| 749589311 | ABA (abscisic acid) | Plant hormone that inhibits growth, closes stomates during times of water stress and counteracts breaking of dormancy | |
| 749589312 | Abiotic | Nonliving and includes temperature, water, sunlight, wind, rocks, and soil | |
| 749589313 | Abscission | The process of leaves falling off a tree or bush | |
| 749589314 | Acetylcholine | One of many neurotransmitters | |
| 749589315 | Acid rain | Caused by pollutants in the air from combustion of fossil fuels; the pH is less than 5.6 | |
| 749589316 | Actin | Thin protein filaments that interact with myosin filaments in the contraction of skeletal muscles | |
| 749589317 | Action potential | A rapid change in the membrane of a nerve or muscle cell when a stimulus causes an impulse to pass | |
| 749589318 | Active immunity | The type of immunity when an individual makes his or her own antibodies after being ill and recovering or after being given an immunization or vaccine | |
| 749589319 | Adaptive radiation | The emergence of numerous species from one common ancestor introduced into an environment | |
| 749589320 | Adenine | A nucleotide that binds to thymine and uracil; it is a purine | |
| 749589321 | Adipose | Fat tissue | |
| 749589322 | Allopatric speciation | The formation of new species caused by separation in geography, such as mountain ranges, canyons, rivers, lakes, glaciers, altitude, or longitude | |
| 749589323 | Allosteric | A type of enzyme that changes its conformation and its function in response to a modifier | |
| 749589324 | Amoebocytes | Found in sponges, these cells are mobile and perform numerous functions, including reproduction, transport of food particles to nonfeeding cells, and secretion of material that forms the spicules | |
| 749589325 | Amphipathic | A molecule with both a positive and negative pole | |
| 749589326 | Anaerobic respiration | The anaerobic breakdown of glucose into pyruvic acid with the release of a small amount of ATP | |
| 749589327 | Analogous structures | Structures, such as a bat's wing and a fly's wing, that have the same function, but the similarity is superficial and reflects an adaptation to similar environments, not a common ancestry | |
| 749589328 | Aneuploidy | Any abnormal number or a particular chromosome | |
| 749589329 | Angiosperms | Flowering plants | |
| 749589330 | Anode | The positive pole in an electrolytic cell | |
| 749589331 | Antenna pigment | Accessory photosynthetic pigment that explains the wavelengths of light that can be used to power photosynthesis | |
| 749589332 | Anterior pituitary | Gland in the brain that releases many hormones, including growth hormone, luteinizing hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, and follicle-stimulating hormone | |
| 749589333 | Anther | Part of a flowering plant that produces male gametophytes | |
| 749589334 | Antheridia | Structures in plants that produce male gametes | |
| 749798049 | Antibodies | Produced by B lymphocytes and destroy antigens | |
| 749798050 | Anticodon | The three-base sequence of nucleotides at one end of a tRNA molecule | |
| 749798051 | Antidiuretic hormone | Released by the posterior pituitary, its target is the collecting tube of the nephron | |
| 749798052 | Apical dominance | The preferential growth of a plant upward (toward the sun), rather than laterally | |
| 749798053 | Apoptosis | Programmed cell death | |
| 749798054 | Aposematic coloration | The bright, often red or orange coloration of poisonous animals as a warning that predators should avoid them | |
| 749798055 | Archegonia | Structures in plants that produce female gametes | |
| 749798056 | Artificial selection | The intentional selection of specific individuals with desired traits for breeding | |
| 749798057 | Associative learning | One type of learning in which one stimulus becomes linked, through experience, to another | |
| 749798058 | ATP-synthase channels | Located in the cristae of mitochondria and thylakoids of chloroplasts, these are membrane channels that allow protons to diffuse down a gradient in the production of ATP | |
| 749798059 | Autonomic nervous system | The branch of the vertebrate peripheral nervous system that controls involuntary muscles | |
| 749798060 | Autotrophs | Organisms that synthesize their own nutrients | |
| 749798061 | Auxin | A plant hormone that stimulated stem elongation and growth, enhances apical dominance, and is responsible for tropisms | |
