336 ❯ Glossary
together when excess exposure to UV light occurs.
This can negatively affect replication of DNA and
assist in the creation of further mutations.
thymosin Hormone involved in the development of
the T cells of the immune system.
thyroid-stimulating hormone (TSH) A hormone that
stimulates the synthesis and secretion of thyroid
hormones, which regulate the rate of metabolism
in the body.
thyroxin Hormone released by the thyroid gland
that functions in the control of metabolic activities
in the body.
tongue Structure that functions to move food
around while we chew and helps to arrange the
food into a swallowable bolus.
trachea The tunnel that leads air into the thoracic
cavity.
tracheid cells Xylem cells in charge of water trans-
port in gymnosperm.
tracheophytes Vascular plants.
transcription factors Helper proteins that assist
RNA polymerase in finding and attaching to the
promoter region.
transduction The movement of genes from one cell
to another by phages.
transformation The transfer of genetic material
from one cell to another, resulting in a genetic
change in the receiving cell.
translocationMovement of the ribosome along the
mRNA in such a way that the A site becomes the
P site and the next tRNA comes into the new
A site carrying the next amino acid.
translocation (plants) Movement of carbohydrates
through the phloem.
transpiration Process by which plants lose water by
evaporation through their leaves.
trichinosis Disease found in humans caused by a
roundworm that infects meat products.
trophic levels Hierarchy of energy levels that
describe the energy distribution of a planet.
trophoblast Forms the placenta for the developing
fetus, and aids in attachment to the endometrium.
This structure also produces human chorionic
gonadotropin (hCG), which maintains the
endometrium by ensuring the continued produc-
tion of progesterone.
tropical forestsThese forests consist primarily of
tall trees that form a thick cover, which blocks the
light from reaching the floor of the forest (where
there is little growth). Tropical rainforests are
known for their rapid recycling of nutrients and
contain the greatest diversity of species.
tropism Plant growth that occurs in response to an
environmental stimulus such as sunlight or gravity.
tropomyosin Regulatory protein known to block the
actin–myosin binding site and prevent muscular
contraction in the absence of calcium.
trypsinEnzyme that cuts protein bonds in the small
intestine.
TSHSeethyroid-stimulating hormone.
tundraThis biome experiences extremely cold win-
ters during which the ground freezes completely.
Short shrubs or grasses that are able to withstand
the difficult conditions dominate.
Turner syndrome Affects females who are missing
an X chromosome.
umbilical cordStructure that transports oxygen,
food, and waste (CO 2 ) between the embryo and
the placenta.
uniform distribution Scenario in which individuals
are evenly spaced out across a given geographic area.
unsaturated fatFat that contains one or more
double bonds; found in plants.
uracilThe nucleotide that replaces thymine in RNA.
urethra Exit point for both urine and sperm from
males and urine for females.
uterus Site of embryo attachment and development
in mammals.
vaccinationInoculation of medicine into a patient
in an effort to prime the immune system to be pre-
pared to fight a specific sickness if confronted in
the future.
vacuole A storage organelle that is large in plant cells
but small in animal cells.
vascular cambium A cylinder of tissue that extends
the length of the stem and root and gives rise to
the secondary xylem and phloem.
vascular cylinder Structure in plants that is com-
posed of cells that produce the lateral roots of the
plant.
vas deferens Tunnel that connects the epididymis
to the urethra.
vector Agent that moves DNA from one source to
another.
veinsStructures that return blood to the heart.
vena cava system System of veins that returns
deoxygenated blood from the body to the heart to
be reoxygenated in the lungs.