Active Transport Mechanism
Transcript of Active Transport Mechanism
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ACTIVE TRANSPORT MECHANISM
PRIMARY ACTIVE TRANSPORT
SECONDARY ACTIVE TRANSPORT
TERTIARY ACTIVE TRANSPORT
AND
SODIUM POTASSIUM PUMP
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PRESENT
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OUTLINE
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TERMINOLOGIES
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INTRODUCTION
The plasma membrane is both a barrier and gateway between the cytoplasm and the extracellular fluid.
It is selective permeable i.e. allows some substances to pass through it such as nutrients and wastes but
usually prevent other substance such as proteins and phosphates from entering or leaving the cells.
All the cells in the body must be supplied with essential substances like nutrients, water, electrolytes, etc.
Cells also must get rid of many unwanted substances like carbon dioxide in order for the cell to survive.
The cells achieve these by means of transport mechanisms across the cell membrane.
Two types of mechanisms are involved in transport of substances across the cell membrane. These arepassive and active transport.
Passive transport is the movement of substances along the concentration gradient or electrical gradient. It
does not require energy expenditure of the cell. In most cases the random molecular motion of the
particles themselves provide the necessary energy . Examples include simple diffusion, osmosis, filtration
and facilitated diffusion which is carrier mediated.
Active transport mechanism however consumes energy in form adenosine triphosphate, example include
active transport and vesicular transport. Example of active transport is the sodium-potassium (Na-K) pump, also known as Na-KATPase because
the carrier is an enzyme that hydrolyzes ATP.
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Fig.1 diagrammatic representation of membrane transport.
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ACTIVE TRANSPORT
Active transport is the carrier-mediated transport of Substancesthrough a membrane against the concentration gradient orelectrical gradient.
It is also referred to as uphill transport of substances since its amovement against the concentration gradient.
Movement of the transported substance to the opposite side of themembrane and its subsequent release from the carrier protein arefueled by the breakdown of ATP.
The maximum rate at which active transport proceeds depends onthe number of carrier proteins in the plasma membrane and theavailability of adequate ATP.
It is usually associated with accumulating high concentrations ofmolecules that the cell needs, such as ions, glucose and amino acids
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Examples;
. Examples of active transport include the
transportation of sodium out of the cell and
potassium into the cell by the sodium-
potassium pump.
Active transport often takes place in the
internal lining of the small intestine.
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Carrier proteins involved in active
transport;
Carrier protein of active transport can be
uniporter
symporter
antiporter.
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Fig.2: diagrammatic representation of carrier proteins involved
in active transport.
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uniporter It is an integral
membrane protein
which is involved in
the movement of
one type ofmolecule or ion
across the plasma
membrane
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Antiporter is an integral
membrane protein
which is involved in
movement of two or
more different
molecules or ionsacross a as the plasma
membrane in opposite
directions.
Example is the Na-K
pumps
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Symporter It is an integral
membrane proteinwhich is involved inmovement of two ormore different
molecules or ionsacross the plasmamembrane in thesame direction.
Example is the
sodium glucosetransporter in theabsorptive cells of thesmall intestine andrenal tubules
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FEATURES OF ACTIVE TRANSPORT
Substances, which are transported actively, are in ionicand non-ionic form. Substances in ionic form aresodium, potassium, calcium, hydrogen, chloride andiodide. Substances in non-ionic form are glucose,
amino acids and urea. It is carrier mediated
Its uphill
It requires energy which is derived mainly from thebreakdown of ATP
It occurs in all human cells.
Process can be modified by drugs such as ouabain,aspirin
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TYPES OF ACTIVE TRANSPORT
Primary active transport
Secondary active transport
Tertiary active transport
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PRIMARY ACTIVE TRANSPORT
This is the type of active transport in which the energy is deriveddirectly from the breakdown of adenosine triphosphate (ATP) withthe process. Ions such as sodium, potassium, calcium, hydrogen,chloride etc. are transported across the plasma membrane.
