Encyclopedia of Chemistry

carbon, hydrogen, oxygen, and nitrogen atoms bonded
together. Organic chemistry is one of two main divi-
sions of chemistry, the other being INORGANIC CHEM-
ISTRY. Branches of these two include analytical,
biochemical, and physical chemistry.

organism A living entity.

organochlorine compounds (chlorinated hydrocar-
bons) Organic pesticides that contain chlorine, car-
bon, and hydrogen, e.g., DDT and endrin. These
pesticides affect the central nervous system.

organophosphorus compound A compound con-
taining phosphorus and carbon, whose physiological
effects include INHIBITIONof acetylcholinesterase. Many
pesticides and most nerve agents are organophosphorus
compounds, e.g., malathion and parathion.

orphan drug A DRUGfor 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 The concentration of a solution in terms
of osmols per liter. An osmol is the amount of sub-
stance that dissociates in solution to form one mole of
osmotically active particles. For example, a mole of
nonelectrolyte forms one osmol of solute, but a sub-
stance that dissociates such as NaCl forms 2 osmols of
solute. Mostly used in medicine.

osmoregulation A process to control water balance
in a cell or organism with respect to the surrounding
environment using osmosis; the ability by which organ-

isms maintain a stable solute concentration by main-

taining osmotic pressure on each side of a semiperme-

able 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 concentrations. Water

moves from areas of high-water/low-solute concentra-

tion to areas of low-water/high-solute concentration.

osmotic pressure Pressure that is generated by a

solution moving by osmosis into and out of a cell and

caused by a concentration gradient.

Ostwald process An industrial process that pro-

duces oxide and nitric acid from ammonia and oxygen.

outer orbital complex Valence bond designation for

a complex in which the metal ion utilizes dorbitals in

the occupied or outermost shell in hybridization.

outer orbital complex 203

Osmosis. The diffusion or movement of water across a selectively

permeable membrane from one aqueous system to another of

different concentrations