glossary G-21segmentation In the small intestine, an oscillating move-
ment produced by rings of muscle that moves digested food
and forces it against the intestinal wall.
segregation The separation of pairs of gametes during
meiosis.selective permeability The capacity of a cell membrane to
let some substances but not others cross it at certain times.
The property arises as an outcome of the membrane’s lipid
bilayer structure and its transport proteins.
semen [L. serere, to sow] Sperm-bearing fluid expelled from
the penis during male orgasm.semicircular canals Fluid-filled canals positioned at differ-
ent angles within the vestibular apparatus of the inner ear
and that contain sensory receptors that detect head move-
ments, deceleration, and acceleration.semiconservative replication [Gk. hemi, half, and L. conser-
vare, to keep] Reproduction of a DNA molecule when a com-
plementary strand forms on each of the unzipping strands of
an existing DNA double helix, the outcome being two “half-
old, half-new” molecules.
semilunar valve A valve in each half of the heart that opens
and closes during each heartbeat in ways that keep blood
flowing in one direction, from the ventricle to the arteries
leading away from it.
seminal vesicle Part of the male reproductive system;
secretes fructose that nourishes sperm.seminiferous tubules Coiled tubes inside the testes where
sperm develop.
senescence (sen-ess-cents) [L. senescere, to grow old] Sum
total of processes leading to the natural death of an organism
or some of its parts.sensation The conscious awareness of a stimulus.sensory adaptation In a sensory system, a state in which the
frequency of action potentials eventually slows or stops even
when the strength of a stimulus is constant.
sensory neuron Any of the nerve cells that act as sensory
receptors, detecting specific stimuli (such as light energy) and
relaying signals to the brain and spinal cord.sensory receptor A sensory cell or specialized cell adjacent
to it that can detect a particular stimulus.
serous membranes Membranes that occur in paired sheets
and anchor internal organs and reduce friction between
organs.
Sertoli cell A type of cell in seminiferous tubules that nour-
ishes and otherwise aids the development of sperm.sex chromosome A chromosome that determines a new
individual’s gender. Compare autosome.
shell model Model of electron distribution in atoms in
which orbitals available to electrons occupy a nested series
of shells.rigor mortis A stiffening of skeletal muscles caused when a
person dies and body cells stop making ATP.
RNA Ribonucleic acid. A category of single-stranded nucleic
acids that function in processes by which genetic instructions
are used to build proteins.
RNA polymerase Enzyme that catalyzes the assembly of
RNA strands on DNA templates.
rod cell Of the retina, a photoreceptor sensitive to very dim
light that contributes to coarse perception of movement.
rRNA See ribosomal RNA.
rugae The crumpled wall folds of an empty stomach.
salivary amylase Starch-degrading enzyme in saliva.
salivary gland Any of the glands that secrete saliva, a fluid
that initially mixes with food in the mouth and starts the
breakdown of starch.
salt Compound that releases ions other than H+ and OH– in
solution.
sampling error Error that develops when an experimenter
uses a sample (or subset) of a population for an experimental
group that is not large enough to be representative of the
whole.
sarcomere (sar-koe-meer) The basic unit of muscle contrac-
tion; a region of myosin and actin filaments organized in par-
allel between two Z lines of a myofibril inside a muscle cell.
sarcoplasmic reticulum (sar-koe-plaz-mik reh-tik-you-
lum) In muscle cells, a membrane system that takes up,
stores, and releases the calcium ions required for cross-bridge
formation in sarcomeres, hence for contraction.
scapula Flat, triangular bone on either side of the pectoral
girdle; the scapulae form the shoulder blades.
Schwann cell A specialized neuroglial cell that grows
around a neuron axon, forming a myelin sheath.
scientific method A systematic way of gathering knowledge
about the natural world.
scientific theory A thoroughly tested explanation of a broad
range of natural events and observations.
secondary immune response Rapid, prolonged response
by white blood cells, memory cells especially, to a previously
encountered antigen. See memory cell.
secondary oocyte An oocyte (unfertilized egg cell) that has
completed meiosis I; it is this haploid cell that is released at
ovulation.
secondary sexual trait Trait associated with maleness or
femaleness, but not directly involved with reproduction.
Beard growth in males and breast development in females are
examples.
second law of thermodynamics The total amount of energy
in the universe is spontaneously flowing from forms rich in
energy (such as glucose) to forms having less and less of it.
second messenger A molecule inside a cell that mediates
and generally triggers an amplified response to a hormone.
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