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BIO NOTES
UNIT 1: Ecology (Key terms)-Chap. 52
Ecology (from the Greek oikos, home, and logos, study). The scientific study of the interactions between
organisms and the environment.
- Organismal ecology. Includes the subdisciplines of physiological, evolutionary, and behavioral
ecology, is concerned with how an organism’s structure, physiology, and behavior meet the
challenges posed by its environment.
- Population. A group of individuals of the same species living in an area. Population ecology
analyzes factors that affect population size and how and why it changes through time.
- Community. A group of populations of different species in an area. Community ecology
examines how interactions between species, such as predation and competition, affect
community structure and organization.
- Ecosystem. The community of organisms in an area and the physical factors with which those
organisms interact. Ecosystem ecology emphasizes energy flow and chemical cycling between
organisms and the environment.
- A landscape (or seascape) is a mosaic of connected ecosystems. Research in landscape ecology
focuses on the factors controlling exchanges of energy, materials, and organisms across multiple
ecosystems.
- The biosphere is the global ecosystem—the sum of all the planet’s ecosystems and landscapes.
Global ecology examines how the regional exchange of energy and materials influences the
functioning and distribution of organisms across the biosphere.
Environment. Refers to other organisms as well as the physical aspects of an organism’s surroundings.
Tropics. Those regions that lie between 23.5° north latitude and 23.5° south latitude.
Climate. The long-term prevailing weather conditions in a given area. Four physical factors—temperature,
precipitation, sunlight, and wind.
Global Climate Patterns. Determined largely by the input of solar energy and Earth’s movement in space.
Microclimate. Very fine, localized patterns in climatic conditions.
Abiotic , or nonliving, factors influence the distribution and abundance of organisms. Similarly, all of the
biotic , or living, factors —the other organisms that are part of an individual’s environment—also influence
the distribution and abundance of life on Earth.
Climate change. A directional change to the global climate that lasts three decades or more (as opposed
to short-term changes in the weather).
Biomes. Major life zones characterized by vegetation type in terrestrial biomes.
Climograph. A plot of the annual mean temperature and precipitation in a particular region.
Ecotone. The area of intergradation, may be wide or narrow.
Disturbance. Is an event such as a storm, fire, or human activity that changes a community, removing
organisms from it and altering resource availability
In tropical rain forests , rainfall is relatively constant, about 200–400 cm annually. In tropical dry forests ,
precipitation is highly seasonal, about 150–200 cm annually, with a six- to seven-month dry season.
Deserts occur in bands near 30° north and south latitude or at other latitudes in the interior of continents.
The savanna is warm year-round, averaging 24–29°C, but with somewhat more seasonal variation than in
tropical forests. Grasses and small nonwoody plants called forbs , which make up most of the ground
cover, grow rapidly in response to seasonal rains.
Chaparral. This biome occurs in midlatitude coastal regions on several continents, and its many names
reflect its far-flung distribution. Chaparral in North America, matorral in Spain and Chile, garigue and
maquis in southern France, and fynbos in South Africa.
The veldts of South Africa, the puszta of Hungary, the pampas of Argentina and Uruguay, the steppes of
Russia, and the plains and prairies of central North America are examples of temperate grasslands.
Extending in a broad band across northern North America and Eurasia to the edge of the arctic tundra,
the northern coniferous forest , or taiga, is the largest terrestrial biome on Earth.
A mature temperate broadleaf forest has distinct vertical layers, including a closed canopy, one or two
strata of understory trees, a shrub layer, and an herb layer.
Tundra. Covers expansive areas of the Arctic, amounting to 20% of Earth’s land surface. High winds and
low temperatures produce similar plant communities, called alpine tundra, on very high mountaintops at
all latitudes, including the tropics.
A permanently frozen layer of soil called permafrost restricts the growth of plant roots.
The lake environment is generally classified on the basis of three physical criteria: light penetration
(photic and aphotic zones), distance from shore and water depth (littoral and limnetic zones), and
whether the environment is open water (pelagic zone) or on the bottom (benthic zone).