| 749798062 | Bacteriophage | A virus that attacks bacteria | |
| 749798063 | Balanced polymorphism | The presence of two or more phenotypically distinct forms of a trait in a single population, such as two varieties of peppered moths, black ones and white ones | |
| 749798064 | Barr body | An inactivated X chromosome seen as a condensed body lying just inside the nuclear envelope | |
| 749798065 | Batesian mimicry | The copycat coloration where one harmless animal mimics the coloration of one that is poisonous; an example is the viceroy butterfly, which is harmless but looks similar to the monarch butterfly | |
| 749798066 | Binomial nomenclature | A scientific naming system where every organism has a unique name consisting of two parts: a genus name and a species name | |
| 749798067 | Biological magnification | A trophic process in which substances in the food chain become more concentrated with each link of the food chain | |
| 749798068 | Biomes | Very large regions of the earth, names for the climatic conditions and for the predominant vegetation; examples are marine, tropical rain forest, and desert | |
| 749798069 | Biotic potential | The maximum rate at which a population could increase under ideal conditions | |
| 749798070 | B lymphocyte | A lymphocyte that produces antibodies | |
| 749798071 | Bottleneck effect | An example of genetic drift that results from the reduction of a population, typically by natural disaster; the surviving population is no longer genetically representative of the original one | |
| 749798072 | Botulinum | The genus name for the bacterium that produces botulism, a very serious form of food poisoning | |
| 749798073 | Bryophytes | Nonvascular plants like mosses | |
| 749798074 | Bulk flow | The general term for the overall movement of a fluid in one direction in an organism, such as sap flowing in a tree or blood flowing in a human | |
| 749798075 | Bundle sheath cell | A type of photosynthetic plant cell that is tightly packed around the veins in a leaf | |
| 749798076 | C-3 plant | The common type of plant, different from C-4 and CAM plants | |
| 749798077 | C-4 plant | A plant with the anatomical and biochemical modifications for a dry environment that differ from C-3 and CAM plants; examples are sugarcane and corn | |
| 749798078 | Calvin cycle | A cyclic metabolic pathway in the dark reactions of photosynthesis that fixes or incorporates carbon into carbon dioxide and produces phosphoglyceraldehyde (PGAL), a three-carbon sugar | |
| 749798079 | CAM (crassulacean acid metabolism) | A form of photosynthesis that is an adaptation to ry conditions; stomates remain closed during the day and open only at night | |
| 749798080 | Capsid | The protein shell that encloses viral DNA or RNA | |
| 749798081 | Carbon fixation | Carbon becomes fixed or incorporated into a molecule of PGAL; this happens during the Calvin cycle | |
| 749798082 | Carbonic acid anhydrase | An enzyme found in red blood cells that catalyzes the conversion of carbon dioxide and water into carbonic acid as part of the system that maintains blood pH at 7.4 | |
| 749798083 | Carotenoid | Accessory photosynthetic pigment that is yellow or orange | |
| 749798084 | Carrying capacity | The limit to the number of individuals that can occupy one area at a particular time | |
| 749798085 | Catalase | An enzyme produced in a ll cells to decompose hydrogen peroxide, a by-product of cell respiration | |
| 749798086 | Cathode | The negative pole in an electrolytic cell | |
| 749798087 | CDKs (cyclin-dependent kinases) | A kinase whose activity depends on the level of cyclins and that controls the timing of cell division | |
| 749798088 | Cell plate | A double membrane down the midline of a dividing plant cell between which the new cell wall will form | |
| 749798089 | Centriole | One of two structures in animal cells involved with cell division | |
| 749798090 | Centromere | A specialized region in a chromosome that holds the two chromatids together | |
| 749798091 | Chemiosmosis | The process by which ATP is produces from the flow of protons through an ATP-sythase channel in the thylakoid membrane of the chloroplast during the light reactions of photosynthesis and in the cristae membrane if the mitochondria during cell respiration | |
| 749798092 | Chemokines | A chemical secreted by blood vessel endothelium and monocytes during an immune response to attract phagocytes to an area | |
