S
Sedimentation coefficient
see svedberg unit.
S
(1) SVEDBERG UNIT.
(2) Serine (see AMINO ACIDS).
S fimbriae
FIMBRIAE which occur on many of
the strains of Escherichia
coli which cause
meningitis in human neonates; they bind to sialyl galactoside receptors on
human erythrocytes.
S-glucan
Alkali-soluble glucans of fungal CELL WALLS. (cf. RGLUCAN.)
s. lat.
SENSU LATO (q.v.).
S layer
In a prokaryotic cell: a continuous layer – often the outermost layer
of the cell – consisting of a repeating pattern of protein or glycoprotein
subunits (‘S proteins’) arranged in squares,
hexagons etc. S layers occur in bacteria
(including strains of e.g. Aeromonas, Campylobacter, Clostridium, Lactobacillus, Nitrosomonas, Pseudomonas, Treponema, some cyanobacteria) and
archaeans.
S matrix
In numerical taxonomy, similarity matrix: the percentage similarity
between each OTU and each of every other OTU (in a given study) expressed in
tabular form – the ordinate and abscissa of the matrix each comprising a
complete list of designations of the OTUs under study.
S organism
see methanobacillus omelianskii.
S phase
The
synthesis phase of the cell cycle, constituting the portion of interphase
during which DNA is replicated.
SA
(sinoatrial) node
The
pacemaker of the heart, located in the wall of the right atrium. At the base of
the wall separating the two atria is another patch of nodal tissue called the
atrioventricular node (AV).
saltatory
conduction
(sahl-tuh-tor-ee)
Rapid transmission of a nerve impulse along an axon resulting from the action potential jumping from one node of Ranvier to another, skipping the myelin-sheathed regions of membrane.
Rapid transmission of a nerve impulse along an axon resulting from the action potential jumping from one node of Ranvier to another, skipping the myelin-sheathed regions of membrane.
saprobe
[Gk. sapros, rotten,
putrid + bios, life]
An organism that acts as a decomposer by absorbing nutrients from dead organic matter.
An organism that acts as a decomposer by absorbing nutrients from dead organic matter.
sarcolemma
[Gk. sarx, the
flesh + lemma, husk]
The specialized plasma membrane surrounding a muscle cell (muscle fiber); capable of propagating action potentials.
The specialized plasma membrane surrounding a muscle cell (muscle fiber); capable of propagating action potentials.
sarcomere
[Gk. sarx, the
flesh + meris, part of, portion]
The fundamental, repeating unit of striated muscle, delimited by the Z lines.
The fundamental, repeating unit of striated muscle, delimited by the Z lines.
sarcoplasmic
reticulum
(sar-koh-plaz-mik
reh-tik-yoo-lum) [Gk. sarx, the flesh + plasma, from
cytoplasm + L.reticulum, network]
A modified form of endoplasmic reticulum in striated muscle cells that stores calcium used to trigger contraction during stimulation.
A modified form of endoplasmic reticulum in striated muscle cells that stores calcium used to trigger contraction during stimulation.
saturated
fatty acid
A fatty
acid in which all carbons in the hydrocarbon tail are connected by single
bonds, thus maximizing the number of hydrogen atoms that can attach to the
carbon skeleton.
savanna
(suh-van-uh)
A tropical grassland biome with scattered individual trees, large herbivores, and three distinct seasons based primarily on rainfall, maintained by occasional fires and drought.
A tropical grassland biome with scattered individual trees, large herbivores, and three distinct seasons based primarily on rainfall, maintained by occasional fires and drought.
Schwann
cells
A chain
of supporting cells enclosing the axons of many neurons and forming an
insulating layer called the myelin sheath.
sclereid
(skler-ee-id)
A short, irregular sclerenchyma cell in nutshells and seed coats and scattered through the parenchyma of some plants.
A short, irregular sclerenchyma cell in nutshells and seed coats and scattered through the parenchyma of some plants.
sclerenchyma
cell
[Gk. skleros, hard]
A rigid, supportive plant cell type usually lacking protoplasts and possessing thick secondary walls strengthened by lignin at maturity.