Like lakes, the marine environment is generally classified on the basis of light penetration (photic and
aphotic zones), distance from shore and water depth (intertidal, neritic, and oceanic zones), and whether
the environment is open water (pelagic zone) or on the bottom (benthic and abyssal zones).
The upper photic zone is the region where there is sufficient light for photosynthesis, while the lower
aphotic zone is the region where little light penetrates. The photic and aphotic zones together make up
the pelagic zone. Deep in the aphotic zone lies the abyssal zone , the part of the ocean 2,000–6,000 m
below the surface. At the bottom of all of these aquatic zones, deep or shallow, is the benthic zone. Made
up of sand and organic and inorganic sediments, the benthic zone is occupied by communities of
organisms collectively called the benthos. A major source of food for many benthic species is dead organic
matter called detritus, which “rains” down from the productive surface waters of the photic zone.
In the ocean and in most lakes, a narrow layer of abrupt temperature change called a thermocline
separates the more uniformly warm upper layer from more uniformly cold deeper waters.
Turnover. Sends oxygenated water from a lake’s surface to the bottom and brings nutrient-rich water
from the bottom to the surface in both spring and autumn.
Winter: ↑cold water, ↓warm water
For a transplant to be successful, some of the organisms must not only survive in the new area but also
reproduce there sustainably. If a transplant is successful, then we can conclude that the potential range
of the species is larger than its actual range ; in other words, the species could live in certain areas where
it currently does not.
An ecological change, such as the expansion of a predator’s range, can alter the selective pressures faced
by prey populations. This could cause evolutionary change , such as an increase in the frequency of a new
defensive mechanism in a prey population; that change, in turn, could alter the outcome of ecological
interactions.
UNIT 2: Flow of Matter and Energy in Ecosystems (Key terms)- Chap. 55
Ecosystem. The sum of all the organisms living in a given area and the abiotic factors with which they
interact.
An ecosystem, regardless of its size, has two key emergent properties: energy flow and chemical cycling.
Unlike chemicals, energy cannot be recycled. Energy flows through ecosystems, whereas chemicals cycle
within them.
The trophic level that ultimately supports all others consists of autotrophs, also called the primary
producers of the ecosystem.
Heterotrophs. Depend directly or indirectly on the outputs of primary producers for their source of
energy. Herbivores, which eat plants and other primary producers, are primary consumers. Carnivores
that eat herbivores are secondary consumers , and carnivores that eat other carnivores are tertiary
consumers.
Another group of heterotrophs is the detritivores , or decomposers, refer to consumers that get their
energy from detritus. Detritus is nonliving organic material, such as the remains of dead organisms, feces,
fallen leaves, and wood.
Ecosystem’s primary production. The amount of light energy converted to chemical energy—in the form
of organic compounds—by autotrophs during a given time period.
Total primary production in an ecosystem is known as that ecosystem’s gross primary production (GPP) —
the amount of energy from light (or chemicals, in chemoautotrophic systems) converted to the chemical
energy of organic molecules per unit time. Sets the spending limit for the global energy budget.
Net primary production (NPP). Equal to gross primary production minus the energy used by the primary
producers (autotrophs) for their cellular respiration (Ra, where “a” stands for autotrophs):
NPP = GPP – Ra
Net ecosystem production (NEP). A measure of the total biomass accumulation during that time. NEP is
defined as gross primary production minus the total respiration of all organisms in the system (RT)—not
just primary producers, as for the calculation of NPP, but decomposers and other heterotrophs as well:
NEP = GPP – RT
The most common way to estimate NEP is to measure the net flux (flow) of CO2 or O2 entering or leaving
the ecosystem. When NEP>0, the ecosystem gains more carbon than it loses; such ecosystems store
carbon and are said to be a carbon sink. In contrast, when NEP<0, the ecosystem loses more carbon than
it gains; such ecosystems are a carbon source.
Limiting nutrient. The element that must be added for production to increase.
Eutrophication (from the Greek eutrophos, well nourished). When the primary producers die, detritivores
decompose them, depleting the water of much or all of its oxygen.
