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 Active transport Passive transport

I- Simple diffusion  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  down  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.  gated  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.  ΔS  In passive transport ΔS is increased ΔG 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.  ΔS ΔG  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. Δ G = Δ H - T Δ S

Two problems to be considered: 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).ATP

2- The impermeable lipid bilayer  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 +, Mg 2+, or Ca 2+, 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 = RT log C 2 /C 1 where C 1 and C 2 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 ( cal/mol V or 96.5 Jole/ mol V) Vm is the membrane potential in volts ΔG° = RT log C 2 /C 1 + 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 M to a compartment in which conc is 0.1 M at 25 °C. ΔGº = RT log C 2 /C 1 = 1.98 x 298 x log 0.1/0.001 = 2680 cal or 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 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 = RT log C 2 /C 1 + zF V membrane = x x 310 log 0.1/5x x 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- 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 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.