A rigid, supportive plant cell type usually lacking protoplasts and possessing thick secondary walls strengthened by lignin at maturity.
second
law of thermodynamics
The
principle whereby every energy transfer or transformation increases the entropy
of the universe. Ordered forms of energy are at least partly converted to heat,
and in spontaneous reactions, the free energy of the system also decreases.
second
messenger
A small,
nonprotein, water-soluble molecule or ion, such as calcium ion or cyclic AMP,
that relays a signal to a cell's interior in response to a signal received by a
signal receptor protein.
secondary
compound
A
chemical compound synthesized through the diversion of products of major
metabolic pathways for use in defense by prey species.
secondary
consumer
A member
of the trophic level of an ecosystem consisting of carnivores that eat
herbivores.
secondary
growth
The
increase in girth of the stems and roots of many plants, especially woody,
perennial dicots.
secondary
immune response
The
immune response elicited when an animal encounters the same antigen at some
later time. The secondary immune response is more rapid, of greater magnitude,
and of longer duration than the primary immune response.
secondary
productivity
he rate
at which all the heterotrophs in an ecosystem incorporate organic material into
new biomass, which can be equated to chemical energy.
secondary
sex characteristics
Characteristics
of animals that distinguish between the two sexes but that do not produce or
convey gametes; includes facial hair of the human male and enlarged hips and
breasts of the female.
secondary
structure
The
localized, repetitive coiling or folding of the polypeptide backbone of a
protein due to hydrogen bond formation between peptide linkages.
secondary
succession
A type of
succession that occurs where an existing community has been severely cleared by
some disturbance.
secretion
[L. secermere, to
sever, separate]
(1) The discharge of molecules synthesized by the cell. (2) In the vertebrate kidney, the discharge of wastes from the blood into the filtrate from the nephron tubules.
(1) The discharge of molecules synthesized by the cell. (2) In the vertebrate kidney, the discharge of wastes from the blood into the filtrate from the nephron tubules.
sedimentary
rock
(sed-eh-men-tar-ee)
Rock formed from sand and mud that once settled in layers on the bottom of seas, lakes, and marshes. Sedimentary rocks are often rich in fossils.
Rock formed from sand and mud that once settled in layers on the bottom of seas, lakes, and marshes. Sedimentary rocks are often rich in fossils.
seed
An
adaptation for terrestrial plants consisting of an embryo packaged along with a
store of food within a resistant coat.
segregation
See Mendel's first law.
selection
The
process by which the forms of organisms in a population that are better adapted
to the environmental conditions increase in frequency relative to less
well-adapted forms over a number of generations.
selection
coefficient
The
difference between two fitness values, representing a relative measure of
selection against an inferior genotype.
selective
permeability
[L. seligere, to
gather apart + permeare, to go through]
A property of biological membranes that allows some substances to cross more easily than others.
A property of biological membranes that allows some substances to cross more easily than others.
selective
pressure
An
environmental factor that favors the survival and reproduction of those genetic
variants within a population that are better adapted to the environment.
self-incompatibility
The
capability of certain flowers to block fertilization by pollen from the same or
a closely related plant.
self-fertilization
The union
of egg and sperm produced by a single hermaphroditic organism.
self-pollination
The
transfer of pollen from anther to stigma in the same flower or to another
flower of the same plant, leading to self-fertilization.
semen
(see-men)
[L. seed]
The fluid that is ejaculated by the male during orgasm; contains sperm and secretions from several glands of the male reproductive tract.
The fluid that is ejaculated by the male during orgasm; contains sperm and secretions from several glands of the male reproductive tract.
semicircular
canals
A
three-part chamber of the inner ear that functions in maintaining equilibrium.
semilunar
valve
A valve
located at the two exits of the heart, where the aorta leaves the left
ventricle and the pulmonary artery leaves the right ventricle.
seminal
vesicle
[L. semen, seed
+ vesicula, a little bladder]
A gland in males that secretes a fluid (a component of semen) that lubricates and nourishes sperm.
