century German physicist Daniel Gabriel FAHRENHEIT.
The Celsius or centigrade temperature scale is based on
0 for the freezing point of water and 100 for the boil-
ing point of water. This scale was invented in 1742 by
the Swedish astronomer Anders CELSIUS. It was once
called the centigrade scale because of the 100-degree
interval between the defined points. The Kelvin temper-
ature scale is the base unit of thermodynamic tempera-
ture measurement and is defined as 1/273.16 of the
triple point (equilibrium among the solid, liquid, and
gaseous phases) of pure water. The Kelvin is also the
base unit of the Kelvin scale, an absolute-temperature
scale named for the British physicist William Thomson,
Baron Kelvin. Such a scale has its zero point at abso-
lute zero, the theoretical temperature at which the
molecules of a substance have their lowest energy.
tendon (sinew) A fibrous connective tissue that
attaches muscles to bones; a collagen-rich substance.
tendril A slender outgrowth of a plant stem that can
clasp or wind around and help support climbing plants.
tepuis Aunique ecological region in South America
(Venezuela, Guyana, Suriname, and Brazil) of about
19,000 square miles, famous for isolated mountains,
each called a tepui, with flat tops covered with species
that may not exist elsewhere. Thirty three percent of
the 2,300 vascular plants found there are endemic.
teratogen A substance that produces a malformation
in a fetus.
territory An area within the living range of an indi-
vidual animal that it will defend against intruders over
food or protection of its young.
tertiaryconsumer Partof the food chain in which
the tertiary consumer (e.g., hawk) eats the secondary
consumer (e.g., snake), which eats the primary con-
sumer (e.g., mouse), which eats a producer (e.g., grass).
Carnivores that eat mainly other carnivores.
Tertiary period The first period of the Cenozoic era
(after the Mesozoic era and before the Quaternary peri-
od), extending from 65 million to 1.8 million years ago.
See alsoGEOLOGICAL TIME.tertiary structure The overall three-dimensional
structure of a BIOPOLYMER. For proteins, this involves
the side-chain interactions and packing of SECONDARY
STRUCTUREmotifs. For NUCLEIC ACIDS, this can be the
packing of stem loops or supercoiling of DOUBLE
HELIXES.testcross A cross of an individual with a potentially
ambiguous or unknown genotype to a homozygous
recessive individual.testis(testes; testicle) The male reproductive organ,
gonad, where sperm is and hormones (testosterone) are
produced; located inside the scrotum, behind and
below the penis.
See alsoSPERM.testosterone Male androgen hormone that regulates
male sexual characteristics along with dihydrotestos-
terone. Testosterone is converted to dihydrotestos-
terone by the enzyme 5-alpha-reductase. Testosterone is
converted to estradiol (the most potent estrogen,
formed in both the ovary and the testes) by the enzyme
17-ketoreductase.tetanus(lockjaw) A disease caused by the release of
exotoxins by a bacterium (clostridium tetani) that caus-
es muscular spasm, contraction, and convulsions;
obtained through a contaminated wound, e.g., from
stepping on a rusting nail. Also any tense muscle con-
traction by rapid and frequent action potentials (like
electric shocks). Approximately 30 percent of reported
cases of tetanus end in death. Tetanus kills 300,000
newborns and 30,000 birth mothers worldwide, from
lack of immunization.tetrahedron SeeCOORDINATION.tetrahedron 323