order A taxonomic grouping between class and fami-
ly. The order consists of groups that are more alike
than those in a class.
See alsoTAXON.
organ A specialized combination of two or more dif-
ferent tissues that performs a particular function. Sever-
al organs work together in an organ system to perform
a set of coordinated functions. There are ten major
organ systems in the human body, and each system is
made up of one or more organs. These systems include
the circulatory, digestive, endocrine, integumentary,
muscular, nervous, reproductive, respiratory, skeletal,
and urinary systems.
organelle The “organs” of the cell. Any membrane-
bound or nonmembrane-bound structure that is spe-
cialized in performing a specific role in the cell.
Examples of organelles are chloroplasts, centrosomes,
Golgi apparatus, ribosomes, mitochondria, and even
the nucleus.
organic chemistry The study of carbon (organic)
compounds; used to study the complex nature of living
things. Organic compounds are composed mostly of
carbon, hydrogen, oxygen, and nitrogen atoms bonded
together. One of two main divisions of chemistry. The
other is inorganic chemistry. Branches of these two
include analytical, biochemical, and physical chemistry.
organism A living entity.
organ of Corti The organ within the cochlea that
contains thousands of hairlike receptor cells that
respond to different sound frequencies and convert
them to nerve impulses through the auditory nerve to
the brain.
organogenesis The formation and advanced period
of embryonic development of plants and animals when
organs are formed from the primary germ layer. Its
study is called organogeny.
orgasm An involuntary and rhythmic contraction
characterized by strong feelings of pleasure by both
sexes during human sexual activity; the highest point of
sexual excitement.orphan drug ADRUGfor the treatment of a rare dis-
ease for which reasonable recovery of the sponsoring
firm’s research and development expenditure is not
expected within a reasonable time. The term is also
used to describe substances intended for such uses.osmoconformer Not actively changing internal
osmolarity (total solute concentration) because an ani-
mal is isotonic (body fluids are of equal concentration
with respect to osmotic pressure) with the environment.osmolarity Solute concentration expressed as molar-
ity. Molarity is the moles of solute dissolved in 1 liter
of solution. A solute is, in the case of a gas or solid dis-
solved in liquid, the gas or solid, but in other examples
it is the part that has the smaller amount. For example,
a beaker of salt water would have the water as the sol-
vent and the salt as the solute. The mole is defined as
the number of carbon 12 atoms in 12 grams of carbon- It allows scientists to weigh substances and tell how
many particles are in that substance.
See alsoMOLARITY.
osmoregulation A process to control water balance
in a cell or organism with respect to the surrounding
environment using osmosis. The ability by which
organisms maintain a stable solute concentration by
maintaining osmotic pressure on each side of a
semipermeable membrane.osmoregulator An organism that must take in or
discharge excess water because its body fluids have a
different osmolarity than the environment.osmosis The diffusion or movement of water across
a selectively permeable membrane from one aqueous
system to another of different concentration. Waterosmosis 255