A1hydrolyzes the acyl group attached to the 1-posi-
tion, while phospholipase A2 hydrolyzes the acyl group
attached to the 2-position.
phospholipases A class of enzymes that catalyze the
hydrolysis of phosphoglycerides or glycerophosphati-
dates.
phospholipids The main component of cell walls; an
amphiphilic molecule (lipid). A glycerol skeleton is
attached to two fatty acids and a phosphate group, and
onto the phosphate is attached one of three nitrogen
groups, so both phosphate and nitrogen groups make
the “polar head” larger and more polar. The phosphate
part of the molecule is water soluble, while the fatty
acid chains are fat soluble. The phospholipids have a
polar hydrophilic head (phosphate) and nonpolar
hydrophobic tail (fatty acids). When in water, phospho-
lipids sort into spherical bilayers; the phosphate groups
point to the cell exterior and interior, while the fatty
acid groups point to the interior of the membrane.
Examples include lecithin, cephalin and sphin-
gomyelin, phosphatidic acid, plasmalogen. Two types
of phospholipids exist: glycerophospholipid and sphin-
gosyl phosphatide. A synthetic phospholipid,
alkylphosphocholine, has been used in biological and
therapeutic areas.
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 link-
ageis formed, for example: ATP + alcohol ÔADP +
phosphate ester
photic 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 zone and much of the sublit-
toral zone fall within the photic zone.
photoautotroph Anorganism that uses sunlight to
provide energy and uses carbon dioxide as the chief
source of carbon, such as photosynthetic bacteria,
cyanobacteria, algae. Green plants are photoau-
totrophs.photoheterotroph Like photoautotrophs, any organ-
ism 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 ener-
gy emitted in the form of electromagnetic radiation. A
particle of light and gamma and X rays are examples.photoperiodism The physiological response to
length of day and night in a 24-hour period, such as
the flowering or budding in plants.photophosphorylation The process of creating
adenosine 5-triphosphate (ATP) from ADP and phos-
phate by using the energy of the sun. Takes place in the
thylakoid membrane of chloroplasts.photorespiration A process that decreases photo-
synthesis and occurs when carbon dioxide levels are
low inside a plant’s leaves; a process that uses oxygen,
releases carbon dioxide, and generates no ATP in the
process. Occurs on hot days when oxygen concentra-
tions in leaves exceed carbon dioxide levels due to
closed stomata.photosynthesis A metabolic process in plants and
certain bacteria that uses light energy absorbed by
CHLOROPHYLLand other photosynthetic pigments for
the reduction of CO 2 , followed by the formation of
organic compounds.
See alsoMETABOLISM;PHOTOSYSTEM.266 phospholipases