Secondary production. The amount of chemical energy in consumers’ food that is converted to their own
new biomass during a given period. Herbivores consume only about one-sixth of total plant production.
𝑁𝑒𝑡 𝑠𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑖𝑜𝑛 × 100%
Assimilation of primary production
Net secondary production. The energy stored in biomass represented by growth and reproduction.
Production efficiency. The percentage of energy stored in assimilated food that is used for growth and
reproduction, not respiration.
Trophic efficiency. The percentage of production transferred from one trophic level to the next. Must
always be less than production efficiencies because they consider not only the energy lost through
respiration and contained in feces, but also the energy in organic material in a lower trophic level that is
not consumed by the next trophic level.
One important ecological consequence of low trophic efficiencies is represented in a biomass pyramid, in
which each tier represents the total dry mass of all organisms in one trophic level.
Because nutrient cycles involve both biotic and abiotic components, they are called biogeochemical
cycles. We can recognize two general scales of biogeochemical cycles: global and local.
The oceans contain 97% of the water in the biosphere. Approximately 2% is bound in glaciers and polar
ice caps, and the remaining 1% is in lakes, rivers, and groundwater, with a negligible amount in the
atmosphere.
The largest reservoir of carbon is sedimentary rocks such as limestone ; however, carbon remains in this
pool for long periods of time.
The major pathway for nitrogen to enter an ecosystem is via nitrogen fixation , the conversion of N2 to
forms that can be used to synthesize organic nitrogen compounds.
Bioremediation. Using organisms—usually prokaryotes, fungi, or plants—to detoxify polluted
ecosystems.
In contrast to bioremediation, which is a strategy for removing harmful substances from an ecosystem,
biological augmentation uses organisms to add essential materials to a degraded ecosystem.
- Herbivory. An individual of one species eats part of a plant or other primary producer.
- Predation. An individual of one species kills and eats another.
- Type I curve: flat at the start, reflecting low death rates during early and middle life, and then
drops steeply as death rates increase among older age-groups.
- Type II curves: are intermediate, with a constant death rate over the organism’s life span.
- Type III curve: drops sharply at the start, reflecting very high death rates for the young, but
flattens out as death rates decline for those few individuals that survive the early period of die-
off.
Change in population size= Births + immigrants entering population – death – Emigrants leaving
population.
Birth (B) − 𝐷𝑒𝑎𝑡ℎ (𝐷) =
𝑇𝑖𝑚𝑒 𝑖𝑛𝑡𝑒𝑟𝑣𝑎𝑙 (t)
Exponential population growth. In some cases, a population that experiences such ideal conditions
increases in size by a constant proportion at each instant in time.
Intrinsic rate of increase. The per capita rate at which an exponentially growing population increases in
size at each instant in time.
Ecologists define the carrying capacity , symbolized by K, as the maximum population size that a
particular environment can sustain.
In the logistic population growth model, the per capita rate of population growth approaches zero as
the population size nears the carrying capacity (K).
The traits that affect an organism’s schedule of reproduction and survival make up its life history.
- Semelparity (from the Latin semel, once, and parere, to beget). Undergo a “one-shot” pattern of
big-bang reproduction. An organism dies after reproducing.
- Iteroparity (from the Latin iterare, to repeat), or repeated reproduction.
K-selection. Selection for traits that are advantageous at high densities. R-selection. Selection for traits
that maximize reproductive success in uncrowded environments (low densities).
A birth rate or death rate that does not change with population density is said to be density independent.
A death rate that increases with population density or a birth rate that falls with rising density is said to
be density dependent.
Such population fluctuations from year to year or place to place, called population dynamics, are
influenced by many factors and in turn affect other species.
Immigration and emigration are particularly important when a number of local populations are linked,
forming a metapopulation.
The movement from high birth and death rates toward low birth and death rates, which tends to
accompany industrialization and improved living conditions, is called the demographic transition.
Age structure. The relative number of individuals of each age in the population.