| 749798093 | Chiasma/chiasmata | The site at which crossover and recombination occurs | |
| 749798094 | Chitin | A structural polysaccharide found in cell walls | |
| 749798095 | Chlorophyll a | One type of chlorophyll that participates directly in the light-dependent reactions of photosynthesis | |
| 749798096 | Chlorophyll b | One type of chlorophyll that acts as an antenna pigment, expanding the wavelengths of light that can be used to power photosynthesis | |
| 749798097 | Chloroplast | The site of photosynthesis in plant cells | |
| 749798098 | Choanocytes | Collar cells that line the body cavity and have flagella that circulate water in sponges | |
| 749798099 | Chromatid | Either of the two strands of a replicated chromosome joined at the centromere | |
| 749798100 | Chromatin network | The complex of DNA and protein that makes up a eukaryotic chromosome; when the cell is not dividing, chromatin exists as long, thin strands, and is not visible with the light microscope | |
| 749798101 | Cilia | Hairlike extensions from the cytoplasm used for cell locomotion | |
| 749798102 | Citric acid cycle | Another name for the Krebs cycle | |
| 749798103 | Cladogenesis | Branching evolution occurs when a new species branches out from a parent species | |
| 749798104 | Classical conditioning | One type of associative learning that is widely accepted because of the ingenious work of Ivan Pavlov associating a novel stimulus with an innately recognized one | |
| 749798105 | Cleavage furrow | A shallow groove in the cell surface in an animal cell where cytokinesis is taking place | |
| 749798106 | Climax community | The final, stable community in an ecosystem | |
| 749798107 | Cline | A variation in some trait of individuals coordinated with some gradual change in temperature or other factor over a geographic range | |
| 749798108 | Clonal selection | A fundamental mechanism in the development of immunity; antigenic molecules select or bind to specific B or T lymphocytes, activating them; the B cells then differentiate into plasma cells and memory cells | |
| 749798109 | Cnidocytes | Stinging cells in all cnidarians | |
| 749798110 | Codominance | The type of inheritance when there is no trait that dominates over another; both traits show | |
| 749798111 | Codons | The three-base sequence of nucleotides in mRNA | |
| 749798112 | Coelom | The body cavity that arises from within the mesoderm and is completely surrounded by mesoderm tissue | |
| 749798113 | Coevolution | Evolution that is caused by two species that interact and influence each other; all predator-prey relationships are examples | |
| 749798114 | Cohesion tension | Force of attraction between molecules of water due to hydrogen bonding | |
| 749798115 | Collaboration | Two genes interact to produce a novel phenotype | |
| 749798116 | Collenchyma cells | Plant cells with unevenly thickened primary cell walls that are alive at maturity and that function to support the growing stem | |
| 749798117 | Commensalism | A symbiotic relationship where one organism benefits and one is unaware of the other organism (+/o) | |
| 749798118 | Community | All the organisms living in one area | |
| 749798119 | Companion cell | Connected to each sieve tube member in the phloem and nurtures the sieve tube elements | |
| 749798120 | Complement | An important part of the immune system, a group of about twenty proteins that assists in lysing cells | |
| 749798121 | Complementary genes | The expression of two or more genes where each depends upon the alleles of the other in order for a trait to show | |
| 749798122 | Conformation | The particular three-dimentional shape of a protein molecule | |
| 749798123 | Conjugation | A primitive form of sexual reproduction that is characteristic of bacteria and some algae | |
| 749798124 | Convergent evolution | Evolution that occurs when unrelated species occupy the same environment and are subjected to similar selective pressures and show similar adaptations | |
| 750676766 | Countercurrent mechanism | A mechanism or strategy to maximize the rate of diffusion; this is a major strategy to transport substances across membranes passively, such as in the nephron | |
| 750676767 | Cristae | The internal membranes of the mitochondria that are the site of the electron transport chain | |
| 750676768 | Crop | Part of the digestive tract of many animals where food is temporarily stored until it can continue to the gizzard | |
