Passive and active Passive and active transporttransport If we have semipermeable membrane If we have semipermeable membrane

separating two aqueous compartments, and add separating two aqueous compartments, and add to one of them a solute that can pass readily to one of them a solute that can pass readily

across the membrane, the solute will starts to across the membrane, the solute will starts to move from the higher concentration move from the higher concentration

compartment across membrane (down gradient) compartment across membrane (down gradient) to the other compartment until we reach to the other compartment until we reach

equilibrium.equilibrium.At this point the rate of transfer of solute from At this point the rate of transfer of solute from the first compartment to the second exactly the first compartment to the second exactly counterbalanced by the transfer of solute in the counterbalanced by the transfer of solute in the opposite direction.opposite direction.

Simple diffusionSimple diffusion

Molecules and ions move spontaneously Molecules and ions move spontaneously down their concentration gradient (i.e., down their concentration gradient (i.e.,

from a region of higher to a region of lower from a region of higher to a region of lower concentration) byconcentration) by simplesimple diffusiondiffusion..

This tendency of movement is the result This tendency of movement is the result of the operation of the second law of of the operation of the second law of

thermodynamics.thermodynamics.

The entropy of the solute molecules The entropy of the solute molecules becomes maximized as they randomize becomes maximized as they randomize

themselves by diffusion through the two themselves by diffusion through the two compartments.compartments.

In passive transport In passive transport ΔΔ S is incresed S is incresed

while while ΔΔ G is decreased G is decreased

Facilitated diffusionFacilitated diffusion

Facilitated diffusion of ions takes place Facilitated diffusion of ions takes place through proteins, or assemblies of proteins, through proteins, or assemblies of proteins, embedded in the plasma membrane. These embedded in the plasma membrane. These

tran-smembrane proteins form a water-filled tran-smembrane proteins form a water-filled channel through which the ion can passchannel through which the ion can pass downdown

its concentration gradientits concentration gradient. .

The trans-membrane channels that permit The trans-membrane channels that permit facilitated diffusion can be opened or closed. facilitated diffusion can be opened or closed. They are said to beThey are said to be ""gatedgated""..

All molecules and ions are in All molecules and ions are in constant motion and it is the energy constant motion and it is the energy of motion - of motion - kinetic energykinetic energy - that - that drives passive transportdrives passive transport. .

Active transportActive transport

Is the movement of solute against or Is the movement of solute against or up a concentration gradient. i.e from up a concentration gradient. i.e from a compartment of low concentration a compartment of low concentration

to a compartment of high to a compartment of high concentration. concentration.

Entropy will decrease (the solute Entropy will decrease (the solute become less random) and the free become less random) and the free

energy of the system will increase.energy of the system will increase. Active transport is a process in Active transport is a process in

which the system gains free energy.which the system gains free energy.

Passive transport is a process in Passive transport is a process in which the system decreases in free which the system decreases in free

energy. So passive transport occurs energy. So passive transport occurs spontaneously, while active spontaneously, while active

transport can not occur by itself.transport can not occur by itself.

ΔΔ G = G = ΔΔ H - T H - T ΔΔ S S

Active transportActive transport Active transport is the pumping of Active transport is the pumping of

molecules or ions through a molecules or ions through a membranemembrane againstagainst their their concentration gradient.concentration gradient.

It requiresIt requires: :

- A transmembrane protein - A transmembrane protein (Ion (Ion Pump).Pump).

- Energy in the form of ATP.- Energy in the form of ATP.

Two problems to be consideredTwo problems to be considered: :

1- Relative concentrations.1- Relative concentrations.

22- - Lipid bilayers which are impermeable Lipid bilayers which are impermeable to most essential molecules and ions. to most essential molecules and ions.

11--Relative concentrationsRelative concentrations

Molecules and ions can be movedMolecules and ions can be moved againstagainst their concentration their concentration

gradient, so this process requires gradient, so this process requires the expenditure of energy (usually the expenditure of energy (usually

from ATP). from ATP).

22 - -The impermeable lipid bilayerThe impermeable lipid bilayer

TheThe lipid bilayerlipid bilayer is permeable tois permeable to waterwater molecules and a few other small, molecules and a few other small, uncharged, molecules like oxygen and uncharged, molecules like oxygen and carbon dioxide.carbon dioxide.

These diffuse freely in and out of the cell. These diffuse freely in and out of the cell. The diffusion of water through the plasma The diffusion of water through the plasma membrane is of such importance to the cell membrane is of such importance to the cell that it is given a special namethat it is given a special name: : osmosisosmosis..

Impermeability of cell membrane Impermeability of cell membrane (continued)(continued)

The lipid bilayer presents a serious energy The lipid bilayer presents a serious energy barrier to an ion crossing it.barrier to an ion crossing it.

This is because ions are energetically more This is because ions are energetically more stable in water than in the oily substance of the stable in water than in the oily substance of the

membrane interior.membrane interior.

The predominant ions in biological systems The predominant ions in biological systems would essentially never cross the membrane would essentially never cross the membrane

unaided. unaided.

