Biological membranes. The mechanism of a transport of the ...€¦ · Functions of Membrane 1) Structural function. The outer membrane of a cell separates its contents from environment,

Biological membranes. The mechanism of a transport of the

substances. Biopotentials.

L.D.Korovina

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General Plan of Cell Structure

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General Plan of Membrane Structure

• Three-layer construction of cytoplasmic membrane; X 300000.

• Dis-symmetry of two stratums can be result of polarization of a membrane.

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Outer medium

Cytoplasm


• Membranes consist of a phospholipid bilayer combined with a variety of proteins in a fluid mosaic arrangement.

• The surfaces of cell membranes are hydrophilic (water-loving); the interiors are hydrophobic.Hydrophilic molecules tend to interact with water and with each other. Hydrophobic molecules avoid interaction with water and tend to interact with other hydrophobic molecules.

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Structure of Membrane• Membranes consist of a phospholipid bilayer combined with a

variety of proteins in a fluid mosaic arrangement.

• The surfaces of cell membranes are hydrophilic (water-loving); the interiors are hydrophobic.Hydrophilic molecules tend to interact with water and with each other. Hydrophobic molecules avoid interaction with water and tend to interact with other hydrophobic molecules.

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Structure of MembraneRough scheme

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Polar water molecules

Polar phospholipid heads

Hydrophobic phospholipid tails

Polar phospholipid heads

Polar water molecules

Membrane Molecules

Representations of membrane molecules:

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Phosphatidate

Movements in

the membrane


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General Plan of Membrane StructureTwo types of membrane proteins: transmembrane or attached to one side.Transmembrane proteins and other proteins that are strongly bound to the

membrane (e.g. by coupling to transmembrane proteins) are known as Integral Membrane Proteins (1-3, 7, 8). Semisubmerged membrane proteins(4) are too.

Other non-spanning membrane proteins are Peripheral Membrane Proteins (5, 6).

The polypeptide chain of transmembrane proteins crosses the lipid bilayer one or more times. These proteins are amphipathic.

Many membrane proteins diffuse in the plane of the membrane.

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Functions of Membrane

1) Structural function.

2) Protective function.

3) Transport function.

4) Enzimatic function.

5) Electrogenic function.

6) Adhesion function.

7) Receptor function.

8) Antigenic function.

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SIGNAL FUNCTION


1) Structural function. The outer membrane of a cell separates its contents from environment, determines the form of a cell. Sheaths (covers) of organoids divide contents of various regions of a cell, providing their different composition, and, accordingly, different biochemical reactions in them.

2) Protective function. If the cytoplasmic membrane protects contents of a cell from undesirable influences of environment that membranes of organoids carry out similar function in relation to own contents. For example, membranes of lysosomes protect a cell from its lytic enzymes, which it is enough to cause a complete autolysis (self-destruction) of a cell.

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Functions of Membrane3) Transport function. It is carried out due to presence of pores, canals and

proteins - transmitting agents. Transport of substances on these structures is carried out selectively and depends on a structure of substances. Pores and canals have small diameter (less than 1 nm), are covered by proteins which polar groupings cooperate with substances penetrating into a pore, promoting or, on the contrary, hindering from their transmission. Hydrophobic groups cooperate with lipids of a membrane. The permeability of membranes can vary in dependence on a cell state.

4) Enzimatic function. The most part of enzymes of a cell is concentrated on membranes.

Thus membranes organize biochemical reactions, including multistage (multiphasic, sequential). They provide strictly certain directivity of reactions due to the certain sequence in a locating of enzymes. A vivid example is respiratory chains of mitochondrions.

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5) Electrogenic function. of membranes consists that the outer membrane of cells due to presence ATPase-carriers of ions creates a difference of concentrations K+, Na+, Са2+ and Сl– between cytoplasm and intercellular medium.

The different permeability of a membrane for these and other ions results in appearance of a superfluous negative charge inside a cell (because of a diffusion of ions K+ in environment) and, hence, to a potential difference between the intrinsic and outside sides of a membrane.

ATPase (adenosine triphosphatase) - the common name of the proteins decomposing ATP on ADP and phosphate. Energy releasing at it is used for realization of endothermic (thermonegative) reactions.

