Transcript Passive and active transport

Introduction
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.

Type of transportation
Simple diffusion
Passive transport
Active transport
I- 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.
II- Facilitated diffusion

Facilitated diffusion of ions takes place through
proteins, or assemblies of proteins, embedded in the
plasma membrane. These trans-membrane 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 “
 Bear in mind, however, that facilitated diffusion is a
passive process, and the solutes still move down the
concentration gradient.
All molecules and ions are in constant motion and it is
the energy of motion - kinetic energy - that drives passive
transport.

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 ΔS is increased
while ΔG is decreased
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.
 It requires :
- A transmembrane protein (Ion Pump).
- Energy in the form of ATP.


Entropy ΔS will decrease (the solute become less random) and
the free energy of the system ΔG will increase.

Active transport is a process in which the system gains free
energy.
Examples of active transport include the uptake of glucose in
the intestines in humans


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
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.

Metal ions, such as Na+, K+, Mg2+, or Ca2+, require
ion pump or ion channel to cross membranes and
distribute through the body.

The pump for sodium and potassium is called
sodium-potassium pump or Na +/K+-ATPase
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.