trace elements Elements required for physiological
functions in very small amounts that vary for different
organisms. Included among the trace elements are Co,
Cu, F, Fe, I, Mn, Mo, Ni, Se, V, W, and Zn. Excessive
mineral intake can produce toxic symptoms.
trachea A linear portion of the respiratory tract that
connects the larynx to the bronchial tubes, forming an
inverted Y; part of it is in the throat, and the other is in
the chest. Called the windpipe.
tracheae Tiny tubes that branch throughout the
body cavity of insects that carry air and exchange
gases.
tracheal system The part of the insect’s respiratory
system composed of the tracheae and tracheole (small
end tubes formed within single cells of the tracheal
epithelium); opens to the outside of the body through
spiracles.tracheid A thick-walled cell of the xylem tissue that
is water conducting and is thickened and hardened by
lignin; also helps in supporting the plant.trans In inorganic nomenclature, a structural prefix
designating two groups directly across a CENTRAL
ATOMfrom each other (not generally recommended for
precise nomenclature purposes of complicated systems).
See also CIS.transcription The process by which the genetic
information encoded in a linear SEQUENCE of
NUCLEOTIDESin one strand of DNA is copied into an
exactly complementary sequence of RNA.
See alsoGENETIC CODE.transcription 327Scanning electron micrograph (SEM) of the internal lining, or epithelium, of the human trachea. The numerous fine hairlike projections,
called cilia, protrude from the tops of specialized epithelial cells. Between the epithelial cells lie goblet cells, whose function is to
release mucus onto the surface of the epithelium. Rhythmic movements of the cilia serve to move bacteria and other particles caught in
the mucus away from the gas-exchanging parts of the lungs, up the trachea, and toward the throat, where they can be swallowed or
coughed up. Magnification: ×4000 at 10 ×8 inch size.(Courtesy © E. Gray/Photo Researchers, Inc.)