Insulin A protein hormone produced in the pancreas by beta cells, located in the islets of Langerhans, that stimulates cellular utilization of glucose by body cells, by converting glucose and other carbohydrates to energy, and helps control blood-sugar levels by acting antagonistically with glucagons, the chief source of stored fuel, in the liver. It is released by various signals that are sensitive to the intake and digestion of food. It also acts as an important regulator of protein and lipid metabolism. Insulin is used as a drug to control insulin-dependent diabetes mellitus, a disorder that is caused by the insufficient production of insulin. Without insulin, cells do not absorb glucose. Diabetic individuals can have type I diabetes (juvenile) comprising about 10 percent of the population, or type II diabetes (adult). Presently some 16 million Americans have diabetes, with 1,700 new cases being diagnosed daily. Diabetes has been linked to the development of a variety of diseases including heart disease, stroke, peripheral vascular disease, and neurological disorders.
Integral protein A protein of biological membranes that penetrates into or spans the lipid bilayer of a cell membrane. The lipid bilayer is a protective membrane that surrounds the cell and consists of two layers of phospholipids.
Inter- A prefix meaning between or among.
Interbreed (crossbreed) To breed with another kind or species; hybridize.
Intercalation compounds Compounds resulting from inclusion, usually without covalent bonding, of one kind of molecule (the guest molecule) in a matrix of another compound (the host compound), which has a layered structure. The host compound, with a rather rigid structure, may be macromolecular, crystalline, or amorphous.
Interferon A chemical messenger of the immune system, composed of a group of cytokine proteins that have antiviral characteristics that are capable of helping the immune response. Three main types of interfer-on—alpha, beta, and gamma—are produced by virus-infected cells and are released to coat uninfected cells, thus preventing them from becoming infected. Alpha interferon is produced by virus-infected mono-cytes and lymphocytes, while beta is produced by virus-infected fibroblasts. Gamma is produced by stimulated T and NK cells.
Interleukin A group of natural chemical glycoprotein messengers, acting as cytokines, that are secreted by different cells of the immune system to make other cells perform specific cellular functions. Interleukin-1 (IL-1) is released early by monocytes, macrophages, T cells, and other immune cells to fight infection. It stimulates T cell proliferation and protein synthesis and causes fever. Can be cleaved into a peptide involved in cell death (apoptosis). Interleukin-2 (IL-2) is produced by T helper and suppresser lymphocytes. IL-2 increases the expression of natural killer and other cytotoxic cells and stimulates helper T cells to proliferate more rapidly. IL-2 is produced commercially by recombinant DNA technology and used for the treatment of metastatic renal (i.e., kidney) cell cancer. Interleukin-4 (IL-4) is released by lymphocytes (TH-2 subset of T helper lymphocytes) and promotes antibody production by stimulating B cells to proliferate and mature and promotes allergic responses via production of the immunoglobulin IgE. Interleukin-6 (IL-6) affects many different cells in the immune system by inducing differentiation and activation. Inter-leukin-10 (IL-10), like IL-4, is released by lymphocytes (the TH-2 subset of T helper lymphocytes). IL-10 enhances the humoral response and increases antibody production. Interleukin-12 (IL-12) induces the production of natural killer and other cytotoxic immune cells.
Intermediate filament A fibrous protein filament of the cytoskeleton that helps form ropelike bundles in animal cells and is about 10 nm in size, falling in the middle between the size of microtubules and microfila-ments; provides tensile strength.
Interneuron one of the three types of neuron networks (along with sensory and motor neurons) that allow information flow by way of impulses and action potentials to travel through the nervous system. Found in the central nervous system, they provide the center of a loop that receives upstream input from thousands of motor neurons, then sends the signals to the brain, processes the returns, and then transmits the downstream output to a similar number of sensory neurons for action.
Internode The segment on a plant stem between the points where leaves are attached; the region or length of stem between two nodes.
Interphase one of the phases of cell division during the process of mitosis. Interphase is the stage between two successive cell divisions and the time when DNA is replicated in the nucleus, followed by mitosis. Inter-phase itself has several phases: the first gap phase (G1) is the time prior to DNA synthesis where the cell increases in mass; the synthesis or S phase is the time DNA is actually synthesized; and gap or G2 is the phase after DNA syntheses and cell protein synthesis and before the start of prophase. See also mitosis.
The internode is the segment on a plant stem between the points leaves are attached. It is the region or length of stem between two nodes.
Interstitial Refers to the space between cells, airways, blood vessels, alveoli, atoms, molecules, and even soil particles.
Interstitial cells Cells that exist in the connective tissues between other tissues and structures; cells among the seminiferous tubules, tiny tubes of the testis where sperm cells are produced and that secrete the male sex hormones testosterone and other androgens.
