Imaginal disc • [L. imagos: image, form]• In insect larvae, groups of cells that develop into specific adult organs.

Imbibition • Water uptake by a seed; first step in germination.

Immune system [L. immunis: exempt from] • A system in vertebrates that recognizes and attempts to eliminate or neutralize foreign substances (e.g., bacteria, viruses, pollutants).

Immunization • The deliberate introduction of antigen to bring about an immune response.

Immunoglobulins • A class of proteins, with a characteristic structure, active as receptors and effectors in the immune system.

Immunological memory • The capacity to more rapidly and massively respond to a second exposure to an antigen than occurred on first exposure.

Immunological tolerance • A mechanism by which an animal does not mount an immune response to the antigenic determinants of its own macromolecules.

Imprinting (1) In genetics, the differential modification of a gene depending on whether it is present in a male or a female. (2) In animal behavior, a rapid form of learning in which an animal comes to make a particular response, which is maintained for life, to some object or other organism.

Inclusive fitness • The sum of an individual's genetic contribution to subsequent generations both via production of its own offspring and via its influence on the survival of relatives who are not direct descendants.

Incomplete dominance • Condition in which the heterozygous phenotype is intermediate between the two homozygous phenotypes.

Incomplete metamorphosis • Insect development in which changes between instars are gradual.

Incus (in´ kus) [L. incus: anvil] • The middle of the three bones that conduct movements of the eardrum to the oval window of the inner ear. (See malleus, stapes.)

Independent assortment • During meiosis, the random separation of genes carried on nonhomologous chromosomes. Articulated by Mendel as his second law.

Individual fitness • That component of inclusive fitness resulting from an organism producing its own offspring. (Contrast with kin selection component.)

Indoleacetic acid • See auxin.

Induced fit • A change in enzyme conformation upon binding to substrate with an increase in the rate of catalysis.

Induced mutation • A mutation resulting from treatment with a chemical or other agent.

Inducer (1) In enzyme systems, a small molecule which, when added to a growth medium, causes a large increase in the level of some enzyme. (2) In embryology, a substance that causes a group of target cells to differentiate in a particular way.

Inducible enzyme • An enzyme that is present in much larger amounts when a particular compound (the inducer) has been added to the system. (Contrast with constitutive enzyme.)

Inflammation • A nonspecific defense against pathogens; characterized by redness, swelling, pain, and increased temperature.

Inflorescence • A structure composed of several flowers.

Inflorescence meristem • A meristem that produces floral meristems as well as other small leafy structures (bracts).

Inhibitor • A substance that binds to the surface of an enzyme and interferes with its action on its substrates.

Inhibitory postsynaptic potential • A change in the resting potential of a postsynaptic membrane in the hyperpolarizing (negative) direction.

Initial cells • In plant meristems, undifferentiated cells that retain the capacity to divide producing both undifferentiated cells (initials) and cells committed to differentiation. (Compare with stem cells.)

Initiation complex • Combination of a ribosomal light subunit, an mRNA molecule, and the tRNA charged with the first amino acid coded for by the mRNA; formed at the onset of translation.

Initiation factors • Proteins that assist in forming the translation initiation complex at the ribosome.

Inositol triphosphate (IP₃) • An intracellular second messenger derived from membrane phospholipids.

Instar (in´ star) • An immature stage of an insect between molts.

Insulin (in´ su lin) [L. insula: island] • A hormone synthesized in islet cells of the pancreas that promotes the conversion of glucose into the storage material, glycogen.

Integral membrane protein • A membrane protein embedded in the bilayer of the membrane. (Contrast with peripheral membrane protein.)

Integrase • An enzyme that integrates retroviral cDNA into the genome of the host cell.

Integrated pest management • Control of pests by the use of natural predators and parasites in conjunction with sparing use of chemicals; an attempt to limit environmental damage.

Integument [L. integumentum: covering] • A protective surface structure. In gymnosperms and angiosperms, a layer of tissue around the ovule which will become the seed coat.