The ecological footprint concept summarizes the aggregate land and water area required by each person,
city, or nation to produce all the resources it consumes and to absorb all the waste it generates. Typically,
this estimate is made using global hectares, where a global hectare (gha) represents a hectare of land or
water with a productivity equal to the average of all biologically productive areas on Earth (1 hectare =
2.47 acres)
UNIT 4 : Community ecology (Key terms)-Chap. 5 4
A group of populations of different species living in close enough proximity to interact is called a biological
community.
Interspecific interactions. Include competition, predation, herbivory, parasitism, mutualism, and
commensalism.
- Competition. A - /- interaction that occurs when individuals of different species compete for a
resource that limits the survival and reproduction of each species.
o Competitive exclusion. A slight reproductive advantage will eventually lead to local
elimination of the inferior competitor.
o Ecological niche. The specific set of biotic and abiotic resources that an organism uses in
its environment.
o The differentiation of niches that enables similar species to coexist in a community is called
resource partitioning.
o A species’ fundamental niche, which is the niche potentially occupied by that species, is
often different from its realized niche, the portion of its fundamental niche that it actually
occupies.
o Character displacement. Tendency for characteristics to diverge more in sympatric than in
allopatric populations of two species.
- Exploitation. A general term for any +/- interaction in which one species benefit by feeding on
the other species, which is harmed by the interaction.
o Predation. A +/- interaction between species in which one species, the predator, kills
and eats the other, the prey.
o Morphological and physiological defensive adaptations:
▪ Aposematic coloration, warning coloration, such as that of poison dart frogs.
Coloration seems to be adaptive because predators often avoid brightly colored
prey. Cryptic coloration, or camouflage, makes prey difficult to see.
▪ Batesian mimicry, a palatable or harmless species mimics an unpalatable or
harmful species to which it is not closely related. Müllerian mimicry, two or
more unpalatable species, such as the cuckoo bee and yellow jacket, resemble
each other.
o Herbivory. Refer to an exploitative (+/-) interaction in which an organism—an
herbivore—eats parts of a plant or alga, thereby harming it.
o Parasitism. A +/- exploitative interaction in which one organism, the parasite, derives its
nourishment from another organism, its host, which is harmed in the process.
▪ Parasites that live within the body of their host, such as tapeworms, are called
endoparasites; parasites that feed on the external surface of a host, such as ticks
and lice, are called ectoparasites.
- Ecological communities are also heavily influenced by positive interactions , a term that refers to
a +/+ or +/0 interaction in which at least one species benefit and neither is harmed.
- Primary succession. When this process begins in a virtually lifeless area where soil has not yet
formed, such as on a new volcanic island or on the rubble (moraine) left by a retreating glacier.
- Secondary succession. when an existing community has been cleared by a disturbance that
leaves the soil intact.
Evapotranspiration. The evaporation of water from soil and plants.
Species-area curve. All other factors being equal, the larger the geographic area of a community, the
more species it has.
Zoonotic pathogens. Those that are transferred to humans from other animals, either through direct
contact with an infected animal or by means of an intermediate species, called a vector
UNIT 5 : Cell Membrane (Key terms)-Chap. 7
Selective permeability. It allows some substances to cross it more easily than others.
A phospholipid is an amphipathic molecule, meaning it has both a hydrophilic (“water-loving”) region and
a hydrophobic (“water-fearing”) region.
Fluid mosaic model. The membrane is a mosaic of protein molecules bobbing in a fluid bilayer of
phospholipids.
Integral proteins. Penetrate the hydrophobic interior of the lipid bilayer. The majority are transmembrane
proteins, which span the membrane; other integral proteins extend only partway into the hydrophobic
interior.
Peripheral proteins. Not embedded in the lipid bilayer at all; they are loosely bound to the surface of
the membrane, often to exposed parts of integral proteins.
Functions of membrane proteins:
- Transport. A protein that spans the membrane may provide a hydrophilic channel across the
membrane that is selective for a particular solute. Other transport proteins shuttle a substance
from one side to the other by changing shape.
- Enzymatic activity. A protein built into the membrane may be an enzyme with its active site
(where the reactant binds) exposed to substances in the adjacent solution.
- Signal transduction. A membrane protein (receptor) may have a binding site with a specific
shape that fits the shape of a chemical messenger, such as a hormone.
