prises light-harvesting pigments such as CHLOROPHYLL;
a primary electron-transfer center, and secondary elec-
tron carriers. In green plant PHOTOSYNTHESIS, photo-
system I transfers electrons from PLASTOCYANINto a
[2FE-2S] FERREDOXIN, and contains IRON-SULFUR PRO-
TEINs. Photosystem II transfers electrons from the OXY-
GEN-EVOLVING COMPLEX to plastoquinone and
contains an iron center.
photovoltaic cell (solar cell) A cell made of thin
wafers of two slightly different types of silicon. One,
the P-type (P for positive), is doped with tiny quantities
of boron and contains positively charged “holes,” that
are missing electrons. The other type of silicon, called
N-type (N for negative), is doped with small amounts
of phosphorus and contains extra electrons. When
these two thin P and N materials are put together, it
produces a junction that, when exposed to light, pro-
duces a movement of electrons, thus producing an elec-
tric current. Photovoltaic cells convert light energy into
electrical energy.
pH-rate profile A plot of observed rate coefficient,
or more usually its decadic logarithm, against pH of
solution, other variables being kept constant.pH scale The concentration of hydrogen ions in a
solution expressed as the negative logarithm of the con-
centration. The scale typically is regarded as running
from 0 to 14, but in principle it can exceed these values
if the hydrogen ion concentration is greater than 1 M
or less than 10–4 M. Low pH corresponds 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.physical change Refers to when a material changes
from one physical state to another without formation
of intermediate substances of different composition in
the process, such as the change from gas to liquid.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 A peptide of higher plants, consisting
of polymers of 2-11 glutathione (γ-glutamyl-cysteinyl-
glycine) groups, that binds heavy metals.phytochrome(red-light-sensitive system) Photore-
ceptor proteins that regulate light-dependent growth
processes; absorb red and far-red light in a reversible
system; one of the two light-sensing systems involvedphytochrome 213Photosynthesis. A metabolic process in plants and certain bacte-
ria, using light energy absorbed by chlorophyll and other photo-
synthetic pigments for the reduction of CO 2 , followed by the
formation of organic compounds