Most of the enzymes that perform this type of transport are trans-
membrane ATPase A primary ATPase universal to all animal life is the sodium-
potassium pump, which helps to maintain the cell potential.
Other sources of energy for Primary active transportare redox energy. An example of primary active transport usingRedox energy is the mitochondrial electron transport chain that
uses the reduction energy of NADH to move protons across theinner mitochondrial membrane against their concentrationgradient.
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OTHER EXAMPLES;
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PRIMARY ACTIVE TRANSPORT OF CALCIUM
IONS:
Calcium ions are normally maintained at extremely low concentration in
the intracellular cytosol of virtually all cells in the body, at a concentration
about 10,000 times less than that in the extracellular fluid.
This is achieved mainly by two primary active transport calcium pumps.
One is in the cell membrane and pumps calcium to the outside of the cell.
The other pumps calcium ions into one or more of the intracellular
vesicular organelles of the cell, such as the sarcoplasmic reticulum of
muscle cells and the mitochondria in all cells.
In each of these instances, the carrier protein penetrates the membrane
and functions as an enzyme ATPase, having the same capability to cleaveATP as the ATPase of the sodium carrier protein.
The difference is that this protein has a highly specific binding site for
calcium instead of for sodium.
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Fig.3: Active transport of calcium ions
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PRIMARY ACTIVE TRANSPORT OF
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PRIMARY ACTIVE TRANSPORT OF
HYDROGEN IONS:
Primary active transport of hydrogen ions at
two places in the body,
In the gastric glands of the stomach.
In the late distal tubules and cortical
collecting ducts of the kidneys.
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Fig.5:Primary active transport of hydrogen ions
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SECONDARY ACTIVE TRANSPORT
This is the type of active transport in which
the energy is derived secondarily from energy
that has been stored in the form of ionic
concentration differences of secondarymolecular or ionic substances between the
two sides of a cell membrane, created
originally by primary active transport.
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forms of secondary active transport
Co- transport
Counter transport
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Fig.6: Secondary Active TransportCo-Transport and Counter-Transport
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CO-TRANSPORT
The concentration gradient developed fromthe primary active transport of sodium ionsserves as a storehouse of energy because the
excess sodium outside the cell membrane isalways attempting to diffuse to the interior.
This diffusion energy of sodium pulls othersubstances (e.g. glucose, amino acids) alongwith the sodium through the cell membrane.
This phenomenon is called co-transport.
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SODIUM GLUCOSE TRANSPORT
The transport carrier protein has two binding sites on itsexterior side, one for sodium and one for glucose.
Due to the very high concentration of sodium ions on theoutside and very low inside, energy is provided for thetransport.
A special property of the transport protein is that aconformational change to allow sodium movement to theinterior will not occur until a glucose molecule alsoattaches.
When they both become attached, the conformationalchange takes place automatically, and the sodium andglucose are transported to the inside of the cell at the sametime.
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Fig.7: diagrammatic representation of sodium glucose
transport
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Counter transport
In counter-transport, sodium ions again attempt to diffuseto the interior of the cell because of their largeconcentration gradient.
However, this time, the sub- stance to be transported is onthe inside of the cell and must be transported to theoutside.
Therefore, the sodium ion binds to the carrier proteinwhere it projects to the exterior surface of the membrane,while the substance to be counter-transported binds to theinterior projection of the carrier protein.
Once both have bound, a conformational change occurs,and energy released by the sodium ion moving to theinterior causes the other substance to move to the exterior.
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Tertiary transport
This is the type of active transport in which
the energy is derived from energy that has
been stored in the form of ionic concentration
differences from secondary active transport ofsubstances between the two sides of a cell
membrane, created originally by primary
active transport.
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EXAMPLE OF TERTIARY ACTIVE
TRANSPORT
In the transcellular reabsorption of Clin the
late proximal tubule where the energetically
uphill influx of Clacross the apical membrane
occurs through an exchange of luminal Clforcellular anions (e.g., formate, oxalate, HCO3
,
and OH).