A gland in males that secretes a fluid (a component of semen) that lubricates and nourishes sperm.
seminiferous
tubules
[L. semen, seed
+ ferre, to bear or carry + dim. of tubus, tube]
Highly coiled tubes in the testes in which sperm are produced.
Highly coiled tubes in the testes in which sperm are produced.
sensation
An
impulse sent to the brain from activated receptors and sensory neurons.
sensory
neuron
A nerve
cell that receives information from the internal and external environments and
transmits the signals to the central nervous system.
sensory
receptor
A
specialized structure that responds to specific stimuli from an animal's
external or internal environment; transmits the information of an environmental
stimulus to the animal's nervous system by converting stimulus energy to the
electrochemical energy of action potentials.
sepal
(see-pul)
A whorl of modified leaves in angiosperms that encloses and protects the flower bud before it opens.
A whorl of modified leaves in angiosperms that encloses and protects the flower bud before it opens.
septum
[L.
fence]
A partition, or cross wall, that divides a structure, such as a fungal hypha, into compartments.
A partition, or cross wall, that divides a structure, such as a fungal hypha, into compartments.
sessile
[L. sedere, to
sit]
Attached; not free to move about.
Attached; not free to move about.
sex
chromosomes
The pair
of chromosomes responsible for determining the sex of an individual.
sex-linked
genes
Genes
located on one sex chromosome but not the other.
sex-linked
trait
An
inherited trait, such as color discrimination, determined by a gene located on
a sex chromosome and that therefore shows a different pattern of inheritance in
males and females.
sexual
dimorphism
(dy-mor-fiz-um)
A special case of polymorphism based on the distinction between the secondary sex characteristics of males and females.
A special case of polymorphism based on the distinction between the secondary sex characteristics of males and females.
sexual
reproduction
A type of
reproduction in which two parents give rise to offspring that have unique
combinations of genes inherited from the gametes of the two parents.
sexual
selection
Selection
based on variation in secondary sex characteristics, leading to the enhancement
of sexual dimorphism.
shoot
system
The
aerial portion of a plant body, consisting of stems, leaves, and flowers.
short-day
plant
A plant
that flowers, usually in late summer, fall, or winter, only when the light
period is shorter than a critical length.
sieve
tube member
A chain
of living cells that form sieve tubes in phloem.
sign
stimulus
An
external sensory stimulus that triggers a fixed action pattern.
signal
peptide
A stretch
of amino acids on polypeptides that targets proteins to specific destinations
in eukaryotic cells.
signal-transduction
pathway
A
mechanism linking a mechanical or chemical stimulus to a cellular response.
sink
habitat
A habitat
where mortality exceeds reproduction.
sinoatrial
node
[L. sinus, fold,
hollow + atrium, yard, court, hall + nodus, knot]
Area of the vertebrate heart that initiates the heartbeat; located where the superior vena cava enters the right atrium; the pacemaker.
Area of the vertebrate heart that initiates the heartbeat; located where the superior vena cava enters the right atrium; the pacemaker.
sister
chromatids
(kroh-muh-tidz)
Replicated forms of a chromosome joined together by the centromere and eventually separated during mitosis or meiosis II.
Replicated forms of a chromosome joined together by the centromere and eventually separated during mitosis or meiosis II.
skeletal
muscle
Striated
muscle generally responsible for the voluntary movements of the body.
sliding-filament
model
The
theory explaining how muscle contracts, based on change within a sarcomere, the basic unit of muscle
organization, stating that thin (actin) filaments slide across thick (myosin) filaments, shortening the
sarcomere; the shortening of all sarcomeres in a myofibril shortens the
entire myofibril.
small
nuclear ribonucleoprotein (snRNP)
(ry-boh-noo-klee-oh-pro-teen)
One of a variety of small particles in the cell nucleus, composed of RNA and protein molecules; functions are not fully understood, but some form parts of spliceosomes, active in RNA splicing.