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Functions of Membrane6) Adhesion function. Membranes carry out a coupling of cells.This property causes existence of multicellular organisms. The adhesion

happens, despite of presence of the same charge of cells. The basic mechanisms of an adhesion:

a) Mechanical interaction - engagement (gearing) - of acting parts of membranes;

b) "Coagglutination" of membranes at participation of organic salts of the calcium cooperating with carboxylic groups of proteins and phosphatic groups of lipids;

c) Interaction of the proteins coating (covering) membranes - formation of peptide bonds.

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Desmosomes (bridge

corpuscles) of calf

epidermis.

Functions of Membrane7) Receptor function. On membranes there are receptor proteins or

complexes which accept influences from environment (for example, cooperate with hormones or mediators) and accordingly vary metabolism of a cell.

8) Antigenic function consists that on a cellular membrane there are peptide structures, characteristic for cells of the given tissue of the given organism. These structures are intended for mutual "recognition" of cells. Due to their presence exercise of a host (immune) defence of an organism is possible.

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Antigenic activity of a

membrane. A human

erythrocyte.

Transport of Substances through the Membrane

Passive and Active TransportPassive transport is such which descends due to presence of various

gradientsActive transport uses energy to move a solute "uphill" against its

gradient, whereas in facilitated diffusion, a solute moves down its concentration gradient and no energy input is required.

Most biologically important solutes require protein carriers to cross cell membranes, by a process of either passive or active transport.

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Comparing Facilitated Diffusion and Active TransportTransport of solutes across cell membranes by protein carriers can

occur in one of two ways:The solute can move "downhill," from regions of higher to lower

concentration, relying on the specificity of the protein carrier to pass through the membrane. This process is called passive transport or facilitated diffusion, and does not require energy.

The solute can move "uphill," from regions of lower to higher concentration. This process is called active transport, and requires some form of chemical energy.

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Passive transport kindsIt is such which descends due to presence of various

gradients.

Filtration is a mass stream of fluid through a membrane. It is carried out owing to presence of a difference of hydrostatic pressure.

Diffusion is locomotion of molecules of solute from area of greater in area with its smaller concentration that is result of thermal movement. So there is a transport of electroneutral molecules with molecular mass up to 100-150.

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Passive transport kinds

Electrodiffusion is locomotion of charged molecules of solute in condition of appearance of concentration and electrical gradients. Direction of movement relate from both factors. If both gradients promote movement in one direction, resulting velocity rise; if gradients promote movement in opposite directions, resulting velocity decrease.

Osmosis is locomotion of a dissolvent (in biological objects –water) from range with smaller concentration of solute in range with the greater concentration. As a matter of fact, it is a diffusion of a dissolvent.


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Diffusion


DiffusionRate of diffusion is determined on Fick equation:

, (1)

where dm/dt – rate of a diffusion (mass of the substances born in unit of time through the area S); dC/dx – gradient of concentrations; x – distance between points in which concentrations C are observed; D – diffusion coefficient which depends by substance nature, densities and viscosity of medium, temperature and other factors and is measured in cm2/sec.

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dx

dCDS

dt

dm


DiffusionRate of diffusion through membrane is determined

Kollender and Berlund:

, (1)

where dm/dt – rate of a diffusion (mass of the substances born in unit of time through the area S); C1 and C2 –concentrationsof substance on both sides of membrane; x – thickness of membrane; D/x – permeability of membrane to substance P.

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)()( 2121 CCPSCCSx

D

dt

dm


OsmosisIt is locomotion of a dissolvent (in biological objects – water) from range with

smaller concentration of solute in range with the greater concentration. As a matter of fact, it is a diffusion of a dissolvent. This appearance is observed in the presence of semipermeablemembrane between specified areas.

• Electroosmosis is locomotion of a dissolvent in an electrical field if moleculas of a dissolvent are charged, or owing to the osmosis accompanying locomotion of dissolved charged particles.

• Abnormal osmosis happens at presence both a concentration gradient, and conditions for electroosmosis. If the electrical gradient interferes with osmosis, and locomotion of a dissolvent goes on an electrical gradient against concentration gradient, negative abnormal osmosis is observed. If electrical gradient only reduces rate of transmission on a concentration gradient or even enlarges, it speak about a positive abnormal osmosis.