Interstitial fluid (intercellular fluid) Fluid that fills spaces between cells and provides pathways for the flow of nutrients, gases, and wastes between capillaries and cells.
Intertidal zone one of the oceanic zones where the ocean meets land; landform can be submerged or exposed; shallow shore area between low and high tide or water marks.
Intra- A prefix meaning within or inside.
Intrinsic activity The maximal stimulatory response induced by a compound in relation to that of a given reference compound.
This term has evolved with common usage. It was introduced by E. J. Ariens as a proportionality factor between tissue response and receptor occupancy. The numerical value of intrinsic activity (alpha) could range from unity (for full agonists, i.e., agonist inducing the tissue maximal response) to zero (for antagonists),the fractional values within this range denoting partial agonists. Ariens’s original definition equates the molecular nature of alpha to maximal response only when response is a linear function of receptor occupancy. This function has been verified. Thus, intrinsic activity, which is a drug and tissue parameter, cannot be used as a characteristic drug parameter for classification of drugs or drug receptors. For this purpose, a proportionality factor derived by null methods, namely, relative efficacy, should be used. Finally, "intrinsic activity" should not be used instead of "intrinsic efficacy." A "partial agonist" should be termed "agonist with intermediate intrinsic efficacy" in a given tissue.
Intrinsic rate of increase (rmax) A mathematical parameter that measures the maximum rate at which a population will grow if resources are unlimited, using birth and death rates in a population as the determinants. It is the difference between the number of births and the number of deaths; the maximum population growth rate.
Introgression The movement of genes from one population into another through hybridization followed by backcrossing.
Intron An intervening section of DNA that occurs almost exclusively within a eukaryotic gene, but which is not translated to amino acid sequences in the gene product. The introns are removed from the premature mRNA through a process called splicing, which leaves the exons untouched, to form an active mRNA.
An intron is a noncoding sequence of DNA that is initially copied into RNA but is cut out of the final RNA transcript.
Inverse agonist (negative antagonist) A drug that acts at the same receptor as that of an agonist, yet produces an opposite effect. See also antagonist.
Inversion A chromosomal aberration or mutation, occurring during meiosis or from mutagens, that involves detaching a chromosome segment, rotating it 180 degrees, and then reinserting it in its original location.
Invertebrates Animals that do not have a backbone or notochord. They are cold-blooded and have solid, external skeletons or hydrostatic skeletons. They include small protozoans, sponges, corals, tapeworms, spiders, mollusks, and others. Most animals on the planet (95 percent of all animal species) are invertebrates, and the majority of those, excepting insects, are aquatic.
In vitro fertilization Fertilization outside the body, whereby a male sperm and female egg are combined in a laboratory. The embryo is then artificially transferred into a female’s uterus.
Ion An atom that acquires a charge by either gaining or losing an electron.
Ion channel Enables ions to flow rapidly through membranes in a thermodynamically downhill direction after an electrical or chemical impulse. Their structures usually consist of four to six membrane-spanning domains. This number determines the size of the pore and thus the size of the ion to be transported.
Ionic bond A chemical bond or link between two atoms due to an attraction between oppositely charged (positive-negative) ions.
Ionophore A compound that can carry specific ions through membranes of cells or organelles.
Ion pump Enables ions to flow through membranes in a thermodynamically uphill direction by the use of an energy source such as ATP or light. They consist of sugar-containing heteropeptide assemblies that open and close upon the binding and subsequent hydrolysis of ATP, usually transporting more than one ion toward the outside or the inside of the membrane.
Iron-responsive element A specific base sequence in certain messenger rnas that code for various proteins of iron metabolism, which allows regulation at translational level by the iron-responsive protein.
Iron-responsive protein (IRP) A protein that responds to the level of iron in the cell and regulates the biosynthesis of proteins of iron metabolism by binding to the iron-responsive element on messenger rna.
Iron-sulfur cluster A unit comprising two or more iron atoms and bridging sulfide ligands in an iron-sulfur protein. The recommended designation of a cluster consists of the iron and sulfide content, in square brackets, e.g., [2fe-2s], [4fe-4s]. The possible oxidation levels are indicated by the net charge excluding the ligands, for example a [4Fe-4S]2+ or [4Fe-4S]x+ or [4Fe-4S]2+; j+ cluster.
Iron-sulfur proteins Proteins in which nonheme iron is coordinated with cysteine sulfur and usually also with inorganic sulfur. Divided into three major categories: rubredoxlns; simple iron-sulfur proteins, containing only iron-sulfur clusters; and complex iron-sulfur proteins, containing additional active redox centers such as flavin, molybdenum, or heme. In most iron-sulfur proteins the clusters function as electron transfer groups, but in others they have other functions such as catalysis of hydratase/dehydratase reactions, maintenance of protein structure, or regulation of activity.