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Fig.8: diagrammatic representation of tertiary active
transport.
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SODIUM-POTASSIUM ATPase
Sodium-potassium (Na-K) pump is an electrogenic pumpthat actively transports sodium ions outward through thecell membrane of all cells and at the same time pumpspotassium ions from the outside to the inside.
It is responsible for;
I. maintaining the sodium and potassium concentrationdifferences across the cell membrane,
II. as well as for establishing a negative electrical voltageinside the cells which is also the basis of nerve function,transmitting nerve signals throughout the nervous
system. It is responsible for cells containing relatively high
concentrations of potassium ions but low concentrationsof sodium ions.
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DISCOVERY
Na+/K+-ATPase was discovered by Jens
Christian Skou in 1957 while working as
assistant professor at the Department of
Physiology, University of Aarhus, Denmark.
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STRUCTURE:
The carrier protein is a complex of two separateglobular proteins: a larger one called the subunit,(approx. 1000 amino acids) with amolecular weight of about 100,000 and a smaller
one called the subunit, (approx.305 aminoacids), with a molecular weight of about 55,000.Although the function of the smaller protein isnot known (except that it might anchor the
protein complex in the lipid membrane), thelarger protein has three specific features that areimportant for the functioning of the pump
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Fig.9: structure of NaKATPase
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Structure contd
It has three receptorsites for binding sodium
ions on the portion of the
protein that protrudes to
the inside of the cell.
It has two receptor sitesfor potassium ions on the
outside.
The inside portion of
this protein near the
sodium binding sites hasATPase activity.
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Fig.10: MECHANISM OF ACTION OF Na+/K+ATPase
ELECTROGENIC NATURE OF THE NAK
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ELECTROGENIC NATURE OF THE NA K
PUMP.
the Na-K pump is said to be electrogenicbecause it creates an electrical potential acrossthe cell membrane.
The fact that the Na-K pump moves three Na
ions to the exterior for every two K ions to theinterior, means that a net of one positive chargeis moved from the interior of the cell to theexterior for each cycle of the pump.
This creates positivity outside the cell but leavesa deficit of positive ions inside the cell; that is, itcauses negativity on the inside.
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REGULATION:
Endogenous regulation
Exogenous regulation
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ENDOGENOUS
The Na+/K+-ATPase is upregulated by cAMP.
Thus, substances causing an increase in cAMP
upregulate the Na+/K+-ATPase. These include
the ligands of the Gs-coupled GPCRs.
In contrast, substances causing a decrease in
cAMP downregulate the Na+/K+- ATPase.
These include the ligands of the Gi-coupledGPCRs.
EXOGENOUS
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EXOGENOUS
The Na
+
/K
+
-ATPase can be pharmacologicallymodified by administrating drugs exogenously.
E.g. digoxinand ouabain
http://en.wikipedia.org/wiki/Digoxinhttp://en.wikipedia.org/wiki/Ouabainhttp://en.wikipedia.org/wiki/Ouabainhttp://en.wikipedia.org/wiki/Digoxin -
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FUNCTIONS
Na+/K+- ATPase helps maintain resting
potential,
avail transport,
regulate cellular volume
Functioning as signal transducer:
Controlling neuron activity states:
Heat production:
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RESTING POTENTIAL
Na+/K+- ATPase, as well as effects of diffusion of theinvolved ions maintain the resting potential across themembranes.
In order to maintain the cell membrane potential, cellskeep a low concentration of sodium ions and high levels ofpotassium ions within the cell (intracellular). The sodium-potassium pump moves 3 sodium ions out and moves 2potassium ions in, thus, in total, removing one positivecharge carrier from the intracellular space.
The action of the sodium-potassium pump is not the only
mechanism responsible for the generation of the restingmembrane potential. Also, the selective permeability of thecell's plasma membrane for the different ions plays animportant role.
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