25.12: Glossary: T

T3 (triiodothyronine)

endocrine hormone secreted by the thyroid gland that increases the rate of metabolism in cells throughout the body

T4 (thyroxine)

endocrine hormone secreted by the thyroid gland that increases the rate of metabolism in cells throughout the body

target cell

type of cell on which a particular hormone has an effect because it has receptor molecules for the hormone

taste bud

small structure on the tongue containing chemoreceptor cells that sense chemicals in food

TATA box

regulatory element that is part of the promotor of most eukaryotic genes

taxon (plural, taxa)

level of organization in a biological classification system such as the Linnaean system; for example, species or genus

taxonomy

science of classifying organisms

T cell

type of lymphocyte that kills infected or cancerous cells (killer T cell) or helps regulate the immune response (helper T cell)

teething

emergence of deciduous (baby) teeth during infancy and early childhood

telomere

region of repetitive nucleotide sequences at the end of a chromosome that becomes shorter each time a cell divides

telophase

last stage of mitosis during which chromosomes uncoil to form chromatin, the spindle breaks down, and new nuclear membranes form

temporal lobe

part of each hemisphere of the cerebrum that controls hearing, memories, and sensations

tendinitis

inflammation of a tendon when it is over-extended or worked too hard without rest

tendon

tough connective tissue that attaches skeletal muscle to bones

terrestrial biome

any land-based biome

testicular cancer

cancer of the testes, which is relatively common in young men

testis (plural, testes)

one of two male reproductive organs that produce sperm and secrete testosterone; male gonad

testosterone

male sex hormone secreted mainly by the testes

thalamus

part of the inner brain that is a major hub for nerve impulses traveling back and forth between the cerebrum and spinal cord

thalassemia

type of inherited anemia caused by abnormal hemoglobin that is a genetic adaptation to malaria in the Mediterranean region, parts of Africa, and South and Southeast Asia

thermal pollution

pollution by heat energy, typically occurring when heated water is discharged into a river or lake

thermoreceptor

type of sensory receptor that senses temperature

thoracic cavity

body cavity in the chest that holds the lungs and heart

thrifty gene hypothesis

idea that “thrifty genes” evolved in some human populations because they allowed people to burn fewer Calories and store the rest as body fat, which would help them survive famines

thymus

organ of the lymphatic system where lymphocytes called T cells mature

thyroid gland

large endocrine gland in the neck whose hormones control the rate of cellular metabolism and help maintain calcium homeostasis

tissue

group of cells, generally of the same kind, that perform a particular function in an organism

toddlerhood

first part of early childhood that covers the ages 1 to 3 years

tonsil

one of four paired secondary lymphatic organs that encircle the inner throat and filter pathogens out of lymph

touch

ability to sense pressure, vibration, temperature, pain, and other tactile stimuli

toxoplasmosis

common parasitic disease that may be a selective agent maintaining both Rhesus-positive (Rh⁺) and Rhesus-negative (Rh^-) blood type alleles in human populations

trachea

tubular organ of the respiratory system that carries air between the larynx and bronchi; also called windpipe

transcription

process in which genetic instructions in DNA are copied to form a complementary strand of mRNA

trans fat

type of harmful, artificial fat that is added to some foods to help preserve freshness and improve taste

transfer RNA (tRNA)

type of RNA that brings amino acids to ribosomes where they are joined together to form proteins

transgenic crop

crop that has been genetically modified with new genes that code for traits useful to humans

translation

process in which genetic instructions in mRNA are “read” to synthesize a protein

transpiration

process in which plants give off water vapor through tiny pores, called stomata, in their leaves

transport protein

protein in a cell membrane that helps other substances cross the membrane to enter or leave the cell

tribe

taxon between the subfamily and genus in the Linnaean system of classification

trichomoniasis

common, sexually transmitted infection that is caused by the protozoan parasite Trichomonas vaginalis

triglyceride

type of lipid that contains glycerol and makes up body fat in animals

trimester

one of three, approximately three-month periods into which a pregnancy is divided

trophic level

feeding position in a food chain or food web, such as producer, primary consumer, or secondary consumer

trophoblast

outer cell layer of a blastocyst that implants in the uterus and eventually develops into the fetal portion of the placenta

tubal ligation

surgical sterilization procedure in females in which the Fallopian tubes are blocked so sperm cannot reach and fertilize an egg

tumor

abnormal mass of cells that may be cancerous

tumor-suppressor gene

gene that normally inhibits division of abnormal cells and may cause cancer if it mutates

type 1 diabetes

autoimmune disorder in which the immune system destroys insulin-secreting beta cells in the pancreas, leading to loss of glucose control and high levels of blood glucose

type 2 diabetes

multifactorial disorder in which a combination of insulin resistance and impaired insulin production lead to loss of glucose control and high levels of blood glucose

typology

system of discrete categories, such as races, that are sometimes used for classifying variation