, y
og
ol
Bi
ll
be
turgor pressure - The force directed against a plant cell wall after the influx of water and
mp
swelling of the cell due to osmosis.
Ca
turnover - The mixing of waters as a result of changing water-temperature profiles in a
lake.
turnover time - The time required to replace the standing crop of a population or group
of populations (for example, of phytoplankton), calculated as the ratio of standing crop
to production.
twin study - A behavioral study in which researchers compare the behavior of identical
twins raised apart with that of identical twins raised in the same household.
tympanic membrane - Another name for the eardrum, the membrane between the outer
and middle ear.
uniformitarianism - The principle that mechanisms of change are constant over time.
Unikonta - One of five supergroups of eukaryotes proposed in a current hypothesis of
the evolutionary history of eukaryotes. This clade, which is supported by studies of
myosin proteins and DNA, consists of amoebozoans and opisthokonts.
unsaturated fatty acid - A fatty acid that has one or more double bonds between
carbons in the hydrocarbon tail. Such bonding reduces the number of hydrogen atoms
attached to the carbon skeleton.
urea - A soluble nitrogenous waste produced in the liver by a metabolic cycle that
combines ammonia with carbon dioxide.
ureter - A duct leading from the kidney to the urinary bladder.
urethra - A tube that releases urine from the mammalian body near the vagina in
females and through the penis in males; also serves in males as the exit tube for the
reproductive system.
uric acid - A product of protein and purine metabolism and the major nitrogenous waste
product of insects, land snails, and many reptiles. It is relatively nontoxic and largely
insoluble.
urinary bladder - The pouch where urine is stored prior to elimination.
uterine cycle - The changes that occur in the uterus during the reproductive cycle of the
human female; also called the menstrual cycle.
uterus - A female organ where eggs are fertilized and/or development of the young
occurs.
, y
og
ol
Bi
ll
be
mp
utricle - In the vertebrate ear, a chamber in the vestibule behind the oval window that
Ca
opens into the three semicircular canals.
vaccine - A harmless variant or derivative of a pathogen that stimulates a host's immune
system to mount defenses against the pathogen.
vacuole - A membrane-bounded vesicle whose specialized function varies in different
kinds of cells.
vagina - Part of the female reproductive system between the uterus and the outside
opening; the birth canal in mammals. During copulation, it accommodates the male's
penis and receives sperm.
valence - The bonding capacity of a given atom; usually equals the number of unpaired
electrons required to complete the atom's outermost shell.
valence electron - An electron in the outermost electron shell.
valence shell - The outermost energy shell of an atom, containing the valence electrons
involved in the chemical reactions of that atom.
van der Waals interactions - Weak attractions between molecules or parts of molecules
that result from transient local partial charges.
variation - Differences between members of the same species.
vas deferens - In mammals, the tube in the male reproductive system in which sperm
travel from the epididymis to the urethra.
vasa recta - The capillary system in the kidney that serves the loop of Henle.
vascular cambium - A cylinder of meristematic tissue in woody plants that adds layers of
secondary vascular tissue called secondary xylem (wood) and secondary phloem.
vascular plant - A plant with vascular tissue. This category includes all living plant
species except liverworts, mosses, and hornworts.
vascular tissue - Plant tissue consisting of cells joined into tubes that transport water
and nutrients throughout the plant body.
vascular tissue system - A transport system formed by xylem and phloem throughout a
vascular plant. Xylem transports water and minerals; phloem transports sugars, the
products of photosynthesis.
, y
og
ol
Bi
ll
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vasectomy - The cutting and sealing of each vas deferens to prevent sperm from
mp
entering the urethra.
Ca
vasocongestion - The filling of a tissue with blood, caused by increased blood flow
through the arteries of that tissue.
vasoconstriction - A decrease in the diameter of blood vessels caused by contraction of
smooth muscles in the vessel walls.
vasodilation - An increase in the diameter of blood vessels caused by relaxation of
smooth muscles in the vessel walls.
vector - An organism that transmits pathogens from one host to another.
vegetal pole - The point at the end of an egg in the hemisphere where most yolk is
concentrated; opposite of animal pole.
vegetative reproduction - Cloning of plants by asexual means.
vein - (1) In animals, a vessel that carries blood toward the heart. (2) In plants, a
vascular bundle in a leaf.
ventilation - The flow of air or water over a respiratory surface.
ventral - Pertaining to the underside, or bottom, of an animal with radial or bilateral
symmetry.
ventricle - (1) A heart chamber that pumps blood out of the heart. (2) A space in the
vertebrate brain, filled with cerebrospinal fluid.
venule - A vessel that conveys blood between a capillary bed and a vein.
vernalization - The use of cold treatment to induce a plant to flower.
vertebrates - A chordate animal with a backbone, including sharks and rays, ray-finned
fishes, coelacanths, lungfishes, amphibians, reptiles, and mammals.
vesicle - A membranous sac in the cytoplasm of a eukaryotic cell.
vessel - A continuous water-conducting micropipe found in most angiosperms and a few
nonflowering vascular plants.
vessel element - A short, wide water-conducting cell found in the xylem of most
angiosperms and a few nonflowering vascular plants. Dead at maturity, these are
aligned end to end to form micropipes called vessels.
og
ol
Bi
ll
be
turgor pressure - The force directed against a plant cell wall after the influx of water and
mp
swelling of the cell due to osmosis.