| 750676769 | Crossing-over | The reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I | |
| 750676770 | Cutin | The main component of the waxy cuticle covering leaves to minimize water loss | |
| 750676771 | Cyclic phosphorylation | Part of the light-dependent reactions in photosynthesis where electrons travel on a short-circuit pathway to replenish ATP levels only | |
| 750676772 | Cyclin | A regulatory protein whose levels fluctuate cyclically in a cell, in part, related to the timing of cell division | |
| 750676773 | Cystic fibrosis | The most common lethal genetic disease in the United States; characterized by a buildup of extracellular fluid in the lungs and digestive tract | |
| 750676774 | Cytochrome | An iron-containing pigment present in the electron transport chain of all aerobes | |
| 750676775 | Cytokines | Chemicals that stimulate helper T cells, B cells, and killer T cells | |
| 750676776 | Cytokinesis | Division of the cytoplasm | |
| 750676777 | Cytokinins | Plant hormone that stimulates cell division and delays senescence (aging) | |
| 750676778 | Cytosine | A nucleotide that binds with guanine; a pyrimidine | |
| 750676779 | Cytoskeleton | A complex network of protein filaments that gives a cell its shape and helps it move | |
| 750676780 | Cytotoxic T cells | A type of lymphocyte that kills infected body cells and cancer cells | |
| 750676781 | Decomposers | Organisms, like bacteria and fungi, that recycle nutrients back to the soil | |
| 750676782 | Deletion | A chromosomal mutation where a fragment is lost during cell division | |
| 750676783 | Dendrites | The sensory processes of a neuron | |
| 750676784 | Denitrifying bacteria | Convert nitrates (NO3) into free atmospheric nitrogen | |
| 750676785 | Density-dependent factors | Factors, such as starvation, that increase directly as the population density increases | |
| 750676786 | Density-dependent inhibition | A characteristic of normal cells grown in culture that causes cell division to cease when the culture becomes too crowded | |
| 750676787 | Density-independent factors | Factors, such as earthquakes, whose occurrence is unrelates to the population density | |
| 750676788 | Depolarization | An electrical state where the inside of an excitable cell is made less negative compared with the outside; if an axon is depolarized, an impulse is passing | |
| 750805480 | Detrivores | Consumers that derive their nutrition from nonliving, organic matter | |
| 750805481 | Deuterostomes | Animals in which the blastopore becomes the anus during early embryonic development | |
| 750805482 | Dicotyledon | A subdivision of flowering plants whose members possess an embryonic seed leaf made of two halves or cotyledons | |
| 750805483 | Dihybrid cross | A cross between individuals that are hybrid for two different traits, such as height and seed color | |
| 750805484 | Diploblastic | An organism whose body is made of only two cell layers, the ectoderm and the endoderm; the two are connected by a noncellular layer called the mesoglea; animal phyla that are this are the Porifera (sponges) and the Cnidaria (jellyfish and hydra) | |
| 750805485 | Directional selection | Selection here one phenotype replaces another in the gene pool | |
| 750805486 | Disruptive selection | Selection that increases the extreme types in a population at the expense of intermediate forms | |
| 750805487 | Divergent evolution | Evolution that occurs when a population becomes isolated (for any reason) from the rest of the species, becomes exposed to new selective pressures, and evolves into a new species | |
| 750805488 | DNA ligase | An enzyme that permanently attaches pieces of DNA together | |
| 750805489 | Dopamine | A neurotransmitter (dopa) | |
| 750805490 | Down syndrome | A genetic condition caused by trisomy 21 | |
| 750805491 | Duodenum | The first 12 inches (30 cm) of the human small intestine | |
| 750805492 | Ecdysone | A hormone that helps control metamorphosis in insects | |
| 750805493 | Ecological succesion | The sequential rebuilding of an entire ecosystem after a disaster | |
| 750805494 | Ecosystem | All the organisms in a given area as well as the abiotic (nonliving) factors with which they interact | |
| 750805495 | Ectoderm | The germ layer that gives rise to the skin and nervous system | |
| 750805496 | Effectors | Muscles or glands | |