Intercalary meristem • A meristematic region in plants which occurs not apically, but between two regions of mature tissue. Intercalary meristems occur in the nodes of grass stems, for example.

Intercostal muscles • Muscles between the ribs that can augment breathing movements by elevating and suppressing the rib cage.

Interferon • A glycoprotein produced by virus-infected animal cells; increases the resistance of neighboring cells to the virus.

Interkinesis • The phase between the first and second meiotic divisions.

Interleukins • Regulatory proteins, produced by macrophages and lymphocytes, that act upon other lymphocytes and direct their development.

Intermediate filaments • Cytoskeletal component with diameters between the larger microtubules and smaller microfilaments.

Internode • The region between two nodes of a plant stem.

Interphase • The period between successive nuclear divisions during which the chromosomes are diffuse and the nuclear envelope is intact. It is during this period that the cell is most active in transcribing and translating genetic information.

Interspecific competition • Competition between members of two or more species. (Contrast with intraspecific competition.)

Intertropical convergence zone • The tropical region where the air rises most strongly; moves north and south with the passage of the sun overhead.

Intraspecific competition • Competition among members of the same species. (Contrast with interspecific competition.)

Intrinsic protein • A membrane protein that is embedded in the phospholipid bilayer of the membrane. (Contrast with extrinsic protein.)

Intrinsic rate of increaseThe rate at which a population can grow when its density is low and environmental conditions are highly favorable.

Intron • A portion of a DNA molecule that, because of RNA splicing, is not involved in coding for part of a polypeptide molecule. (Contrast with exon.)

Invagination • An infolding of cells during animal embryonic development.

Inversion • A rare 180° reversal of the order of genes within a segment of a chromosome.

Invertebrate • Any animal that is not a vertebrate, that is, whose nerve cord is not enclosed in a backbone of bony segments.

In vitro [L.: in glass] • In a test tube, rather than in a living organism. (Contrast with in vivo.)

In vivo [L.: in the living state] • In a living organism. Many processes that occur in vivo can be reproduced in vitro with the right selection of cellular components. (Contrast with in vitro.)

Ion (eye´ on) [Gr.: ion: wanderer] • An atom or group of atoms with electrons added or removed, giving it a negative or positive electrical charge.

Ion channel • A membrane protein that can let ions diffuse across the membrane. The channel can be ion-selective, and it can be voltage-gated or ligand-gated.

Ionic bond • An electrostatic attraction between positively and negatively charged ions. Usually a strong bond.

Iris (eye´ ris) [Gr. iris: rainbow] • The round, pigmented membrane that surrounds the pupil of the eye and adjusts its aperture to regulate the amount of light entering the eye.

Irruption • A rapid increase in the density of a population. Often followed by massive emigration.

Islets of Langerhans • Clusters of hormone-producing cells in the pancreas.

Iso- [Gr. isos: equal] • Prefix used two separate entities that share some element of identity.

Isogamous • Describes male and female gametes that are morphologically identical.

Isolating mechanism • Geographical, physiological, ecological, or behavioral mechanisms that lead to a reduction in the frequency of successful matings between individuals in separate populations of a species. Can lead to the eventual evolution of separate species.

Isomers • Molecules consisting of the same numbers and kinds of atoms, but differing in the bonding patterns by which the atoms are held together.

Isomorphic (eye so more´ fik) [Gr. isos: equal + morphe: form] • Having the same form or appearance, as when the haploid and diploid life stages of an organism appear identical. (Contrast with heteromorphic.)

Isotonic • Having the same solute concentration; said of two solutions. (Contrast with hypertonic, hypotonic.)

Isotope (eye´ so tope) [Gr. isos: equal + topos: place] • Isotopes of a given chemical element have the same number of protons in their nuclei (and thus are in the same position on the periodic table), but differ in the number of neutrons

Isozymes • Forms of an enzyme that have somewhat different amino acid sequences but catalyze the same reaction.