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Difference of incoming and outcoming flows.

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Osmotic pressureThe pressure difference between solutions on two sizes of the

membrane, necessary for the stopping of osmosis, refers to osmotic pressure.

The osmotic pressure of solution is usual determine in relation to a pure dissolvent.

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Movement of water in different parts of blood vessel

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Mechanisms of Movement Across Cell Membranes

Solutes fall into one of three groups:

Small lipophilic (lipid soluble) molecules that cross the membrane by diffusion alone;

Molecules that cross the membrane due to protein-mediated transport;

Molecules, usually of very large size, that do not cross the membrane at all.

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Selective Permeability of MembranesCell membranes are selectively permeable. Some solutes cross the

membrane freely, some cross with assistance, and others do not cross at all.

A few lipophilic substances move freely across the cell membrane by passive diffusion. Most small molecules or ions require the assistance of specific protein carriers to transport them across the membrane. Large molecules do not cross intact cell membranes, except in certain special cases.

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Porter proteins


A Closer Look at Facilitated Diffusion CarriersThere are two types of facilitated diffusion carriers:1. Channel proteins transport only water or certain ions. They do so

by forming a protein-lined passageway across the membrane. 2. Many water molecules or ions can pass in single file through such

channels at very fast rates. Each type of channel protein is very selective for passage of a specific

substance; for example, some channels allow only K+ ions to pass. Many of these channels are gated; they are closed and unavailable for transport except when certain signals are present. Gated channels are important in regulating nerve conduction in animals.

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Facilitated diffusion• Uniporters (not conjugate transport) normally transport

organic molecules, such as sugars and amino acids. These specific protein carriers move molecules one at a time down a concentration gradient.

• Most tissues in our body have a variety of uniporters that carry glucose and amino acids into their cells.

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The transport process a cell uses depends on its specific needs.

For example, red blood cells rely on facilitated diffusion to move glucose across membranes, whereas intestinal epithelial cells use active transport to take in glucose from the gut.

Facilitated diffusion is effective for red blood cells because the concentration of glucose in the blood is stable and higher than the cellular concentration.

On the other hand, active transport is needed in the gut (intestine) because there are large fluctuations of glucose concentration as a result of eating.

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Facilitated diffusion• Symport (coupled transfer )• To transport some substances against a concentration gradient,

cells use energy already stored in ion gradients, such as proton (H+) or sodium (Na+) gradients, to power membrane proteins called transporters. When the transported molecule and the co-transported ion move in the same direction, the process is known as symport.

• An example of a symport process is the transport of amino acids across the intestinal lining in the human gut and glucose in erythrocytes .

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Facilitated diffusion

• Antiport (exchange diffusion)

• In antiport, a cell uses movement of an ion across a membrane and down its concentration gradient to power the transport of a second substance "uphill" against its gradient. In this process, the two substances move across the membrane in opposite directions.

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Facilitated Diffusion and Active Transport of Glucose• By contrast, erythrocytes (red blood cells) and most other tissues move

glucose by facilitated diffusion carriers, not by active transport. • The environment is different for red blood cells and the gut. • Glucose concentration in the blood is carefully regulated so that it is

normally higher than intracellular concentrations. Glucose is transported across erythrocyte membranes by a uniporter, a type of facilitated diffusion protein.

• As soon as glucose enters the cell, it is converted into other chemicals needed for by the cell for energy production or biosynthesis, so the intracellular concentration of glucose remains much lower than the 5 mM glucose level normally maintained in blood.

• In this situation, diffusion alone can ensure a constant flow of glucose into the erythrocyte, so it would be wasteful and unnecessary for erythrocytes to use active transport for glucose.

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Mechanisms of Active TransportActive transport can occur as a direct result of ATP hydrolysis (ATP

pump) or by coupling the movement of one substance with that of another (symport or antiport).

Active transport may move solutes into the cell or out of the cell, but energy is always used to move the solute against its concentration gradient.

Uncoupled active transport. As its example awake conclusion Са2+ from cytoplasm of muscular cells in environment or sarcoplasmic reticulum can serve.

It is carried out by calcium-dependent adenosinetriphosphatase (Са2+-ATPase) which decompose ATP, simultaneously transferring ions due to released energy.

