330 ❯ Glossary
permafrost Frozen layer of soil just underneath the
upper soil layer, found in the tundra biome.
peroxisome Organelle that functions to break down
fatty acids, and detoxify.
petals Structures that serve to attract pollinators.
PGAL (phosphoglyceraldehyde) Molecule important
to energy-producing reactions photosynthesis and
respiration.
phage A virus that infects bacteria.
phagocytes Immune cells (macrophages and
neutrophils) that use endocytosis to engulf and
eliminate foreign invaders.
pharynx Tube through which both food and air pass
after leaving the mouth.
phenotype The physical expression of the trait asso-
ciated with a particular genotype. Some examples
of the phenotypes for Mendel’s peas were round or
wrinkled, green or yellow, purple flower or white
flower.
phenylketonuria (PKU) An autosomal recessive dis-
ease caused by a single gene defect that leaves a
person unable to break down phenylalanine,
which results in a by-product that can accumu-
late to toxic levels in the blood and cause mental
retardation.
pheromones Chemical signals important to
communication.
phloem Important part of plant vascular tissue that
functions to transport sugars from their produc-
tion site to the rest of the plant.
phosphate group An acidic functional group that is
a vital component of molecules that serve as cellu-
lar energy sources: ATP, ADP, and GTP.
phospholipid Lipid with both a hydrophobic tail
anda hydrophilic head; the major component of
cell membranes with the hydrophilic phosphate
group forming the outside portion and the
hydrophobic tail forming the interior of the wall.
photoautotrophs Photosynthetic autotrophs that
produce energy from light.
photolysisProcess by which water is broken up by an
enzyme into hydrogen ions and oxygen atoms.
Occurs during the light reactions of photosynthesis.
photoperiodism The response by a plant to the
change in the length of days.
photophosphorylation Process by which ATP is
made during the light-dependent reactions of pho-
tosynthesis. It is the chloroplast equivalent of
oxidative phosphorylation.
photorespirationProcess by which oxygen com-
petes with carbon dioxide and attaches to RuBP.
Plants that experience photorespiration have a
lowered capacity for growth.
photosynthesisThe process by which plants gener-
ate energy from light and inorganic raw materials.
This occurs in the chloroplasts and involves two
stages: the light-dependent reactions and the light-
independent reactions.
photosystem Cluster of light-trapping pigments
involved in the process of photosynthesis.
phototaxis Reflex movement toward light at night.
phototropism A plant’s growth in response to light.
Auxin is the hormone involved with this process.
phycobilin Photosynthetic pigment.
phylogeny The evolutionary history of a species.
phytochromeImportant pigment in the process of
flowering. Leads to the production of florigen.
pigment A molecule that absorbs light of a particu-
lar wavelength.
pioneer species A species that is able to survive in
resource-poor conditions and takes hold of a
barren area such as a volcanic island. Pioneer
species do the grunt work, adding nutrients and
other improvements to the once-uninhabited vol-
canic rock until future species take over.
PKU Seephenylketonuria.
placenta In humans, this structure provides the
nutrients for the developing embryo.
planarians Free-living platyhelminthe carnivores
that live in the water.
plasma The liquid portion of the blood that con-
tains minerals, hormones, antibodies, and nutri-
tional materials.
plasma cells The factories that produce antibodies
that eliminate any cell containing on its surface the
antigen that the plasma cell has been summoned
to kill.
plasma membrane Selective barrier around a cell
composed of a double layer of phospholipids that
controls what is able to enter and exit a cell.
plasmids Extra circles of DNA in bacteria that
contain just a few genes and have been useful in
genetic engineering. Plasmids replicate indepen-
dently of the main chromosome.
plasmodial slime molds Nonphotosynthetic het-
erotrophic funguslike protists. They eat and grow
as a unified clumped unicellular mass known as
aplasmodium.