compound PhX relative to that of a single position in
benzene, then the selectivity factor Sf(expressing dis-
crimination between p- and m-positions in PhX) is
defined as
Sf= lg (fpX/fmX).selectron An electron that occupies spherical space
about the nucleus of an atom.
semiconductor A material that conducts electricity
at high temperatures but not at low temperature. A
material having a small number of mobile (free) elec-
trons (n-type) or a small number of electron vacancies
(holes; p-type) in an otherwise filled band, and which
can conduct electricity as a result of the electrons
moving under an applied field. In the n-type, elec-
trons carry the charge (negative charge carriers). In
the p-type, electrons move into a “hole,” thereby cre-
ating a new hole in the previous place, so that it is the
holes that seem to move relative to all the other nega-
tive electrons. Hence the holes are considered the
charge carriers and are taken to be positive relative to
the surrounding electrons. A characteristic of semi-
conductor conductivity behavior is that conductivity
increases with increasing temperature rather than
decreasing, as is characteristic of regular conductors
or metallic conductivity (which also is much greater
because of the larger number of electrons free to
move).
semimetal(metalloid) Any element with properties
between those of a metal and nonmetal, e.g., boron (B),
silicon (Si), germanium (Ge), arsenic (As), and tel-
lurium (Te); they are electrical SEMICONDUCTORs.
semipermeable A membrane where some substances
will pass through but others will not.
sensory neuron A specialized neuron that sends mes-
sages that it receives from external or internal stimuli
such as light, sound, smell, and chemicals to the central
nervous system.
sensory receptor A cell or organ that converts a
stimulus from a form of sound, light, smell or from
thermal, chemical, or mechanical stimuli into a signal,
or action potential, that can be transmitted through the
organism. Sensory receptors are specific to the stimulus
to which they are responding and fall into specific
types such as chemoreceptors, photoreceptors, ther-
moreceptors, mechanoreceptors, and pain receptors, to
name a few. Each changes the polarization of the cell
that may eventually cause an action potential. Phasic
receptors send action potentials quickly when first
stimulated and then soon reduce the frequency of
action potentials even if the stimulus continues, e.g.,
odor or pressure. They are useful for signaling sudden
changes in the environment. Receptors that respond to
light or mechanics are tonic receptors and produce a
constant signal, after an initial amount of high-fre-
quency action potentials, while the stimulus is being
applied.sequence The order of neighboring amino acids in a
protein or the purine and pyrimidine bases in RNA or
DNA.
See alsoPRIMARY STRUCTURE.sequence-directed mutagenesis SeeMUTAGENESIS.serotonin(5-Hydroxy tryptamine (5-HT)) A bio-
genic monoamine (like histamine, epinephrine, nore-
pinephrine), a neurotransmitter, and an important
vasoactive substance. A mediator of immediate hyper-
sensitivity. Serotonin is one of many mediators released
by circulating basophils and tissue mast cells. Found in
blood platelets, in the gastrointestinal tract, and in cer-
tain regions of the brain.
Serotonin plays a major role in blood clotting,
stimulating strong heartbeats, initiating sleep, fighting
depression, and causing migraine headaches.serum SeePLASMA.shell (of electrons) The layers in which electrons
orbit the nucleus of an atom. Shell(s) of electrons are244 selectron