Science / Biology Glossary
Huntington Disease: A progressive and fatal disorder of the nervous system that develops between the ages of 30 and 50 years; caused by an expansion of a trinucleotide repeat and inherited as a dominant trait.
Hydrogen Bond: A weak bond between two atoms (one of which is hydrogen) with partial but opposite electrical charges.
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 Earths surface.
Hydrostatic Skeleton: Fluid-filled closed chambers that give support and shape to the body in organisms such as jellyfish and earthworms. No to be confused with the water-vascular system of echinoderms.
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.: hypha).
Hypothalamus: A region in the brain beneath the thalamus; consists of many aggregations of nerve cells and controls a variety of autonomic functions aimed at maintaining homeostasis.
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 hemisphere experienced several episodes of continental glacial advance and retreat along with a cli . . . View Full Definition
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 the joint from moving; e.g., the bones of the cranium.
Immune System: One of the eleven major body organ systems in vertebrates; defends the internal environment against invading microorganisms and viruses and provides defense against the growth of cancer cells.
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 homozygous parents.
Incomplete Flower: Condition in which one or more typical flower parts are absent. Example: grass flowers such as corn tassels which are male.
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 the respiratory, digestive, or urinary system; characterized by four signs: redness, swellin . . . View Full Definition
Inflammatory Response: The bodys reaction to invading infectious microorganisms; includes an increase in blood fiow to the affected area, the release of chemicals that draw white blood cells, an increased fiow of . . . View Full Definition
Ingestive Feeders: Animals that ingest food through a mouth.