trophic structure feeding relationships among
members of a community or ecosystem. (13.2)
tubular secretion a process that uses active
transport to draw substances, such as hydrogen
ions, creatinine, and drugs (e.g., penicillin) out of
the blood and into the filtrate of the kidney. (4.2)
uniformitarianism Charles Lyell’s theory that
geological processes repeat themselves. The key
to understanding geological processes of the past
lies in understanding geological processes of the
present. (10.2)
uracil (U) one of four nitrogenous bases in a
nucleotide that make up RNA. Uracil is not
found in DNA. (7.1)
urban sprawl a pattern of scattered, uncontrolled
development in which working, living, and
recreational areas are widely separated. (15.1)
ureters tubes that carry waste fluid (urine) from the
kidneys to the urinary bladder for temporary
storage. (4.2)
urethra the tube through which urine exits the
bladder (4.2)
valence electrons the electrons in the outermost
occupied shell, or energy level, of an atom in its
lowest energy state; valence electrons dictate the
chemical properties of the element. (1.1)
valence level the outermost energy level of an atom.
(1.1)
valence shell the outermost occupied shell or
energy level of an atom in its lowest energy state.
(1.1)
vasoconstriction decrease in the diameter of blood
vessels near the skin to conserve body heat.
Blood circulation is concentrated in the core of
the body to keep the major organs functioning.
(4.1)
vasodilation expansion of the diameter of blood
vessels near the skin to bring more blood to the
surface to help reduce body temperature. (4.1)
vasomotor centre part of the medulla oblongata
that adjusts blood pressure by controlling the
diameter of blood vessels. (5.1)
vestigial structures body parts that were functional
in a species’ ancestors, yet appear to have no
current function (e.g., pelvic bones in baleen
whales). (10.3)
vitreous humour the gel filling the compartment
behind the lens in the eye. The gel helps
maintain the shape of the eyeball. (5.3)wave of depolarization nervous impulse
transmitted along the axon of a nerve cell
(neuron). (5.1)
whole genome shotgun sequencing computer
program designed to sequence a large genome,
developed in 1996 by Craig Venter. The program
skips the genome mapping stage and breaks the
entire genome into random fragments of first
about 2000 and then about 1000 base pairs. The
program then sequences and analyzes these
fragments, and assembles nucleotide sequences
corresponding to chromosomes. (9.2)zero population growth situation in which the
population is not increasing or decreasing
because birth and death rates are approximately
equal and immigration and emigration rates are
approximately equal. (15.1)Z
W
V
U
Glossary • MHR 589