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Active Transport

Coupled active transport descends the same way as, but substances of two types in opposite directions in one cycle of work of an enzyme only are transfer.

An example is K+-Na+ pump (K+-Na+-ATPase).

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• Active Transport• Most living cells maintain internal environments that are different from

their extracellular environment, as well as concentration differences between the cytosol and internal compartments.

• In all human cells a concentration of Na+ outside the cell is higher than inside, and a concentration of K+ inside the cell is higher than outside.

• These concentration gradients of Na+ and K+ represent a form of energy storage.

• An other example: in the lysosome (internal compartment) the concentration of hydrogen ions (H+) can be 100 to 1000 times greater than the concentration outside, in the cytosol. Like pushing an object uphill, moving a molecule against a concentration gradient requires energy.

• Cells have evolved active transport proteins that can use energy to establish and maintain concentration gradients.

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ATP-powered Pumps• ATP-powered pumps (ATPases) couple the splitting, or hydrolysis, of ATP

with the movement of ions across a membrane against a concentration gradient. ATP is hydrolyzed directly to ADP and inorganic phosphate, and the energy released is used to move one or more ions across the cell membrane. As much as 25% of a cell's ATP reserves may be spent in such ion transport.

Examples include:• The Na+-K+ ATP-ase pumps Na+ out of the cell while it pumps K+ in.

Because the pump moves three Na+ to the outside for every two K+ that are moved to the inside, it creates an overall charge separation known as polarization. This electrical potential is required for nervous system activity, and supplies energy needed for other types of transport such as symport and antiport.

• Ca++ ATP-ases are responsible for keeping intracellular Ca++ at low levels, a necessary precondition for muscle contractionю

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Electrogenesis of the Membrane• Pumps of such type can transfer ions Nа+ and K+ in a

proportion 1:1. In that case only gradients of ions concentration are formed. If ions are transfer not fifty-fifty, the pump participates in formation of membrane potential.

• In all cases quantity of positive charges (Na+ ions) removed from the cell is equal or more than quantity of positive charges (K+ ions) entered into the cell.

• Such pumps refer to electrogenic pump. For example, Nа+:К+

exchange can occur in proportion 3:2.

• Proton pump works in inner membrane of mitochondrions. Energy for it is delivered by respiratory enzymes

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Electrogenesis of the Membrane• Diffusion

at selective permeability:electrical gradientproduction

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Electrogenesis• The sodium (Na+) and chloride (CI–) ions are more outside and potassium

(K+) ions are more inside.• Inorganic ions (such as Na+ and K+) are able to penetrate through pores

within integral proteins that span the thickness of the double phospholipid layers.

• Besides the cell membrane has a selective permeability for ions. The ion channels available in a membrane, can be open or closed, that depends on transmembrane potential and a membrane state. So, in a cell which is taking place in rest, sodium channels are closed, potassium channels - are open. Therefore the membrane permeability for different ions is various. An interrelation of permeabilities for ions К+, Na+ and Сl– in rest (we accept a permeability for К+ for 1): К+:Na+: Сl– = 1 : 0,04 : 0,45 (data of Hodjkin and Katz).Consequently, K+ diffises much more rapidly than Na+.

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Electrogenesis. A rest potentialIt results to that K+ ions diffuse from a cell in extracellular space on a

concentrations gradient. The counter current of Na+ ions is very small (in 25 times below) and it is compensated by work of Na+-К+-pump.

In result between an internal cell environment where there is an anion excess, and the outside medium where cation excess collects, potential difference is created which for different cells is peer from –60 mV to –100 mV and results in the stoppage of the further K+ ions diffusion. This transmembrane potential difference also refers to as a rest potential.

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ElectrogenesisThe membranous potential is a particular case of the

diffusion potential incipient on a boundary of two fluid mediums as a result of various ionic mobility.

If mobility of anion V=0 the membranous potential difference is calculated with the help of Nernst equation:

,(6)

where R - gas constant, T - Kelvin temperature, n - valence of ions, F - a Faraday constant, a1 and a2 - ion activity in areas, whence (a1) and where (a2) there is a diffusion.

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a

aln

nF

RTE

ElectrogenesisThe membranous potential is a particular case of the

diffusion potential incipient on a boundary of two fluid mediums as a result of various ionic mobility.