Energy of requirement of Energy of requirement of active transportactive transport

For 1.0 mole of an uncharged solute to For 1.0 mole of an uncharged solute to move from one compartment to anothermove from one compartment to another

ΔΔGº = 2.303 RT log CGº = 2.303 RT log C22/C/C11

where Cwhere C11 and and CC2 2 are the conc of free are the conc of free solute at the beginning and end of the solute at the beginning and end of the transport process.transport process.

R is gas constantR is gas constant

T is absolute temperatureT is absolute temperature

If a charged molecule is actively transported , If a charged molecule is actively transported , this will be done against 2 gradients:this will be done against 2 gradients:

1- Concentration or chemical gradient.1- Concentration or chemical gradient.

2- Electrical gradient2- Electrical gradient

Then the equation become:Then the equation become:

ΔΔG° = 2.303 RT log CG° = 2.303 RT log C22/C/C1 1 + zF V membrane+ zF V membrane

z is the charge of transported molecule. z is the charge of transported molecule.

F is the Faraday constant (23.062 cal/mol V or F is the Faraday constant (23.062 cal/mol V or 96.5 Jole/ mol V) 96.5 Jole/ mol V)

Vm is the membrane potential in volts Vm is the membrane potential in volts

Calculate the change in free energy in Calculate the change in free energy in transporting one gram molecular weight of transporting one gram molecular weight of glucose up a hundred fold gradient from a glucose up a hundred fold gradient from a

compartment in which its conc is 0.001 M to a compartment in which its conc is 0.001 M to a compartment in which conc is 0.1 M at 25 °C.compartment in which conc is 0.1 M at 25 °C.

ΔΔGº = 2.303 RT log CGº = 2.303 RT log C22/C/C11

= 1.98 x 298 x 2.303 log 0.1/0.001= 1.98 x 298 x 2.303 log 0.1/0.001 = 2680 cal or 2.680 K cal.= 2680 cal or 2.680 K cal.

Since the free energy change is Since the free energy change is positivepositive, so , so the process is one of the process is one of active transportactive transport i.e i.e

endergonic reaction.endergonic reaction. If same energy is calculated but down If same energy is calculated but down

gradient i.e from 0.1 M to 0.001 M then gradient i.e from 0.1 M to 0.001 M then ΔΔGº Gº is is negativenegative indicating a spontaneous reaction indicating a spontaneous reaction

or or passive transportpassive transport. .

Example:Example: The conc of KThe conc of K++ ions in the glomerular filtrate is 5 ions in the glomerular filtrate is 5

mM and that of the renal tubule cells is 0.1 M at mM and that of the renal tubule cells is 0.1 M at 37 ºC . The membrane potential across active 37 ºC . The membrane potential across active renal tubule cells is 0.04 Vrenal tubule cells is 0.04 V

ΔΔG = 2.303 RT log CG = 2.303 RT log C22/C/C1 1 + zF V + zF V membranemembrane

= 2.303 x 8.314 x 310 log 0.1/5x10 = 2.303 x 8.314 x 310 log 0.1/5x10 --

3 3 + 1x 96.5 x 0.04+ 1x 96.5 x 0.04

Characteristics of active Characteristics of active transporttransport

1- It depends on a source of metabolic 1- It depends on a source of metabolic energy to pump a solute against a gradient energy to pump a solute against a gradient of concentration.of concentration.

e.g: Red blood cells obtain the energy e.g: Red blood cells obtain the energy required to pump Krequired to pump K++ into the cell across the into the cell across the membrane and this needs a highly active membrane and this needs a highly active glycolytic pathway to provide ATP needed glycolytic pathway to provide ATP needed to this transport.to this transport.

When we add fluoride which inhibits When we add fluoride which inhibits glycolysis, the intracellular conc of Kglycolysis, the intracellular conc of K++ will will decrease and Nadecrease and Na++ will rise. will rise.

2- They are specific for given solutes. Some 2- They are specific for given solutes. Some cells have a pump specific for certain amino cells have a pump specific for certain amino acids but can not transport glucose. Others acids but can not transport glucose. Others

can pump glucose but not amino acids.can pump glucose but not amino acids.

3- The active transport system depends on 3- The active transport system depends on the conc of substance being transported. the conc of substance being transported. e.g: when glucose is actively transported e.g: when glucose is actively transported

into a cell, the rate of glucose influx into a cell, the rate of glucose influx increases with the external conc of glucose. increases with the external conc of glucose. However, a characteristic plateaue is soon However, a characteristic plateaue is soon

reached, so that any further increase in the reached, so that any further increase in the external glucose produce no increase in the external glucose produce no increase in the

influx.influx.

4-Active transport have a specific 4-Active transport have a specific directionalitydirectionality

KK++ is pumbed only inward is pumbed only inward

NaNa++ is pumbed outword is pumbed outword

5- They may be selectively poisoned.5- They may be selectively poisoned.

e.g: e.g:

-active transport of glucose in the -active transport of glucose in the kidney is poisoned by phlorizin.kidney is poisoned by phlorizin.

- Active transport of Na out of RBCs is - Active transport of Na out of RBCs is inhibited by the toxic ouabain.inhibited by the toxic ouabain.