induced by gravitational pull; hydrotropism is a
response to water gradients; and thigmotropism is a
response to touch.
troposphere The lowest layer of the atmosphere. It
ranges in thickness from 8 km at the poles to 16 km
over the equator and is bounded above by the
tropopause, a boundary marked by stable tempera-
tures. The troposphere is the layer where most of the
world’s weather takes place.
Trouton’s rule At the normal boiling temperature,
the entropy of vaporization is constant.
tunneling The process by which a particle or a set of
particles crosses a barrier on its POTENTIAL-ENERGY
SURFACEwithout having the energy required to sur-
mount this barrier. Since the rate of tunneling decreases
with increasing reduced mass, it is significant in the
context of ISOTOPE EFFECTSof hydrogen isotopes.
turgor pressure Like air pressure in a car tire, it is
the outward pressure that is exerted against the inside
surface of a plant cell wall under the conditions of
water flowing into the cell by osmosis, and the result-
ing resistance by the cell wall to further expansion.Tyndall effect Light scattering by colloidal particles.type 1,2,3 copper Different classes of copper-BIND-
ING SITEs in proteins, classified by their spectroscopic
properties as Cu(II). In type 1, or BLUE COPPERcenters,
the copper is COORDINATEDto at least two imidazole
nitrogens from histidine and one sulfur from cysteine.
They are characterized by small copper HYPERFINEcou-
plings and a strong visible absorption in the Cu(II)
state. In type 2, or non-blue copper sites, the copper is
mainly bound to imidazole nitrogens from histidine.
Type 3 copper centers comprise two spin-coupled cop-
per ions bound to imidazole nitrogens.tyrosinase A copper protein containing an antiferro-
magnetically coupled DINUCLEAR copper unit (TYPE
3–like site) that oxygenates the tyrosine group to cate-
chol and further oxidizes this to the quinone.tyrosine kinase(PTK) Protein enzymes that modu-
late a wide variety of cellular events, including differen-
tiation, growth, metabolism, and apoptosis. Protein
kinases add phosphate groups to proteins. Enzymes
that add phosphate groups to tyrosine residues are
called protein tyrosine kinases. These enzymes have
important roles in signal transduction and regulation of
cell growth, and their activity is regulated by a set of
molecules called protein tyrosine phosphatases that
remove the phosphate from the tyrosine residues. A
tyrosine kinase is an enzyme that specifically phospho-
rylates (attaches phosphate groups to) tyrosine residues
in proteins and is critical in T- and B-cell activation.tyrosine kinase receptor Proteins found in the
plasma membrane of the cell that can phosphorylate
(attach phosphate groups to) a tyrosine residue in a
protein. Insulin is an example of a hormone whose
receptor is a TYROSINE KINASE. Following binding of
the hormone, the receptor undergoes a conformational
change, phosphorylates itself, then phosphorylates a
variety of intracellular targets.270 troposphere
Triple point. Temperature and pressure at which the three phases
of a substance are in equilibrium