Irregular Refers to flowers that are bilaterally symmetrical, i.e., that are divisible into equal halves only in one plane.
Irruption A rapid and temporary increase in population density, often followed by a mass emigration; common in bird species.
Ischemia Local deficiency of blood supply and dioxygen to an organ or tissue owing to constriction of the blood vessels or to an obstruction.
Isoenzymes Multiple forms of enzymes arising from genetically determined differences in primary structure. The term does not apply to those derived by modification of the same primary sequence.
Isogamy Sexual reproduction involving the fusion of gametes that are similar in size or are morphologically indistinguishable, e.g., fungi such as zygomycetes.
Isolating mechanism Any environmental, behavioral, mechanical, or physiological barriers or characteristics that will prevent two individuals of different populations from producing viable progeny. Important for the development of new species.
Isomerase An enzyme of EC class 5, which catalyzes the isomerization of a substrate.
Isomers Compounds that have the same number and type of atoms (same molecular formula) but differ in the way they are combined with each other. They can differ by the bonding sequence, called structural or constitutional isomerism, or the way their atoms are arranged spatially, called stereoisomerism. other types include conformational, configurational, geometric, optical, enantiomers, and diastereomers.
Isomorphic alternation of generations When gametophyte and sporophyte generations are morphologically alike, but differ in the number of chromosomes.
Isoptera Termites, a social order of insects that are soft bodied and comprise about 2,300 species worldwide. They obtain their nutrition from eating wood and digesting cellulose with the help of bacteria and protozoans found in their intestinal area.
Isosteres Molecules or ions of similar size containing the same number of atoms and valence electrons, e.g., O2-, F-, Ne.
Isotonic solutions Solutions having identical osmotic pressures, i.e., a solution where cells do not swell or shrink.
Isotope A different form of a single element that has the same number of protons, but has a different number of neutrons in its nucleus. Radioactive isotopes are unstable and break down until they become stable. Carbon 14 is a radioactive isotope of carbon that is used to date fossilized organic matter.
Isotropy Lack of anisotropy; the property of molecules and materials of having identical physical properties in all directions.
Ixodes A genus of ticks. Ixodes scopularis is the vector for Lyme disease because it can carry a spirochete known as Borrelia burgdorferi that is transmitted upon the bite of the tick.
joint The contact area between two bones. There are many different types of joint, including hinge, ball and socket, universal, sliding, and slightly movable.
Joly, John An Irish geologist (1857-1933) who, in 1899, tried to determine the Earth’s age by calculating how long it would take for the rivers to dump salt into the ocean to reach the present salinity levels. It was based on the assumption that oceans were freshwater when first formed. He estimated 90 million years for the age of the Earth. He was more accurate with dating a geological time period. Working with Sir Ernest Rutherford in Cambridge in 1913, and using the radioactive decay in minerals, he estimated that the beginning of the Devonian peri-od—the time between the Silurian and Carbonifer-ous—was not less than 400 million years ago, an age that is pretty well accepted today. Joly collaborated with Henry Horatio Dixon and was the first to explain how sap rises in plants largely due to evaporation from leaves.
Joule (J) A unit of work and energy; 1 J = 0.239 cal; 1 cal = 4.184 J. It is defined as being equal to the work done when the point of application of a force of 1 newton (N) moves in the direction of the force, a distance of 1 meter (m).
Junk DNA (noncoding DNA) Genomic DNA that serves no apparent purpose; stretches of DNA that do not code for genes.
Jurassic period The middle period of the Mesozoic era, 213 to 145 million years ago. Age of the dinosaurs. Named after the Jura Mountains between France and Switzerland, where rocks of this age were first studied.
Juvenile A stage in development prior to adult stage.
Juvenile hormone A chemical hormone in insects secreted by a pair of endocrine glands, corpora allata, close to the brain. It inhibits metamorphosis and maintains larva or nymph characteristics during development and is responsible for determining the molt type. One of three major insect development hormones. Used as a pesticide, it retards the development of insects.
Juxtaglomerular apparatus A group of specialized cells or tissue in a kidney nephron that is located near the point where the afferent arteriole meets the distal tubule. It is composed of the macula densa, a specialized group of cells in the distal tubule, and the juxtaglomerular cells, epithelioid cells in the media of the afferent arterioles just as they enter the glomerulus. The juxta-glomerular apparatus supplies blood to the glomeru-lus, controls the glomerular filtration rate (volume of plasma filtered through the glomerulus per second), blood pressure, and circulating volume through the release of rennin, which activates angiotensin, a family of peptide hormones that control blood pressure and body fluid levels.