| 750805497 | Electron transport chain | A sequence of membrane proteins that carry electrons through a series of redox reactions to produce ATP | |
| 750805498 | Endergonic | Any process that absorbs energy | |
| 750805499 | Endoderm | The embryonic germ layer that gives rise to the viscera, the digestive tract, and other internal organs | |
| 750805500 | Endodermis | The tightly packed layer of cells that surrounds the vascular cylinder in the root of a plant | |
| 750805501 | Endoplasmic reticulum | A system of transport channels inside a eukaryotic cell | |
| 750805502 | Endosperm | The food source for the growing embryo in monocots | |
| 750805503 | Endosymbiosis | This theory states that mitochondria and chloroplasts were once free-living prokaryotes that took up residence inside larger prokaryotic cells in a permanent, symbiotic relationship | |
| 750805504 | Endotherms | Animals that can raise their body temperature, although they cannot maintain a stable body temperature | |
| 750805505 | Envelope | Cloaks the capsid of a virus and aids the virus in infecting the host; the envelope is derived from membranes of host cells | |
| 750805506 | Enzyme | A protein that serves as a catalyst | |
| 750805507 | Epicotyl | Part of the developing embryo that will become the upper part of the stem and the leaves of a plant | |
| 750805508 | Epinephrine | A neurotransmitter (EpiPen) | |
| 750805509 | Epiphytes | Photosynthetic plants that grow on other trees rather than supporting themselves | |
| 750805510 | Epistasis | Two separate genes control one trait, but one gene masks the expression of the other gene | |
| 750805511 | Esterase | An enzyme that breaks down excess neurotransmitter | |
| 750805512 | Ethylene | A gaseous plant hormone that promotes fruit ripening and opposes auxins in its actions | |
| 750805513 | Eukaryotes | Cells with internal membranes | |
| 750805514 | Eutrophication | Translates as "true feeding"; a process begun by the entrance of large amounts of nutrients into a lake, ultimately ending with the death of the lake | |
| 750805515 | Exocytosis | The process by which cells expel substances | |
| 750805516 | Exons | Stands for expressed sequences of DNA; these are genes | |
| 750805517 | Exothermic | Any process that gives off energy | |
| 750805518 | Expressivity | The range of expression of mutant genes | |
| 750805519 | Extranuclear genes | Genes outside the nucleus, in the mitochondria and chloroplasts | |
| 750805520 | Facultative anaerobes | Organisms that can live without oxygen in the environment | |
| 750805521 | FAD (flavin adenine dinucleotide) | A coenzyme that carries protons or electrons from glycolysis and the Krebs cycle to the electron transport chain | |
| 750805522 | Fermentation | A synonym for anaerobic respiration; the anaerobic breakdown of glucose into pyruvic acid | |
| 750805523 | Fixed action pattern | An innate, highly stereotypic behavior, which when begun is continued to completion, no matter how useless | |
| 750805524 | Flagella | The tail-like structure that propels some single-celled organisms; these consist of microtubules | |
| 750805525 | Follicle-stimulating hormone (FSH) | A hormone released from the anterior pituitary that stimulates the ovarian follicle | |
| 750805526 | Food chain | The pathway along which food is transferred from one trophic level to the next | |
| 750805527 | Food pyramid | A model of the food chain that demonstrates the interaction of the organisms and the loss of energy | |
| 750805528 | Food web | The interconnected feeding relationships of organisms in an ecosystem | |
| 750805529 | Founder effect | An example of genetic drift, when a small population breaks away from a larger one to colonize a new area; it is most likely not genetically representative of the original larger population | |
| 750805530 | Frameshift | One type of mutation caused by a deletion or addition where the entire reading sequence of DNA is shifted; AAA TTT CCC GGG could become AAT TTC CCG GG | |
| 750805531 | Frequency-dependent selection | A form of selection that acts to decrease the frequency of the more-common phenotypes and increase the frequency of the less-common ones | |
| 750805532 | Fruit | A ripened ovary of a flowering plant | |
| 750805533 | Fungi | The kingdom that consists of heterotrophs that carry out extracellular digestion and have walls made of chitin; includes mushrooms and yeast | |
| 750805534 | GABA (gamma-aminobutyric acid) | A neurotransmitter (gamma-aminobutyric acid) | |