- Cell-cell recognition. Some glycoproteins serve as identification tags that are specifically
recognized by membrane proteins of other cells.
- Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of
junctions, such as gap junctions or tight junctions.
- Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other
elements of the cytoskeleton may be noncovalently bound to membrane proteins, a function
that helps maintain cell shape and stabilizes the location of certain membrane proteins.
Glycolipids. Membrane carbohydrates that are covalently bonded to lipids. Glyco refers to
“carbohydrate”. However, most are covalently bonded to proteins, which are thereby glycoproteins.
Nonpolar molecules , such as hydrocarbons, CO2, and O2, are hydrophobic, as are lipids. They can all
therefore dissolve in the lipid bilayer of the membrane and cross it easily, without the aid of membrane
proteins.
Polar molecules such as glucose and other sugars pass only slowly through a lipid bilayer, and even water,
a very small polar molecule, does not cross rapidly relative to nonpolar molecules.
Hydrophilic substances can avoid contact with the lipid bilayer by passing through transport proteins that
span the membrane. Some transport proteins, called channel proteins, function by having a hydrophilic
channel that certain molecules or atomic ions use as a tunnel through the membrane.
- Channel proteins simply provide corridors that allow specific molecules or ions to cross the
membrane.
o Aquaporins. Channel proteins that greatly facilitates the passage of water molecules
through the membrane in certain cells.
o Channel proteins that transport ions are called ion channels. Many ion channels function
as gated channels, which open or close in response to a stimulus.
- Carrier proteins. Hold onto their passengers and change shape in a way that shuttles them across
the membrane.
Diffusion. The movement of particles of any substance so that they spread out into the available space.
In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is
less concentrated.
- Diffusion of one solute. The membrane has pores large enough for molecules of dye to pass
through. The solute molecules continue to cross the membrane, but at roughly equal rates in both
directions.
- Diffusion of two solutes. Solutions of two different dyes are separated by a membrane that is
permeable to both. Each dye diffuses down its own concentration gradient.
Concentration gradient. The region along which the density of a chemical substance increases or
decreases.
The diffusion of a substance across a biological membrane is called passive transport because the cell
does not have to expend energy to make it happen. The concentration gradient itself represents potential
energy.
Osmosis. The diffusion of free water across a selectively permeable membrane, whether artificial or
cellular. The solution with a higher solute concentration has a lower free water concentration. Water
diffuses across the membrane from the region of higher free water concentration (lower solute
concentration) to that of lower free water concentration (higher solute concentration) until the solute
concentrations on both sides of the membrane are more nearly equal.
Tonicity. The ability of a surrounding solution to cause a cell to gain or lose water.
- Pinocytosis (Cellular drinking). A cell continually “gulps” droplets of extracellular fluid into tiny
vesicles, formed by infoldings of the plasma membrane.
- Receptor-mediated endocytosis. A specialized type of pinocytosis that enables the cell to acquire
bulk quantities of specific substances, even though those substances may not be very
concentrated in the extracellular fluid.
UNIT 6 : Cell-to-Cell Communication (Key terms)-Chap. 11
Apoptosis. A mechanism of programmed cell death that integrates input from multiple signaling
pathways. Protects neighboring cells from damage that they would otherwise suffer if a dying cell merely
leaked out all its contents, including its many digestive enzymes.
Quorum sensing. Allows bacterial populations to coordinate their behaviors in activities that require a
given number of cells.
Biofilm. An aggregation of bacterial cells adhered to a surface.
Communication by direct contact between cells:
- Cell junctions. Both animals and plants have cell junctions that allow molecules, including signaling
molecules, to pass readily between adjacent cells without crossing plasma membranes.
- Cell-cell recognition. Two cells in an animal may communicate by interaction between molecules
protruding from their surfaces.
Local signaling:
- Paracrine signaling. A signaling cell acts on nearby target cells by secreting molecules of a local
regulator (a growth factor, for example). Some molecules travel only short distances; such local
regulators influence cells in the vicinity.
Growth factors. Are compounds that stimulate nearby target cells to grow and divide.
