Hydrolysis
A chemical process that lyses or splits molecules by the addition of water; an essential
process in digestion.
Hydrogen ion
A single proton with a charge of +1. The dissociation of a water molecule (H 2 O) leads to the
generation of a hydroxide ion (OH –) and a hydrogen ion (H +).
Hydrophilic
Having an affinity for water.
Hydrophobic
Having an aversion to water; tending to coalesce and form droplets in water.
Hydrostatic skeleton
A skeletal system composed of fluid held under pressure in a closed body compartment; the
main skeleton of most cnidarians, flatworms, nematodes, and annelids.
Hydroxyl group
A functional group consisting of a hydrogen atom joined to an oxygen atom by a polar
covalent bond. Molecules possessing this group are soluble in water and are called alcohols.
Hyperpolarization
An electrical state whereby the inside of the cell is made more negative relative to the
outside than at the resting membrane potential. A neuron membrane is hyperpolarized if a
stimulus increases its voltage from the resting potential of –70 mV, reducing the chance that
the neuron will transmit a nerve impulse.
Hypertonic solution
A solution with a greater solute concentration than another, a hypotonic solution.
Hypha
A filament that collectively makes up the body of a fungus.
Hypodermis
An epidermal layer of cells that secretes an overlying chitinous cuticle, as in arthropods.
Hypogynous
Having the floral parts, such as sepals, petals, and stamens, borne on the receptacle
beneath the ovary.
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