One of a variety of small particles in the cell nucleus, composed of RNA and protein molecules; functions are not fully understood, but some form parts of spliceosomes, active in RNA splicing.
smooth ER
That
portion of the endoplasmic reticulum that is free of ribosomes.
smooth
muscle
A type of
muscle lacking the striations of skeletal and cardiac muscle because of the
uniform distribution of myosin filaments in the cell.
social
dominance
A
hierarchical pattern of social organization involving domination of some
members of a group by other members in a relatively orderly and long-lasting
pattern.
society
[L. socius, companion]
An organization of individuals of the same species in which there are divisions of resources, divisions of labor, and mutual dependence; a society is held together by stimuli exchanged among members of the group.
An organization of individuals of the same species in which there are divisions of resources, divisions of labor, and mutual dependence; a society is held together by stimuli exchanged among members of the group.
sociobiology
The study
of social behavior based on evolutionary theory.
sodium-potassium
pump
A special
transport protein in the plasma membrane of animal cells that transports sodium
out of and potassium into the cell against their concentration gradients.
solute
(sol-yoot)
A substance that is dissolved in a solution.
A substance that is dissolved in a solution.
solution
A
homogeneous, liquid mixture of two or more substances.solvent The dissolving
agent of a solution. Water is the most versatile solvent known.
somatic
cell
[Gk. soma, body]
Any cell in a multicellular organism except a sperm or egg cell.
Any cell in a multicellular organism except a sperm or egg cell.
somatic
nervous system
[Gk. soma, body]
The branch of the motor division of the vertebrate peripheral nervous system composed of motor neurons that carry signals to skeletal muscles in response to external stimuli.
The branch of the motor division of the vertebrate peripheral nervous system composed of motor neurons that carry signals to skeletal muscles in response to external stimuli.
somatotropin
[Gk. soma, body
+ trope, a turning]
A hormone, produced by the pituitary gland, that stimulates protein synthesis and promotes the growth of bone; also known as growth hormone.
A hormone, produced by the pituitary gland, that stimulates protein synthesis and promotes the growth of bone; also known as growth hormone.
source
habitat
A habitat
where reproduction exceeds mortality and from which excess individuals disperse.
Southern
blotting
A
hybridization technique that enables researchers to determine the presence of
certain nucleotide sequences in a sample of DNA.
specialized
(1) Of
cells, having particular functions in a multicellular organism. (2) Of
organisms, having special adaptations to a particular habitat or mode of life.
speciation
The
origin of new species in evolution.
species pl. species
[L. kind,
sort]
A particular kind of organism; members possess similar anatomical characteristics and have the ability to interbreed.
A particular kind of organism; members possess similar anatomical characteristics and have the ability to interbreed.
species
diversity
The
number and relative abundance of species in a biological community.
species
richness
The
number of species in a biological community.
species
selection
A theory
maintaining that species living the longest and generating the greatest number
of species determine the direction of major evolutionary trends.
species-specific
Characteristic
of (and limited to) a particular species.
specific
Unique;
for example, the proteins in a given organism, the enzyme catalyzing a given
reaction, or the antibody to a given antigen.
specific
heat
The
amount of heat that must be absorbed or lost for 1 g of a substance to change
its temperature 1°C.
spectrophotometer
An
instrument that measures the proportions of light of different wavelengths absorbed
and transmitted by a pigment solution.
sperm
[Gk. sperma, seed]
A male gamete.
A male gamete.
spermatid
[Gk. sperma, seed]
Each of four haploid (n) cells resulting from the meiotic divisions of a spermatocyte; each spermatid becomes differentiated into a sperm cell.
Each of four haploid (n) cells resulting from the meiotic divisions of a spermatocyte; each spermatid becomes differentiated into a sperm cell.
spermatocytes
[Gk. sperma, seed
+ kytos, vessel]
The diploid (2n) cells formed by the enlargement and differentiation of the spermatogonia; they give rise by meiotic division to the spermatids.