If mobility of anion V=0 the membranous potential difference is calculated with the help of Nernst equation:

,(6)

where R - gas constant, T - Kelvin temperature, n - valence of ions, F - a Faraday constant, a1 and a2 - ion activity in areas, whence (a1) and where (a2) there is a diffusion.

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2

1

a

a

nF

RTE ln

mVa

a

nE

2

1158 lg

After substitution of constants in this

equation and transition from natural

logarithms to decimal logarithms:

Electrogenesis

To more precise definition of potential apply Goldman the generalized equation:

,

where PK, PNa, PCl – membrane permeability coefficients to ions of potassium, sodium and chlorine;

[K], [Na], [Cl] – their activities in (i) and out (e) of cell.

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iCleNaeK

eCliNaiK

ClPNaPKP

ClPNaPKP

nF

RTE ln

Electrogenesis

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ElectrogenesisDielectric breakdown occurs when a charge buildup

exceeds the electrical limit or dielectric strength of a material.

The dielectric strength of air is approximately 3 kV/mm. Its exact value varies with the shape and size of the electrodes and increases with the pressure of the air.

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mVm

V

d

VE /

The bioelectric phenomena. A rest potential

GeneralizationResting Membrane Potential - positivity outside and more

negativity inside the cell.

The ionic imbalance is produced mainly by two transport mechanisms in the cell membrane:

• Sodium-potassium pump and

• Selective permeability of cell membrane.

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The bioelectric phenomena. Action potential

Action potential is a process of change of a transmembrane potential difference down to change of its sign, proceeding on the rules determined for the given tissue even after the terminal of irritator action.

The tissue, capable to respond generating of action potential for irritation, refers to excitable tissue, process of generating refers to excitation.

To excitable tissues carry all kinds of a muscular, nervous and glandular tissue.

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The bioelectric phenomena. Action potentialRest potential of a nervous tissue can make about –70 mV.

The phase of depolarization is caused by opening of fast Na+-canals and a current of Na+ ions inside of a cell.

On its termination Na+-canals are inactivated. Duration of inactivation, i.e. impossibility of canals to be opened in reply to irritation, is various in cells of different kinds. In one cell kind inactivation lasts less, than the phase of repolarization, in others - is much longer.

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The transmembrane potential during

peak of action potential (overshoot)

can achieve +30 mV.

During repolarization the

strengthened exit of К+ ions (on

concentrations gradient, and in the

beginning – and on electrical gradient

too) is gradually slowed down, the

fast repolarization is replaced by slow

repolarization.


Some time membrane potential can change about a rest potential level. These processes (trace phenomena) can be as after depolarization (negative afterpotential), and an after hyperpolarization (positive afterpotential).

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Overshoot

Firing level

The bioelectric phenomena. Action potentialThe threshold of stimulation (the least irritator force, capable to cause action

potential) varies during exaltation.

When a stimulus(irritator) is applied, there is a slight irregular deflection of baseline for a very short period. This is called local potential.

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The bioelectric phenomena. Action potential• Latent Period• The local potential is followed by a short period

without any change. This period is called latent period, which is about 0.5 to 1 millisecond.

• If it irritate cell in time of slow depolarization (when membrane potential lower than in the rest but upper than threshold potential) that even subliminal potential can call acceleration of polarization and appearance of action potential. That is, cell excitability is raised in this time.

• Firing Level (Threshold Potential, Critical Depolarization Level)

• Depolarization starts after the latent period. The point at which, the rate of depolarization increases is called firing level or critical depolarization level.

• Overshoot• From firing level, the curve reaches the isoelectric

potential (zero potential) rapidly and then overshoots the zero line up to +55 mV (spike potential).

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For excitable tissues specific reactions during exaltation are characteristic also.

For a muscular tissue is a contraction,

for glandular – secretion of secret,

for nervous - carrying out of exaltation.

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Propagation of Action PotentialFor nervous tissues and long muscles cells

AP propagation is typical process.

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Propagation of Action Potential

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Propagation of Action Potential:Saltatory conduction

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Propagation of Action Potential:Saltatory conduction

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Documents

Biological membranes. The mechanism of a transport of the ...€¦ · Functions of Membrane 1) Structural function. The outer membrane of a cell separates its contents from environment,