oxygen One of the most important elements for
biological systems and for other processes, such as
reacting with other substances to release energy. One
tree can produce enough oxygen in one week to meet
the demands of a person’s daily oxygen need. Oxygen
is needed in oxidation-reduction reactions within
cells. Cellular respiration is the process that releases
energy by breaking down food molecules in the pres-
ence of oxygen. Atomic symbol is O; atomic number
is 8.
oxygen-evolving complex (OEC) The ENZYME
that catalyzes the formation of O 2 in PHOTOSYNTHE-
SIS. Contains a CLUSTERof probably four manganese
ions.
ozone(O 3 ) A form of oxygen containing three atoms
instead of the common two; formed by ultraviolet radia-
tion reacting with oxygen. Ozone accounts for the dis-
tinctive odor of the air after a thunderstorm or around
electrical equipment, first reported as early as 1785;
ozone’s chemical constitution was established in 1872.
The ozone layer in the upper atmosphere blocks harmful
ultraviolet radiation that normally causes skin cancer.
Ozone is an oxidizer and a disinfectant, and it forms
hydrogen peroxide when mixed with water. The Earth’s
ozone layer protects all life from the sun’s harmful radia-
tion, but human activities have damaged this shield. The
United States, in cooperation with over 140 other coun-
tries, is phasing out the production of ozone-depleting
substances in an effort to safeguard the ozone layer.
See alsoOXYGEN.258 oxygen