molecule is attracted to the partially negative atom of a
polar covalent bond in another.
hydrogen ion(hydron) A single proton with a
charge of +1.
See alsoHYDRON;ION.
hydrolase An ENZYMEof EC class 3, also known
as a hydro-LYASE, that catalyzes the HYDROLYSISof a
SUBSTRATE.
See alsoEC NOMENCLATURE FOR ENZYMES.
hydrolysis SOLVOLYSISby water.
hydron General name for the ion H+, either in natu-
ral abundance or where it is not desired to distinguish
between the isotopes, such as proton for^1 H+, deuteron
for^2 H+, and triton for^3 H+.
hydrophilic “Water loving.” The capacity of a
molecular entity or of a substituent to interact with
polar solvents, in particular with water, or with other
polar groups. Hydrophilic molecules dissolve easily in
water, but not in fats or oils.
hydrophilicity The tendency of a molecule to be sol-
vated by water.
hydrophobic A molecule or substance that does not
associate, bond, or dissolve in water. Hydrophobic
molecules dissolve easily in fats and oils.
hydrophobic interaction The tendency of hydrocar-
bons (or of lipophilic hydrocarbonlike groups in
solutes) to formintermolecular aggregates in an aque-
ous medium as well as analogous intramolecular inter-
actions. The name arises from the attribution of the
phenomenon to the apparent repulsion between water
and hydrocarbons. Use of the misleading alternative
term hydrophobic bondis discouraged.
hydrophobicity The association of nonpolar groups
or molecules in an aqueous environment that arises
from the tendency of water to exclude nonpolar
molecules.
See alsoLIPOPHILICITY.hydrostatic skeleton Askeletal system created by
the pressure caused by fluid-filled closed areas that sup-
port rigidity in an organism or one of its parts. Many
invertebrates have hydrostatic skeletons. Earthworms
are an example.hydroxyl group A functional group that has a
hydrogen atom joined to an oxygen atom by a polar
covalent bond (–OH). When put in solution with
water, they form alcohols.hydroxyl ion (–OH). One atom each of oxygen and
hydrogen bonded into an ion that carries a negative
charge.
See alsoION.Hymenoptera Alarge order of insects having two
pairs of membranous wings (hymen means “mem-
brane”) coupled by a row of tiny hooks. Examples
include ants, bees, sawflies, and wasps.hyperactive A state of excessive muscular activity or
a condition when a particular portion of the body is
excessively active, e.g., a gland that produces too much
of its particular hormone. Often referred to in attention
deficit hyperactivity disorder (ADHD) in children.hyperendemic Disease organisms that exist in a host
population at very high rates. The human papillo-
mavirus is a large group of viruses that are hyperen-
demic in humans. They cause common warts, such as
plantar and genital warts.hyperfine SeeELECTRON PARAMAGNETIC RESONANCE
SPECTROSCOPY.hyperfine 169