system of membranous sacs and tubules (cisternae).
Protein molecules move from the rough ER into the
smooth ER, which forwards them as enclosed vesi-
cles, usually to the Golgi complex. Smooth ER func-
tions in metabolic processes: the synthesis of lipids,
carbohydrate metabolism, and the detoxification of
drugs and other toxins. Enzymes in the smooth ER
deal with synthesis of fats, phospholipids, steroids,
and other lipids, and they serve to detoxify drugs and
other toxins in the liver cells so they can be dis-
charged from the body. In muscle cells, the smooth
ER stores calcium that is released upon stimulation to
effect cell contraction.
See alsoROUGH ER.
smooth muscle Smooth muscle is composed of sin-
gle, spindle-shaped cells that have no visible striations.
Each smooth muscle cell contains thick (myosin) and
thin (actin) filaments that slide against each other to
produce contraction of the cell and are anchored near
the plasma membrane. Smooth muscle contraction is
slower than that of striated muscle and can be sus-
tained for long periods of time. Smooth muscle can be
found in blood vessels, the gastrointestinal tract,
bronchi of the lungs, the bladder, and the uterus.
Smooth muscle is primarily under the control of the
autonomic nervous system.
smuts Smuts are a group of organisms, along
with rusts, belonging to the microfungi, class Usto-
mycetes. Microfungi also include molds and mildews.
Smuts are parasites that appear on plants such as
grain, corn, and other economically important
plants as small galls that break open and spread
spores. In the United States, about 13,000 species of
microfungi on plants or plant products have been
described.
society An interdependent system of organisms,
plant or animal; in insects, two or more individuals
that constitute a social unit (bee hive, ant hill).
sociobiology The study of the behavior of social ani-
mals, including humans, with a premise that genetics is
the sole factor responsible for the behavior in humans
and animals.sodium-potassium pump An active transport
mechanism of cell membranes to regulate pressure
between the inside and outside of the cell and to pump
potassium ions into the cell and keep sodium out,
thereby preventing water retention and swelling within
the cell. The sodium-potassium pump also maintains
the electrical charge within each cell. ATP is used as the
energy source for the pump.
See alsoACTIVE TRANSPORT.soft acid SeeHARD ACID.soft base SeeHARD BASE.soft drug Acompound that is degraded in vivo to
predictable nontoxic and inactive METABOLITEs after
having achieved its therapeutic role.solute Any dissolved substance in a solution.solution Any liquid mixture of two or more sub-
stances that is homogeneous. A solution can be basic,
that is, have more OH– ions than H+ions with a pH
greater than 7, or it can be acidic with more H+ions
and have a pH lower than 7.solvation Any stabilizing interaction of a solute (or
solute moiety) and the solvent, or a similar interaction
of solvent with groups of an insoluble material (e.g.,
the ionic groups of an ion-exchange resin). They gener-
ally involve electrostatic forces and Van der Waals
forces, as well as more specific chemical effects such as
hydrogen bond formation.solvent Any liquid that dissolves another solute and
forms a homogeneous solution. Several types of sol-
vents exist such as organic solvents (acetone, ethanol)solvent 309