Ca
turnover - The mixing of waters as a result of changing water-temperature profiles in a
lake.
turnover time - The time required to replace the standing crop of a population or group
of populations (for example, of phytoplankton), calculated as the ratio of standing crop
to production.
twin study - A behavioral study in which researchers compare the behavior of identical
twins raised apart with that of identical twins raised in the same household.
tympanic membrane - Another name for the eardrum, the membrane between the outer
and middle ear.
uniformitarianism - The principle that mechanisms of change are constant over time.
Unikonta - One of five supergroups of eukaryotes proposed in a current hypothesis of
the evolutionary history of eukaryotes. This clade, which is supported by studies of
myosin proteins and DNA, consists of amoebozoans and opisthokonts.
unsaturated fatty acid - A fatty acid that has one or more double bonds between
carbons in the hydrocarbon tail. Such bonding reduces the number of hydrogen atoms
attached to the carbon skeleton.
urea - A soluble nitrogenous waste produced in the liver by a metabolic cycle that
combines ammonia with carbon dioxide.
ureter - A duct leading from the kidney to the urinary bladder.
urethra - A tube that releases urine from the mammalian body near the vagina in
females and through the penis in males; also serves in males as the exit tube for the
reproductive system.
uric acid - A product of protein and purine metabolism and the major nitrogenous waste
product of insects, land snails, and many reptiles. It is relatively nontoxic and largely
insoluble.
urinary bladder - The pouch where urine is stored prior to elimination.
uterine cycle - The changes that occur in the uterus during the reproductive cycle of the
human female; also called the menstrual cycle.
uterus - A female organ where eggs are fertilized and/or development of the young
occurs.
, y
og
ol
Bi
ll
be
mp
utricle - In the vertebrate ear, a chamber in the vestibule behind the oval window that
Ca
opens into the three semicircular canals.
vaccine - A harmless variant or derivative of a pathogen that stimulates a host's immune
system to mount defenses against the pathogen.
vacuole - A membrane-bounded vesicle whose specialized function varies in different
kinds of cells.
vagina - Part of the female reproductive system between the uterus and the outside
opening; the birth canal in mammals. During copulation, it accommodates the male's
penis and receives sperm.
valence - The bonding capacity of a given atom; usually equals the number of unpaired
electrons required to complete the atom's outermost shell.
valence electron - An electron in the outermost electron shell.
valence shell - The outermost energy shell of an atom, containing the valence electrons
involved in the chemical reactions of that atom.
van der Waals interactions - Weak attractions between molecules or parts of molecules
that result from transient local partial charges.
variation - Differences between members of the same species.
vas deferens - In mammals, the tube in the male reproductive system in which sperm
travel from the epididymis to the urethra.
vasa recta - The capillary system in the kidney that serves the loop of Henle.
vascular cambium - A cylinder of meristematic tissue in woody plants that adds layers of
secondary vascular tissue called secondary xylem (wood) and secondary phloem.
vascular plant - A plant with vascular tissue. This category includes all living plant
species except liverworts, mosses, and hornworts.
vascular tissue - Plant tissue consisting of cells joined into tubes that transport water
and nutrients throughout the plant body.
vascular tissue system - A transport system formed by xylem and phloem throughout a
vascular plant. Xylem transports water and minerals; phloem transports sugars, the
products of photosynthesis.
, y
og
ol
Bi
ll
be
vasectomy - The cutting and sealing of each vas deferens to prevent sperm from
mp
entering the urethra.
Ca
vasocongestion - The filling of a tissue with blood, caused by increased blood flow
through the arteries of that tissue.
vasoconstriction - A decrease in the diameter of blood vessels caused by contraction of
smooth muscles in the vessel walls.
vasodilation - An increase in the diameter of blood vessels caused by relaxation of
smooth muscles in the vessel walls.
vector - An organism that transmits pathogens from one host to another.
vegetal pole - The point at the end of an egg in the hemisphere where most yolk is
concentrated; opposite of animal pole.
vegetative reproduction - Cloning of plants by asexual means.
vein - (1) In animals, a vessel that carries blood toward the heart. (2) In plants, a
vascular bundle in a leaf.
ventilation - The flow of air or water over a respiratory surface.
ventral - Pertaining to the underside, or bottom, of an animal with radial or bilateral
symmetry.
ventricle - (1) A heart chamber that pumps blood out of the heart. (2) A space in the
vertebrate brain, filled with cerebrospinal fluid.
venule - A vessel that conveys blood between a capillary bed and a vein.
vernalization - The use of cold treatment to induce a plant to flower.
vertebrates - A chordate animal with a backbone, including sharks and rays, ray-finned
fishes, coelacanths, lungfishes, amphibians, reptiles, and mammals.
vesicle - A membranous sac in the cytoplasm of a eukaryotic cell.
vessel - A continuous water-conducting micropipe found in most angiosperms and a few
nonflowering vascular plants.
vessel element - A short, wide water-conducting cell found in the xylem of most
angiosperms and a few nonflowering vascular plants. Dead at maturity, these are
aligned end to end to form micropipes called vessels.