- Synaptic signaling. A nerve cell releases neurotransmitter molecule into a synapse, stimulating
the target cell, such as a muscle or another nerve cell.
Long-distance signaling:
- Endocrine (hormonal) signaling. Specialized endocrine cells secrete hormones into body fluids,
often blood. Hormones reach most body cells, but are bound by and affect only some cells.
- Both animals and plants use molecules called hormones for long-distance signaling.
- In hormonal signaling in animals, also known as endocrine signaling, specialized cells release
hormones, which travel via the circulatory system to other parts of the body, where they reach
target cells that can recognize and respond to them.
The Three Stages of Cell Signaling:
- Reception. Reception is the target cell’s detection of a signaling molecule coming from outside
the cell.
- Transduction. The transduction stage converts the signal to a form that can bring about a specific
cellular response. Transduction sometimes occurs in a single step but more often requires a
sequence of changes in a series of different molecules—a signal transduction pathway.
- Relay molecules
- Response. In the third stage of cell signaling, the transduced signal finally triggers a specific
cellular response.
Ligand. The term for a molecule that specifically binds to another (often larger) molecule.
Three major types of cell-surface transmembrane receptors:
- A G protein-coupled receptor (GPCR) is a cell-surface transmembrane receptor that works with
the help of a G protein , a protein that binds the energy-rich molecule GTP.
- Receptor tyrosine kinases (RTKs ) belong to a major class of plasma membrane receptors
characterized by having enzymatic activity. An RTK is a protein kinase—an enzyme that catalyzes
the transfer of phosphate groups from ATP to another protein.
- A ligand-gated ion channel is a type of membrane channel receptor containing a region that can
act as a “gate,” opening or closing the channel when the receptor changes shape.
Special proteins called transcription factors control which genes are turned on—that is, which genes are
transcribed into mRNA— in a particular cell at a particular time.
Protein kinase. An enzyme that transfers phosphate groups from ATP to a protein.
Phosphorylation cascade. Depicted from a hypothetical pathway containing two different protein
kinases. Many of the relay molecules in signal transduction pathways are protein kinases, and they often
act on other protein kinases in the pathway. Add phosphate group to the next one in line.
Protein phosphatases. Enzymes that can rapidly remove phosphate groups from proteins, a process called
dephosphorylation.
Not all components of signal transduction pathways are proteins. Many signaling pathways also involve
small, nonprotein, water-soluble molecules or ions called second messengers. (The pathway’s “first
messenger” is considered to be the extracellular signaling molecule— the ligand —that binds to the
membrane receptor.)
The binding of epinephrine to the plasma membrane of a liver cell elevates the cytosolic concentration of
cyclic AMP (cAMP; cyclic adenosine monophosphate ). An enzyme embedded in the plasma membrane,
adenylyl cyclase (also known as adenylate cyclase ), converts ATP to cAMP in response to an extracellular
signal.
The pathways leading to calcium release involve two other second messengers, inositol trisphosphate
(IP3) and diacylglycerol (DAG).
Scaffolding proteins. Large relay proteins to which several other relay proteins are simultaneously
attached.
UNIT 7 : Neurons, Synapses, and Signaling (Key terms)-Chap. 48
The magnitude of the membrane voltage at equilibrium for a particular ion is called that ion’s equilibrium
potential (Eion).
Gated ion channels. Ion channels that open or close in response to stimuli. Voltage-gated ion channel, a
channel that opens or closes in response to a shift in the voltage across the plasma membrane of the
neuron.
This increase in the magnitude of the membrane potential, called a hyperpolarization , makes the inside
of the membrane more negative. A reduction in the magnitude of the membrane potential is a
depolarization.
Graded potential. A shift in the membrane potential. Has a magnitude that varies with the strength of the
stimulus: A larger stimulus causes a greater change in the membrane potential.
Action potential. If a depolarization shifts the membrane potential sufficiently, the result is a massive
change in membrane voltage. Has a constant magnitude and can regenerate in adjacent regions of the
membrane.
If a depolarization increases the membrane potential to a level called threshold , the voltage-gated sodium
channels open.