| 750805535 | Gametangia | A protective jacket of cells that prevents some plants' gametes and zygotes from dryingout | |
| 750805536 | Gametophyte | The monoploid generation of a plant | |
| 750805537 | Gastrodermis | Cells that line the gastrovascular cavity in cnidarians | |
| 750805538 | Gastrovascular cavity | A digestive cavity with only one opening, characteristic of cnidarians | |
| 750805539 | Gated-ion channel | A channel in the plasma membrane for one specific ion, such as sodium or calcium; in the terminal branch of a neuron, it is responsible for the release of neurotransmitter into the synapse | |
| 750805540 | Gene flow | the movement of alleles into or out of a population | |
| 750805541 | Genetic drift | Change in the gene pool due to chance | |
| 750805542 | Genetic engineering | The technology of manipulating genes for practical purposes | |
| 750805543 | Genomic imprinting | Certain traits whose expression varies, depending on the parent from which they are inherited; diseases that result from imprinting are Prader-Willi and Angelman syndromes | |
| 750805544 | Genotype | The types of genes an organism has | |
| 750805545 | Gibberellins | Plant hormone that promotes stem elongation | |
| 750805546 | Gizzard | Part of the digestive tract of many animals; it is the site of mechanical digestion | |
| 750805547 | Glial cells | Cells that nourish neurons | |
| 750805548 | Gluteraldehyde | A chemical fixative often used in the preparation of tissue for electron microscopy | |
| 751511251 | Glycocalyx | The external surface of a plasma membrane that is important for cell-to-cell communication | |
| 751511252 | Glycolysis | A nine-step, anaerobic process that breaks down one glucose molecule into two pyruvates and four ATP | |
| 751511253 | Golgi apparatus | An organelle in eukaryotes that lis near the nucleus and that packages and secretes substances for the cell | |
| 751511254 | Gonadotropic-releasing hormone | A hormone released by the hypothalamus that stimulates other glands to release their hormones | |
| 751511255 | Gradualism | The theory that organisms descend from a common ancestor gradually, over a long period of time, in a linear or branching fashion | |
| 751582470 | Grana | Stacks of thylakoid disks in the chloroplast where light reactions occur | |
| 751582471 | Greenhouse effect | The warming of the planet because of the accumulation of atmospheric carbon dioxide | |
| 751582472 | Ground tissue | The most common tissue type in a plant, functions mainly in support and consists of parenchyma, collenchyma, and sclerenchyma in cells | |
| 751582473 | GTP (guanosine triphosphate) | A molecule closely related to ATP that provides the energy for translation | |
| 751582474 | Guanine | A nucleotide that bonds with cytosine; a purine | |
| 751582475 | Guttation | Due to root pressure, droplets of water appear in the morning on the leaf tips of some herbaceous plants | |
| 751582476 | Gymnosperms | Conifers or cone-bearing plants | |
| 751582477 | Habitat isolation | Separation of two or more organisms of the same species living in the same area but in separate habitats, such as in the water and on land | |
| 751582478 | Habituation | One of the simplest forms of learning in which an animal comes to ignore a persistent stimulus | |
| 751582479 | Halophiles (halobacteria) | Anaerobic bacteria that thrive in environments with very high salt concentrations | |
| 751582480 | Hatch-Slack pathway | An alternate biochemical pathway found in C-4 plants; its purpose is to remove CO2 from the airspace near the stomate | |
| 751582481 | Head-foot | The part of the body of mollusks that contains both sensory and motor organs | |
| 751582482 | Helicase | An enzyme that untwists the double helix at the replication fork | |
| 751582483 | Helper T cells | One type of T lymphocyte that activates B cells and other T lymphocytes | |
| 751582484 | Hemocoels | Blood-filled cavities within the body of arthropods and mollusks with open circulatory systems | |
| 751582485 | Hemophilia | An inherited genetic disease caused by the absence of one or more proteins necessary for normal blood clotting | |
| 751582486 | Hermaphrodite | Organisms possessing both male and female sex organs | |
| 751582487 | Heterosporous | A plant that produces two kinds of spores, male and female | |
| 751582488 | Heterotroph hypothesis | This theory states that the first cells on earth were heterotrophic prokaryotes | |