The diploid (2n) cells formed by the enlargement and differentiation of the spermatogonia; they give rise by meiotic division to the spermatids.
spermatogonia
[Gk. sperma, seed
+ gonos, a child, the young]
The unspecialized diploid (2n) cells on the walls of the seminiferous tubules that, by enlargement, differentiation, and meiotic division, become spermatocytes, then spermatids, then sperm cells.
The unspecialized diploid (2n) cells on the walls of the seminiferous tubules that, by enlargement, differentiation, and meiotic division, become spermatocytes, then spermatids, then sperm cells.
spermatogenesis
The
continuous and prolific production of mature sperm cells in the testis.
sphincter
(sfink-ter)
[Gk. sphinkter, a band]
A ringlike valve, consisting of modified muscles in a muscular tube, such as a digestive tract; closes off the tube like a drawstring.
A ringlike valve, consisting of modified muscles in a muscular tube, such as a digestive tract; closes off the tube like a drawstring.
spinal
cord
Part of
the vertebrate central nervous system; consists of a thick, dorsal,
longitudinal bundle of nerve fibers extending posteriorly from the brain.
spindle
An
assemblage of microtubules that orchestrates chromosome movement during
eukaryotic cell division.
spiracle
[L. spirare, to
breathe]
One of the external openings of the respiratory system in terrestrial arthropods.
One of the external openings of the respiratory system in terrestrial arthropods.
spiral
cleavage
A type of
embryonic development in protostomes, in which the planes of cell division that
transform the zygote into a ball of cells occur obliquely to the polar axis,
resulting in cells of each tier sitting in the grooves between cells of
adjacent tiers.
spliceosome
(sply-see-oh-some)
A complex assembly that interacts with the ends of an RNA intron in splicing RNA; releases an intron and joins two adjacent exons.
A complex assembly that interacts with the ends of an RNA intron in splicing RNA; releases an intron and joins two adjacent exons.
splitting
evolution
See Cladogenesis.
spongy
parenchyma
In plant
leaves, a tissue composed of loosely arranged chloroplast-containing parenchyma
cells.
sporangiophore
(spo-ran-ji-o-for)
[Gk. spora, seed + phore, from phorein, to
bear]
A specialized hypha or a branch bearing one or more sporangia.
A specialized hypha or a branch bearing one or more sporangia.
sporangium pl. sporangia
[Gk. spora, seed]
A capsule in fungi and plants in which meiosis occurs and haploid spores develop.
A capsule in fungi and plants in which meiosis occurs and haploid spores develop.
spore
[Gk. spora, seed]
In the life cycle of a plant or alga undergoing alternation of generations, a meiotically produced haploid cell that divides mitotically, generating a multicellular individual, the gametophyte, without fusing with another cell.
In the life cycle of a plant or alga undergoing alternation of generations, a meiotically produced haploid cell that divides mitotically, generating a multicellular individual, the gametophyte, without fusing with another cell.
sporophyte
[Gk. spora, seed
+ phytos, growing]
The multicellular diploid form in organisms undergoing alternation of generations that results from a union of gametes and that meiotically produces haploid spores that grow into the gametophyte generation.
The multicellular diploid form in organisms undergoing alternation of generations that results from a union of gametes and that meiotically produces haploid spores that grow into the gametophyte generation.
sporopollenin
A
secondary product, a polymer synthesized by a side branch of a major metabolic
pathway of plants that is resistant to almost all kinds of environmental
damage; especially important in the evolutionary move of plants onto land.
stablizing
selection
Natural
selection that favors intermediate variants by acting against extreme
phenotypes.
stamen
[L. a
thread]
The pollen-producing male reproductive organ of a flower, consisting of an anther and filament.
The pollen-producing male reproductive organ of a flower, consisting of an anther and filament.
starch
[M.E. sterchen, to
stiffen]
A storage polysaccharide in plants consisting entirely of glucose.
A storage polysaccharide in plants consisting entirely of glucose.
statocyst
[Gk. statos, standing
+ kystis, sac]
A type of mechanoreceptor that functions in equilibrium in invertebrates through the use of statoliths, which stimulate hair cells in relation to gravity.
