IntroductionIntroduction 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 transportationType of transportation

Simple Simple diffusiondiffusion

Active Active transporttransport

Passive Passive transporttransport

I- Simple diffusionI- 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 simplesimple diffusiondiffusion..

II- Facilitated diffusionII- 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 downdown its concentration gradient.

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

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 ΔΔSS is increased

while ΔΔGG is decreased

Active transportActive 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 ΔΔS S will decrease (the solute become less random) and the free energy of the system Δ ΔGG 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.

Δ G = Δ H - T Δ SΔ G = Δ H - T Δ S

Two problems to be consideredTwo problems to be considered: :

1- Relative concentrations.

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

11--Relative concentrationsRelative concentrations

Molecules and ions can be moved against their concentration gradient, so this process requires the expenditure of energy (usually

from ATP).

22 - -The impermeable lipid bilayerThe impermeable lipid bilayer

The lipid bilayer 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 Impermeability of cell membrane (continued)(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 Energy of requirement of active transportactive transport

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

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

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:

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

ΔG° = 2.303 RT log C2/C1 + zF V membraneΔG° = 2.303 RT log C2/C1 + zF V membrane

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.

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

= 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: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 Characteristics of active transporttransport

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- 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.

3-3- 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.