| 751582489 | Heterotrophs | Organisms that must ingest nutrients rather than synthesize them | |
| 751582490 | Histamine | A chemical released by the body during an inflammatory response that causes the blood vessels to dilate | |
| 751582491 | Homeotherms | Organisms that maintain a consistent body temperature | |
| 751582492 | Homologous structures | Structures in different species that are similar because they have a common origin | |
| 751582493 | Homosporous | A plant that produces a singe bisexual spore | |
| 751582494 | Huntington's disease | A degenerative, inherited, dominant disease of the nervous system that results in certain and early death | |
| 751582495 | Hybrid vigor | A phenomenon in which the hybrid state is selected because it has greater survival and reproductive success; also known as heterosis | |
| 751582496 | Hydrophilic | Having an affinity for water | |
| 751582497 | Hyperpolarized | An electrical state where the inside of the excitable cell is made more negative compared with the outside of the cell and the electric potential of the membrane increases (gets more negative) | |
| 751582498 | Hypertonic | Having a greater concentration of solute than another solution | |
| 751582499 | Hypocotyl | Part of the developing embryo that will become the lower part of the stem and roots | |
| 751582500 | Hypothalamus | Gland locates in the brain above the pituitary that is the bridge between the endocrine and the nervous systems | |
| 751582501 | Hypotonic | Having a lesser concentration of solute than another solution | |
| 751582502 | Immunological memory | The capacity of the immune system to generate a secondary immune response against a specific antigen for a lifetime | |
| 751582503 | Imprinting | A type of learning that is responsible for the bonding between the mother and offspring; common in birds, it occurs during a sensitive or critical period in early life | |
| 751582504 | Incomplete dominance | The type of inheritance that is characterized by blending traits; for instance, one gene for red plus one gene for white results in a pink four o'clock flower | |
| 751582505 | Indoleacetic acid (IAA) | IAA; a naturally occurring auxin | |
| 751582506 | Inflammatory response | A nonspecific defensive reaction of the body to invasion by a foreign substance that is accompanied by the release of histamine, fever, and red, itchy areas | |
| 751582507 | Interferons | A class of chemicals that block viral infections | |
| 751582508 | Interneuron | AKA an association neuron, resides within the spinal chord and receives sensory stimuli and transfers the information directly to a motor neuron or to the brain for processing | |
| 751582509 | Interphase | The longest stage of the life cycle of a cell; it consists of G1, S, and G2 | |
| 751582510 | Introns | Intervening sequences, the noncoding regions of DNA that are sometimes referred to as junk | |
| 751582511 | Inversion | A chromosome mutation where a chromosomal fragment reattaches to its original chromosome but in the reverse orientation | |
| 751582512 | In vitro | In the laboratory | |
| 751582513 | In vivo | In the living thing | |
| 751582514 | Isotonic | Two solutions containing equal concentrations of solutes | |
| 751582515 | Karyotype | A procedure that analyzes the size, number, and shape of chromosomes | |
| 751582516 | Kinase | An enzyme that transfers phosphate ions from one molecule to another | |
| 751582517 | Kinetochore | A disc-shaped protein on the centromere that attaches the chromatid to the mitotic spindle during cell division | |
| 751582518 | Klinefelter's syndrome | A genetic condition in males in which there is an extra X chromosome; the genotype is XXY | |
| 751582519 | Kranz anatomy | Refers to the structure of C-4 leaves and differs from C-3 leaves. In C-4 leaves, the bundle sheath cells lie under the mesophyll cells, tightly wrapping the vein deep within the leaf, where CO2 is sequestered | |
| 751582520 | Krebs cycle | AKA the citric acid cycle, it completes the breakdown of pyruvic acid into CO2, with the release of a small amount of ATP | |
| 751582521 | Lactic acid fermentation | The process by which pyruvate from glycolysis is reduced to form lactic acid or lactate; this is the process that the dairy industry uses to produce yogurt and cheese | |
| 751582522 | Lateral meristem | Growth region of a plant that provides secondary growth, increase in girth | |
