Structurally, phospholipids are similar to triglyc-
erides except that a phosphate group replaces one of
the fatty acids.
phosphorylation A process involving the transfer of
a phosphate group (catalyzed by ENZYMEs) from a
donor to a suitable acceptor; in general an ester linkage
is formed, for example:
ATP + alcohol →ADP + phosphate esterphotic zone The upper layer within bodies of water
reaching down to about 200 meters, where sunlight
penetrates and promotes the production of photosyn-
thesis; the richest and most diverse area of the ocean. A
region where photosynthetic floating creatures (phyto-
plankton) are primary producers as well as a major
food source. The LITTORAL ZONEand much of the sub-
littoral zone falls within the photic zone.
See alsoAPHOTIC ZONE; OCEANIC ZONE.
photoautotroph An organism that uses sunlight to
provide energy and carbon dioxide as the chief source
of carbon, such as photosynthetic bacteria, cyanobacte-
ria, and algae. Green plants are photoautotrophs.
See alsoPHOTOHETEROTROPH.
photochemical oxidants Photochemically produced
oxidizing agents capable of causing damage to plants
and animals. Formed when sunlight reacts to a mixture
of chemicals in the atmosphere.
See alsoPHOTOCHEMICAL SMOG.
photochemical smog A brownish-colored smog in
urban areas that receive large amounts of sunlight;
caused by photochemical (light-induced) reactions from
nitrogen oxides, hydrocarbons, and other components
of polluted air that produce PHOTOCHEMICAL OXI-
DANTS.
photochemistry The branch of chemistry concerned
with the chemical effects of light (far UV to IR).
photodissociation The splitting of a molecule by a
photon.photoelectric effect The emission of electrons from
the surface of a metal caused by light hitting the sur-
face.photoheterotroph Like PHOTOAUTOTROPHs, any
organism that uses light as a source of energy but must
use organic compounds as a source of carbon, for
example, green and purple nonsulfur bacteria.photolysis A light-induced bond cleavage. The term
is often used incorrectly to describe irradiation of a
sample.photon Name given to a quantum or packet of
energy emitted in the form of electromagnetic radia-
tion. A particle of light, gamma and X rays are exam-
ples.photoperiodism The physiological response to
length of day and night in a 24-hour period, such as
flowering or budding in plants.photophosphorylation The process of creating
ADENOSINE-5′-TRIPHOSPHATE (ATP) from ADP and
phosphate by using the energy of the sun. Takes place
in the thylakoid membrane of chloroplasts.photosynthesis A METABOLICprocess in plants and
certain bacteria, using light energy absorbed by
CHLOROPHYLLand other photosynthetic pigments for
the reduction of CO 2 , followed by the formation of
organic compounds.
See alsoPHOTOSYSTEM.photosystem A membrane-bound protein complex
in plants and photosynthetic bacteria, responsible for
light harvesting and primary electron transfer. Com-212 phosphorylation