334 ❯ Glossary
sex ratio Proportion of males and females in a given
population.
sexual selection The process by which certain char-
acters are selected for because they aid in mate
acquisition.
shoots Parts of a plant that are above the ground.
short-day plants Plants, such as poinsettias, that
flower if exposed to nighttime conditions longer
than a critical period of length.
sickle cell anemiaA recessive disease caused by the
substitution of a single amino acid in the hemoglo-
bin protein of red blood cells, leaving hemoglobin
less able to carry oxygen and also causing the
hemoglobin to deform to a sickle shape when the
oxygen content of the blood is low. The sickling
causes pain, muscle weakness, and fatigue.
sieve-tube elements Functionally mature cells of
the phloem that are alive.
sink Site of carbohydrate consumption in plants.
skeletal muscle Striated muscle that controls volun-
tary activities and contains multiple nuclei.
smooth endoplasmic reticulum (SER) Membrane-
bound organelle involved in lipid synthesis, detox-
ification, and carbohydrate metabolism; has no
ribosomes on its cytoplasmic surface.
smooth muscle Involuntary muscle that contracts
slowly and is controlled by the autonomic nervous
system (ANS).
sodium–potassium pump A mechanism that actively
moves potassium intothe cell and sodium out ofthe
cell against their respective concentration gradients
to maintain appropriate levels inside the cell.
solute A substance dissolved in a solution.
somatotropic hormone (STH) A hormone that stim-
ulates protein synthesis and growth in the body.
somite Structure that gives rise to the muscles and
vertebrae in mammals.
source Site of carbohydrate creation in plants.
Southern blottingProcedure used to determine if a
particular sequence of nucleotides is present in a
sample of DNA.
specialized transductionTransduction involving a
virus in the lysogenic cycle that shifts to the lytic
cycle. If it accidentally brings with it a piece of the
host DNA as it pulls out of the host chromosome,
this DNA could find its way into another cell.
speciation The process by which new species evolve.
speciesA group of interbreeding (or potentially
interbreeding) organisms.
specific immunityComplicated multilayered defense
mechanism that protects a host against foreign
invasion.
spectrophotometer Machine used to determine
how much light can pass through a sample.
spermatids Immature sperm that enter the epi-
didymis, where their waiting game begins and
maturation is completed.
spermatogenesisProcess by which the male
gametes are formed. Four haploid sperm are pro-
duced during each meiotic cycle. This does not
begin until puberty, and it occurs in the seminifer-
ous tubules.
S phase The DNA is copied so that each daughter
cell has a complete set of chromosomes at the con-
clusion of the cell cycle.
spongy mesophyllRegion of a plant where the cells
are more loosely arranged, aiding in the passage of
CO 2 to cells performing photosynthesis.
sporophyte The diploid multicellular stage of the
plant life cycle.
sporozoite Small infectious form that apicomplexa
protists take to spread from place to place.
stabilizing selection This describes selection for the
mean of a population for a given allele; has the
effect of reducing variation in a given population.
stamenMale structure of a flower that contains the
pollen-producing anther.
starch Storage polysaccharide made of glucose mol-
ecules; seen in plants.
start codon (AUG) Codon that establishes the read-
ing frame for protein formation.
stem cells Cells that give rise to the immune cells of
the human body.
steroid hormonesLipid-soluble molecules that pass
through the cell membrane and combine with
cytoplasmic proteins. These complexes pass through
to the nucleus to interact with chromosomal pro-
teins and directly affect transcription in the nucleus.
steroids Lipids composed of four carbon rings.
Examples include cholesterol, estrogen, proges-
terone, and testosterone.
STHSeesomatotropic hormone.
sticky ends Single-stranded DNA fragments formed
when DNA is treated with restriction enzymes.
These fragments find and reconnect with other
fragments with the same ends.
stigma Flower structure that functions as the receiver
of pollen.