G-22 glossary
genetic differences prevent them from interbreeding success-
fully under natural conditions.
species (spee-ceez) [L. species, a kind] A unit consisting of
one or more populations of individuals that can interbreed
under natural conditions to produce fertile offspring that are
reproductively isolated from other such units.
sperm [Gk. sperma, seed] A mature male gamete.
spermatogenesis (sperm-at-oh-jen-ih-sis) Formation of a
male gamete, from a germ cell to a mature sperm.
sphere of hydration A clustering of water molecules around
the individual molecules of a substance placed in water.
Compare solute.
sphincter (sfink-tur) Ring of smooth muscle between
regions of a tubelike system (as between the stomach and
small intestine).
spinal cavity Body cavity that houses the spinal cord.
spinal cord The portion of the central nervous system
threading through a canal inside the vertebral column. It pro-
vides direct reflex connections between sensory and motor
neurons, as well as communication lines to and from the
brain.
spindle A structure that forms during mitosis or meiosis and
that moves the chromosomes. It consists of two sets of micro-
tubules that extend from the opposite poles and that overlap
at the spindle’s equator.
spleen The largest lymphoid organ; it is a filtering station
for blood and a reservoir of lymphocytes, red blood cells, and
macrophages.
spongy bone Type of bone tissue in which hard, needlelike
struts separate large spaces filled with marrow. Spongy
bone occurs inside the shaft and at the ends of long bones
and within the breastbone (sternum), pelvis, and bones of
the skull.
sporadic disease A disease that breaks out irregularly and
affects relatively few people.
start codon Of protein synthesis, a base triplet in a strand of
mRNA that serves as the start signal for mRNA translation.
stem cell Unspecialized cell that can give rise to descendants
that differentiate into specialized cells.
sternum Elongated flat bone (also called the breastbone) to
which the upper ribs attach and so form the rib cage.
steroid (stair-oid) A lipid with a backbone of four carbon
rings and with no fatty acid tails. Steroids differ in their func-
tional groups. Different types have roles in metabolism, inter-
cellular communication, and cell membranes.
steroid hormone A type of lipid-soluble hormone synthe-
sized from cholesterol. Many steroid hormones move into
the nucleus and bind to receptors there; others bind to recep-
tors in the cytoplasm, and the entire complex moves into the
nucleus.
sterol A type of lipid with a rigid backbone of four fused
carbon rings. Sterols occur in cell membranes; cholesterol is
the main type in human tissues.sinoatrial (SA) node Region of conducting cells in the upper
wall of the right atrium; the cells generate periodic waves of
excitation that stimulate the atria to contract.
sinus In the skull, an air-filled space lined with mucous
membrane that reduces the weight of the skull.
sister chromatid Of a duplicated chromosome, one of
two DNA molecules (and associated proteins) that remain
attached at their centromere during nuclear division. Each
ends up in a separate daughter nucleus.
skeletal muscle Type of muscle that interacts with the
skeleton to bring about body movements. A skeletal muscle
typically consists of bundles of many long cylindrical cells
encapsulated by connective tissue.
sliding filament mechanism The mechanism by which
skeletal muscles contract; sarcomeres contract (shorten) when
myosin filaments slide along and pull actin filaments toward
the center of the sarcomere.
small intestine The portion of the digestive system where
digestion is completed and most nutrients are absorbed.
smog A general term for air pollution; originally the term
meant “fog” infused with “smoke” and other pollutants.
smooth muscle One of the three main muscle types; occurs
in the walls of internal organs and generally is not under vol-
untary control.
sodium–potassium pump A transport protein spanning the
lipid bilayer of the plasma membrane. When activated by
ATP, its shape changes and it selectively transports sodium
ions out of the cell and potassium ions in.
solute (sol-yoot) [L. solvere, to loosen] Any substance
dissolved in a solution. In water, this means spheres of hydra-
tion surround the charged parts of individual ions or mol-
ecules and keep them dispersed.
solvent Fluid in which ions and polar molecules easily
dissolve.
somatic cell (so-mat-ik) [Gk. soma, body] Any body cell that
is not a germ cell; that is, a body cell that does not give rise to
gametes.
somatic nerves Nerves leading from the central nervous
system to skeletal muscles.
somatic sensation Awareness of touch, pressure, heat, cold,
pain, and limb movement.
somatosensory cortex Part of the gray matter of the cere-
bral hemispheres that controls somatic sensations.
somites In a developing embryo, paired blocks of mesoderm
that will give rise to most bones and to the skeletal muscles of
the neck and trunk.
special senses Vision, hearing, olfaction, or other sensation
that arises from a particular location, such as the eyes, ears, or
nose.
speciation (spee-cee-ay-shun) The evolutionary process
by which species originate. One speciation route starts with
divergence of two reproductively isolated populations of a
species. They become separate species when accumulated
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