. Pangaea began to breakup
Trichocysts: Barbed, thread-like organelles of ciliated protozoans that can be discharged for defense or to captu
Trichomes: Extensions from the epidermis of the plant that provide shade and protection for the plant.
Trilobites: A group of benthonic, detritus-feeding, extinct marine invertebrate animals (phylum Arthropoda), hav
Triplet: Three-base sequence of mRNA that codes for a specific amino acid or termination codon.
Trisomy: A condition where a cell has an extra chromosome.
Trophoblast: The outer layer of cells of a blastocyst that adhere to the endometrium during implantation.
Tropic Hormone: Hormone made by one gland that causes another gland to secrete a hormone.
Tropical Rain Forest Biome: The most complex and diverse biome; found near the equator in South America and Africa; characterize
Tropism: The movement of plant parts toward or away from a stimulus in the plants environment. Plant movement
True-Breeding: Occurs when self-fertilization gives rise to the same traits in all offspring, generation after gene
Trypanosomes: A type of roundworm, responsible for human disease associated with eating raw or undercooked pork.
Tubal Ligation: A contraceptive procedure in women in which the oviducts are cut, preventing the ova from reaching t
Tubal Pregnancy: Occurs when the morula remains in the oviduct and does not descend into the uterus.
Tube Nucleus: One of the cells in the male gametophyte in seed plants. The tube nucleus grows through the stigma,
Tube-Within-A-Tube System: A type of body plan in animals. The organism has two openings&emdash;one for food and one for the el
Tubers: Swollen underground stems in plants that store food, such as the irish potato.
Tubular Secretion: The process in which ions and other waste products are transported into the distal tubules of the ne
Tubulins: The protein subunits from which microtubules are assembled.
Tumor Suppressor Genes: Genes that normally keep cell division under control, preventing the cell from responding to interna
Tundra Biome: Extensive treeless plain across northern Europe, Asia, and North American between the taiga to the s
Turgor Pressure: Pressure caused by the cytoplasm pressing against the cell wall.
Turner Syndrome: In humans, a genetically determined condition in which an individual has only one sex chromosome (an
Umbilical Cord: The structure that connects the placenta and the embryo; contains the umbilical arteries and the umb
Unicellular: Single-celled.
Uniformitarianism: The idea that geological processes have remained uniform over time and that slight changes over long
Uninucleate: Term applied to cells having only a single nucleus.
Unsaturated Fat: A triglyceride with double coavent bonds between some carbon atoms.
Uracil: The pyrimidine that replaces thymine in RNA molecules and nucleotides.
Ureter: A muscular tube that transports urine by peristaltic contractions from the kidney to the bladder.
Urethra: A narrow tube that transports urine from the bladder to the outside of the body. In males, it also c
Urine: Fluid containing various wastes that is produced in the kidney and excreted from the bladder.
Uterus: The organ that houses and nourishes the developing embryo and fetus. The womb. Female reproductive o
Vaccination: The process of protecting against infectious disease by introducing into the body a vaccine that sti
Vaccine: A preparation containing dead or weakened pathogens that when injected into the body elicit an immun
Vacuoles: Membrane-bound fiuid-filled spaces in plant and animal cells that remove waste products and store in
Vagina: The tubular organ that is the site of sperm deposition and also serves as the birth canal.
Vas Deferens: The duct that carries sperm from the epididymis to the ejaculatory duct and urethra. The tube connec
Vascular Bundle: Groups of xylem, phloem and cambium cells in stems of plants descended from the procambium embryonic
Vascular Cambium: A layer of lateral meristematic tissue between the xylem and phloem in the stems of woody plants. La
Vascular Cylinder: A central column formed by the vascular tissue of a plant root; surrounded by parenchymal ground tis
Vascular Parenchyma: Specialized parenchyma cells in the phloem of plants.
Vascular Plants: Group of plants having lignified conducting tissue (xylem vessels or tracheids).
Vascular System: Specialized tissues for transporting fiuids and nutrients in plants; also plays a role in supporting
Vasectomy: A contraceptive procedure in men in which the vas deferens is cut and the cut ends are sealed to pre
Vectors: Self-replicating DNA molecules that can be joined with DNA fragments to form recombinant DNA molecul
Veins: Thin-walled vessels that carry blood to the heart. Units of the circulatory system that carry blood
Ventilation: The mechanics of breathing in and out through the use of the diaphragm and muscles in the wall of th
Ventral: Term applied to the lower side of a fish, or to the chest of a land vertebrate.
Ventricle: The chamber of the heart that pumps the blood into the blood vessels that carry it away from the hea
Venules: The smallest veins. Blood fiows into them from the capillary beds. Small veins that connect a vein w
Vernalization: Artifcial exposure of seeds or seedlings to cold to enable the plant to fiower.
Vertebrae: The segments of the spinal column; separated by disks made of connective tissue (sing.: vertebra).
Vertebrate: Any animal having a segmented vertebral column; members of the subphylum Vertebrata; include reptile
Vesicles: Small membrane-bound spaces in most plant and animal cells that transport macromolecules into and ou
Vessel Elements: Short, wide cells arranged end to end, forming a system of tubes in the xylem that moves water and s
Vestigial Structures: Nonfunctional remains of organs that were functional in ancestral species and may still be functiona
Villi: Finger-like projections of the lining of the small intestine that increase the surface area availabl
Viroids: Infective forms of nucleic acid without a protective coat of protein; unencapsulated single-stranded
Virus: Infectious chemical agent composed of a nucleic acid (DNA or RNA) inside a protein coat.
Vitamins: A diverse group of organic molecules that are required for metabolic reactions and generally cannot
Vulva: A collective term for the external genitals in women.
White Blood Cell: Component of the blood that functions in the immune system. Also known as a leukocyte.
Wood: The inner layer of the stems of woody plants; composed of xylem.
X-Chromosome: One of the sex chromosomes.
X-Ray Diffraction: Technique utilized to study atomic structure of crystalline substances by noting the patterns produc
Xerophytic Leaves: The leaves of plants that grow under arid conditions with low levels of soil and water. Usually char
Xylem: Tissue in the vascular system of plants that moves water and dissolved nutrients from the roots to t
Z Lines: Dense areas in myofibrils that mark the beginning of the sarcomeres. The actin filaments of the sarc
Zebroid: A hybrid animal that results from breeding zebras and horses.
Zone Of Differentiation: Area in plant roots where recently produced cells develop into different cell types.
Zone Of Elongation: Area in plant roots where recently produced cells grow and elongate prior to differentiation.
Zone Of Intolerance: The area outside the geographic range where a population is absent; grades into the zone of physiolo
Zone Of Physiological Stress: The area in a populations geographic range where members of population are rare due to physical and
Zygomycetes: One of the division of the fungi, characterized by the production of zygospores; includes the bread
Zygospore: In fungi, a structure that forms from the diploid zygote created by the fusion of haploid hyphae of