Sodium channels open first, initiating the action potential. As the action potential proceeds, sodium
channels remain open but become inactivated: a portion of the channel protein called an inactivation loop
blocks ion flow through the open channel. Sodium channels remain inactivated until after the membrane
returns to the resting potential and the channels close. Potassium channels open more slowly than sodium
channels, but remain open and functional until the end of the action potential.
The “downtime” when a second action potential cannot be initiated is called the refractory period. Due
to the inactivation of sodium channels, not to a change in the ion gradients across the plasma membrane.
The electrical insulation that surrounds vertebrate axons is called a myelin sheath. Myelin sheaths are
produced by glia: oligodendrocytes in the CNS and Schwann cells in the PNS.
In myelinated axons, voltage-gated sodium channels are restricted to gaps in the myelin sheath called
nodes of Ranvier. Action potentials propagate more rapidly in myelinated axons because the time-
consuming process of opening and closing of ion channels occurs at only a limited number of positions
along the axon. This mechanism for propagating action potentials is called saltatory conduction (from the
Latin saltare, to leap) because the action potential appears to jump from node to node along the axon.
Electrical synapses contain gap junctions that allow electrical current to flow directly from one neuron to
another. The majority of synapses are chemical synapses , which rely on the release of a chemical
neurotransmitter by the presynaptic neuron to transfer information to the target cell. At rest, the
presynaptic neuron synthesizes the neurotransmitter at each synaptic terminal, packaging it in multiple
membrane-enclosed compartments called synaptic vesicles.
Neurotransmitter released from the synaptic terminus diffuses across the synaptic cleft , the gap that
separates the presynaptic neuron from the postsynaptic cell.
The receptor protein that binds and responds to neurotransmitters is a ligand-gated ion channel , often
called an ionotropic receptor.
Postsynaptic potential. A graded potential in the postsynaptic cell. The depolarization that brings the
membrane potential toward threshold, it is called an excitatory postsynaptic potential (EPSP). A
hyperpolarization produced when the membrane potential moves further from threshold is an inhibitory
postsynaptic potential (IPSP).
Individual postsynaptic potentials combine to produce a larger postsynaptic potential, a process called
summation. If the second EPSP arises before the postsynaptic membrane potential returns to its resting
value, the EPSPs add together through temporal summation. Summation can also involve multiple
synapses on the same postsynaptic neuron. If such synapses are active at the same time, the resulting
EPSPs can add together through spatial summation.
The resulting opening or closing of ion channels depends on one or more metabolic steps, these G protein-
coupled receptors are also called metabotropic receptors.
Acetylcholine. A common neurotransmitter in both invertebrates and vertebrates.
- Ligand-gated ion channel. Function at the vertebrate neuromuscular junction, the site where a
motor neuron forms a synapse with a skeletal muscle cell.
- G protein-coupled receptor for acetylcholine is found at locations that include the vertebrate
CNS and heart. In heart muscle, acetylcholine released by neurons activates a signal transduction
pathway.
Amino Acids:
- Glutamate. One of several amino acids that can act as a neurotransmitter.
- Glycine. Acts at inhibitory synapses in parts of the CNS that lie outside of the brain.
- Within the brain, the amino acid gamma-aminobutyric acid (GABA) is the neurotransmitter at
most inhibitory synapses in the brain.
Biogenic amines:
- The neurotransmitters grouped as biogenic amines are synthesized from amino acids and include
norepinephrine , which is made from tyrosine.
- The biogenic amines dopamine (made from tyrosine) and serotonin ( made from tryptophan) are
released at many sites in the brain and affect sleep, mood, attention, and learning.
Neuropeptides. Relatively short chains of amino acids, serve as neurotransmitters that operate via G
protein-coupled receptors. Endorphins, function as natural analgesics, decreasing pain perception
REFERENCE:
Urry, L. A., Cain, M. L. 1., Wasserman, S. A., Minorsky, P. V., Reece, J. B., & Campbell, N. A. (2017). Campbell
biology. Eleventh edition. New York, NY, Pearson Education, Inc.