A type of mechanoreceptor that functions in equilibrium in invertebrates through the use of statoliths, which stimulate hair cells in relation to gravity.
stele
The
central vascular cylinder in roots where xylem and phloem are located.
stem
The
aboveground part of the axis of vascular plants, as well as anatomically
similar portions below ground (such as rhizomes).
stem
cells
The
common, self-regenerating cells in the marrow of long bones that give rise, by
differentiation and division, to red blood cells and all of the different types
of white blood cells.
stereoisomer
A
molecule that is a mirror image of another molecule with the same molecular
formula.
stereoscopic
vision
[Gk. stereos, solid
+ optikos, pertaining to the eye]
Ability to perceive a single, three-dimensional image from the simultaneous but separate images delivered to the brain by each eye.
Ability to perceive a single, three-dimensional image from the simultaneous but separate images delivered to the brain by each eye.
steroids
A class
of lipids characterized by a carbon skeleton consisting of four rings with
various functional groups attached.
stigma
[Gk. stigme, a
prick mark, puncture]
In plants, the region of a carpel serving as a receptive surface for pollen grains, which germinate on it.
In plants, the region of a carpel serving as a receptive surface for pollen grains, which germinate on it.
stimulus
[L. goad,
incentive]
Any internal or external change or signal that influences the activity of an organism or of part of an organism.
Any internal or external change or signal that influences the activity of an organism or of part of an organism.
stoma pl. stomata
[Gk.
mouth]
A microscopic pore surrounded by guard cells in the epidermis of leaves and stems that allows gas exchange between the environment and the interior of the plant.
A microscopic pore surrounded by guard cells in the epidermis of leaves and stems that allows gas exchange between the environment and the interior of the plant.
strategy
[Gk. strategein, to
maneuver]
A group of related traits, evolved under the influence of natural selection, that solve particular problems encountered by living organisms; often includes anatomical, physiological, and behavioral characteristics.
A group of related traits, evolved under the influence of natural selection, that solve particular problems encountered by living organisms; often includes anatomical, physiological, and behavioral characteristics.
striated
muscle
[L.
from striare, to groove]
Skeletal voluntary muscle and cardiac muscle. The name derives from the striped appearance, which reflects the arrangement of contractile elements.
Skeletal voluntary muscle and cardiac muscle. The name derives from the striped appearance, which reflects the arrangement of contractile elements.
strict
aerobe
An
organism that can survive only in an atmosphere of oxygen, which is used in
aerobic respiration.
strict
anaerobe
An organism
that cannot survive in an atmosphere of oxygen. Other substances, such as
sulfate or nitrate, are the terminal electron acceptors in the electron
transport chains that generate their ATP.
stroma
[Gk. a
bed, from stronnymi, to spread out]
The fluid of the chloroplast surrounding the thylakoid membrane; involved in the synthesis of organic molecules from carbon dioxide and water.
The fluid of the chloroplast surrounding the thylakoid membrane; involved in the synthesis of organic molecules from carbon dioxide and water.
stromatolite
Rock made
of banded domes of sediment in which are found the most ancient forms of life:
prokaryotes dating back as far as 3.5 billion years.
structural
formula
A type of
molecular notation in which the constituent atoms are joined by lines
representing covalent bonds.
structural
gene
A gene
that codes for a polypeptide.
style
[L. stilus, stake,
stalk]
In angiosperms, the stalk of a carpel, down which the pollen tube grows.
In angiosperms, the stalk of a carpel, down which the pollen tube grows.
substrate
[L. substratus, strewn
under]
(1) The substance on which an enzyme works. (2) The foundation to which an organism is attached.
