photosystem A membrane-bound protein complex
in plants and photosynthetic bacteria, responsible for
light harvesting and primary electron transfer. Com-
prises light-harvesting pigments such as CHLOROPHYLL;
a primary electron-transfer center and a secondary
electron carrier. In green-plant photosynthesis, photo-
system I transfers electrons from PLASTOCYANIN to
a [2FE-2S] FERREDOXIN and contains IRON-SULFUR
PROTEINS. Photosystem II transfers electrons from the
OXYGEN-EVOLVING COMPLEX to plastoquinone and
contains an iron center.
See alsoPHOTOSYNTHESIS.
phototropism Growth movement by plants that is
induced by light. If growth is toward the light source it
is called positive phototropism; if it is away from the
source it is termed negative phototropism.
pH scale The concentration of hydrogen ions in a
solution, based on a scale from 0 to 14. Low pH corre-
sponds to high hydrogen ion concentration, and high
pH refers to low hydrogen concentration. A substance
added to water that increases the concentration of
hydrogen ions (i.e., lowers the pH) is called an acid,
while a substance that reduces the concentration of
hydrogen ions (i.e., raises the pH) is called a base. Acid
in the stomach has a pH of 1, while a liquid drainer
has a pH of 14. Pure water is neutral with a pH of 7.
Compounds called buffers can be added to a solution
that will resist pH changes when an acid or base is
added.
See alsoACID;BASE.
phylogeny The evolutionary tree that connects a
group of organisms.
phylum A taxonomic category between kingdom
and class.
See alsoTAXON.
phytoalexin A toxic substance that acts like an
antibiotic that is produced by plants to inhibit or kill
the growth of microorganisms, such as certain fungi
that would otherwise infect them; e.g., pisatin (pro-
duced by peas), phaseollin (produced by beans [Phase-
olus]), camalexin (produced by Arabidopsis thaliana),
resveratrol (grapes).phytochelatin Apeptide of higher plants consisting
of polymers of 2–11 glutathione (γ, glutamyl, cysteinyl,
glycine) groups, which binds heavy metals.phytochrome(red-light-sensitive system) Photore-
ceptor proteins that regulate light-dependent growth
processes; absorbs red and far-red light in a reversible
system; one of the two light-sensing systems involved
in photoperiodism and photomorphogenesis. Plant
responses regulated by phytochrome include
photoperiodic induction of flowering, chloroplast
development (minus chlorophyll synthesis), leaf
senescence, leaf abscission, seed germination, and
flower induction.picket-fence porphyrin APORPHYRINwith a pro-
tective enclosure for binding oxygen at one side of the
ring that is used to mimic the dioxygen-carrying prop-
erties of the HEMEgroup.
See alsoBIOMIMETIC.pilus A hairlike projection, composed of a protein
pilin and found on the surface of certain gram-negative
bacteria, that functions to adhere to carbohydrate
receptors on host cells. Specialized sex pili conjugate
with other bacteria to transfer DNA. Also called fim-
briae.pineal gland (pineal body; epiphysis) A small
endocrine gland shaped like a pine cone and located in
the middle of the brain. It functions as a regulator of the
biological clock. At night, the hormone serotonin con-
verts into melatonin (n-acetyl-5-methoxy-tryptamine
[NA-5-MT]) by enzymatic interaction. Melatonin is a
neurotransmitter and neurohormone.
The pineal helps to regulate the function of all
organs of the endocrine system in the body (pituitary
gland, thyroid + parathyroid glands, thymus, pancreas,pineal gland 267