Transcript Passive and active transport

Passive and active transport
If we have semipermeable membrane
separating two aqueous compartments, and add
to one of them a solute that can pass readily
across the membrane, the solute will starts to
move
from
the
higher
concentration
compartment across membrane (down gradient)
to the other compartment until we reach
equilibrium.
At this point the rate of transfer of solute from
the first compartment to the second exactly
counterbalanced by the transfer of solute in the
opposite direction.

Simple diffusion

Molecules and ions move spontaneously
down their concentration gradient (i.e.,
from a region of higher to a region of lower
concentration) by simple diffusion.

This tendency of movement is the result of the
operation of the second law of thermodynamics.

The entropy of the solute molecules becomes
maximized as they randomize themselves by
diffusion through the two compartments.
In passive transport Δ S is incresed
while Δ G is decreased

Facilitated diffusion

Facilitated diffusion of ions takes place
through proteins, or assemblies of proteins,
embedded in the plasma membrane. These
tran-smembrane proteins form a water-filled
channel through which the ion can pass down
its concentration gradient .

The trans-membrane channels that permit
facilitated diffusion can be opened or closed.
They are said to be" gated ."

All molecules and ions are in constant motion
and it is the energy of motion - kinetic energy that drives passive transport .
Active transport



Is the movement of solute against or up a
concentration gradient. i.e from a compartment
of low concentration to a compartment of high
concentration.
Entropy will decrease (the solute become less
random) and the free energy of the system will
increase.
Active transport is a process in which the system
gains free energy.

Passive transport is a process in which the
system decreases in free energy. So passive
transport occurs spontaneously, while active
transport can not occur by itself.
ΔG=ΔH-TΔS
Active transport
Active transport is the pumping of molecules or
ions through a membrane against their
concentration gradient.
It requires :
- A transmembrane protein (Ion Pump).
- Energy in the form of ATP.

Two problems to be considered :
1- Relative concentrations.
2- Lipid bilayers which are impermeable
to most essential molecules and ions.
1-Relative concentrations

Molecules and ions can be moved against their
concentration gradient, so this process requires
the expenditure of energy (usually from ATP).
2- The impermeable lipid bilayer
The lipid bilayer is permeable to water molecules and a
few other small, uncharged, molecules like oxygen and
carbon dioxide.
These diffuse freely in and out of the cell. The diffusion
of water through the plasma membrane is of such
importance to the cell that it is given a special name :
osmosis.
Impermeability of cell membrane
(continued)

The lipid bilayer presents a serious energy
barrier to an ion crossing it.

This is because ions are energetically more
stable in water than in the oily substance of the
membrane interior.

The predominant ions in biological systems
would essentially never cross the membrane
unaided.
Energy of requirement of active
transport
For 1.0 mole of an uncharged solute to move
from one compartment to another
ΔGº = 2.303 RT log C2/C1

where C1 and C2 are the conc of free solute at the
beginning and end of the transport process.
R is gas constant
T is absolute temperature

If a charged molecule is actively transported , this will be
done against 2 gradients:
1- Concentration or chemical gradient.
2- Electrical gradient
Then the equation become:
ΔG° = 2.303 RT log C2/C1 + zF V membrane
z is the charge of transported molecule.
F is the Faraday constant (23.062 cal/mol V or 96.5 Jole/ mol
V)
Vm is the membrane potential in volts
Calculate the change in free energy in transporting one
gram molecular weight of glucose up a hundred fold
gradient from a compartment in which its conc is
0.001 M to a compartment in which conc is 0.1 M at
25 °C.
ΔGº = 2.303 RT log C2/C1
= 1.98 x 298 x 2.303 log 0.1/0.001
= 2680 cal or 2.680 K cal.
 Since the free energy change is positive, so the process
is one of active transport i.e endergonic reaction.
 If same energy is calculated but down gradient i.e
from 0.1 M to 0.001 M then ΔGº is negative indicating
a spontaneous reaction or passive transport.

Example:
The conc of K+ ions in the glomerular filtrate
is 5 mM and that of the renal tubule cells is 0.1
M at 37 ºC . The membrane potential across
active renal tubule cells is 0.04 V
ΔG = 2.303 RT log C2/C1 + zF V membrane
= 2.303 x 8.314 x 310 log 0.1/5x10 -3 + 1x
96.5 x 0.04

Characteristics of active transport
1- It depends on a source of metabolic energy to
pump a solute against a gradient of concentration.
e.g: Red blood cells obtain the energy required to
pump K+ into the cell across the membrane and this
needs a highly active glycolytic pathway to provide
ATP needed to this transport.
 When we add fluoride which inhibits glycolysis, the
intracellular conc of K+ will decrease and Na+ will
rise.


2- They are specific for given solutes. Some cells have
a pump specific for certain amino acids but can not
transport glucose. Others can pump glucose but not
amino acids.

3- The active transport system depends on the conc of
substance being transported. e.g: when glucose is
actively transported into a cell, the rate of glucose
influx increases with the external conc of glucose.
However, a characteristic plateaue is soon reached, so
that any further increase in the external glucose
produce no increase in the influx.

4-Active transport have a specific directionality
K+ is pumbed only inward
Na+ is pumbed outword

5- They may be selectively poisoned.
e.g:
-active transport of glucose in the kidney is poisoned
by phlorizin.
- Active transport of Na out of RBCs is inhibited by
the toxic ouabain.