(1) The substance on which an enzyme works. (2) The foundation to which an organism is attached.
substrate-level
phosphorylation
The
formation of ATP by directly transferring a phosphate group to ADP from an
intermediate substrate in catabolism.
succession
See Ecological succession.
sucrose
Cane
sugar; a common disaccharide found in many plants; a molecule of glucose linked
to a molecule of fructose.
sugar
Any
monosaccharide or disaccharide.
summation
A
phenomenon of neural integration in which the membrane potential of the
postsynaptic cell in a chemical synapse is determined by the total activity of
all excitatory and inhibitory presynaptic impulses acting on it at any one
time.
suppressor
T cell (TS)
A type of
T cell that causes B cells as well as other cells to ignore antigens.
surface
tension
A measure
of how difficult it is to stretch or break the surface of a liquid. Water has a
high surface tension because of the hydrogen bonding of surface molecules.
survivorship
curve
A plot of
the number of members of a cohort that are still alive at each age; one way to
represent age-specific mortality.
suspension-feeder
An
aquatic animal, such as a clam or a baleen whale, that sifts small food
particles from the water.
sustainable
agriculture
Long-term
productive farming methods that are environmentally safe.
sustainable
development
The
long-term prosperity of human societies and the ecosystems that support them.
swim
bladder
An
adaptation, derived from a lung, that enables bony fishes to adjust their
density and thereby control their buoyancy.
symbiont
(sim-by-ont)
The smaller participant in a symbiotic relationship, living in or on the host.
The smaller participant in a symbiotic relationship, living in or on the host.
symbiosis
[Gk. syn, together
with + bioonai, to live]
An ecological relationship between organisms of two different species that live together in direct contact.
An ecological relationship between organisms of two different species that live together in direct contact.
sympathetic
division
One of
two divisions of the autonomic nervous system of vertebrates; generally
increases energy expenditure and prepares the body for action.
sympatric
speciation
[Gk. syn, together
with + patra, fatherland, country]
A mode of speciation occurring as a result of a radical change in the genome that produces a reproductively isolated subpopulation in the midst of its parent population.
A mode of speciation occurring as a result of a radical change in the genome that produces a reproductively isolated subpopulation in the midst of its parent population.
symplast
In
plants, the continuum of cytoplasm connected by plasmodesmata between cells.
synapomorphies
Shared
derived characters; homologies that evolved in an ancestor common to all
species on one branch of a fork in a cladogram, but not common to species on
the other branch.
synapse
(sin-aps)
[Gk. synapsis, a union]
The locus where one neuron communicates with another neuron in a neural pathway; a narrow gap between a synaptic terminal of an axon and a signal-receiving portion (dendrite or cell body) of another neuron or effector cell. Neurotransmitter molecules released by synaptic terminals diffuse across the synapse, relaying messages to the dendrite or effector.
The locus where one neuron communicates with another neuron in a neural pathway; a narrow gap between a synaptic terminal of an axon and a signal-receiving portion (dendrite or cell body) of another neuron or effector cell. Neurotransmitter molecules released by synaptic terminals diffuse across the synapse, relaying messages to the dendrite or effector.
synapsis
The
pairing of replicated homologous chromosomes during prophase I of meiosis.
synaptic
cleft
A narrow
gap separating the synaptic knob of a transmitting neuron from a receiving
neutron to an effector.
synaptic
knob
The relay
point at the tip of a transmitting neuron's axon, where signals are sent to
another neuron or to an effector.
synaptic
terminal
A bulb at
the end of an axon in which neurotransmitter molecules are stored and released.
syngamy
(sin-gam-ee)
The process of cellular union during fertilization.
The process of cellular union during fertilization.
synthesis
[Gk. syntheke, a
putting together]
The formation of a more complex substance from simpler ones.
The formation of a more complex substance from simpler ones.
synthesis
phase
In the
cell cycle, the phase in which the DNA of the chromosomes is replicated and
DNA-associated proteins, such as histones, are synthesized.
systematics
The
branch of biology that studies the diversity of life; encompasses taxonomy and
is involved in reconstructing phylogenetic history.
systemic
acquired resistance (SAR)
A
defensive response in infected plants that helps protect healthy tissue from
pathogenic invasion.
systole
(sis-toh-lee)
The stage of the heart cycle in which the heart muscle contracts and the chambers pump blood.
The stage of the heart cycle in which the heart muscle contracts and the chambers pump blood.
systolic
pressure
The
pressure in an artery during the ventricular contraction phase of the heart
cycle.
