1. 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. 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. 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.

3. 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. 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.

4. This tendency of movement is the result of the operation of the second law of thermodynamics. 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. 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 In passive transport Δ S is incresed while Δ G is decreased while Δ G is decreased

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

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

9. 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. 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. 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. Active transport is a process in which the system gains free energy.

10. 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. 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 Δ G = Δ H - T Δ S

11. Active transport Active transport is the pumping of molecules or ions through a membrane against their concentration gradient. Active transport is the pumping of molecules or ions through a membrane against their concentration gradient. It requires: It requires: - A transmembrane protein (Ion Pump). - A transmembrane protein (Ion Pump). - Energy in the form of ATP. - Energy in the form of ATP.

12. Two problems to be considered: Two problems to be considered: 1- Relative concentrations. 1- Relative concentrations. 2- Lipid bilayers which are impermeable to most essential molecules and ions. 2- Lipid bilayers which are impermeable to most essential molecules and ions.

13. 1-Relative concentrations Molecules and ions can be moved against their concentration gradient, so this process requires the expenditure of energy (usually from ATP). Molecules and ions can be moved against their concentration gradient, so this process requires the expenditure of energy (usually from ATP).ATP

14. 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. The lipid bilayer is permeable to water molecules and a few other small, uncharged, molecules like oxygen and carbon dioxide.lipid bilayerlipid bilayer 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. 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. osmosis

15. Impermeability of cell membrane (continued) The lipid bilayer presents a serious energy barrier to an ion crossing it. 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. 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. The predominant ions in biological systems would essentially never cross the membrane unaided.

18. Energy of requirement of active transport For 1.0 mole of an uncharged solute to move from one compartment to another For 1.0 mole of an uncharged solute to move from one compartment to another ΔGº = 2.303 RT log C 2 /C 1 ΔGº = 2.303 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. 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 R is gas constant T is absolute temperature T is absolute temperature

19. If a charged molecule is actively transported, this will be done against 2 gradients: If a charged molecule is actively transported, this will be done against 2 gradients: 1- Concentration or chemical gradient. 1- Concentration or chemical gradient. 2- Electrical gradient 2- Electrical gradient Then the equation become: Then the equation become: ΔG° = 2.303 RT log C 2 /C 1 + 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

21. 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. 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 C 2 /C 1 = 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 positive, so the process is one of active transport i.e endergonic reaction. 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. 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.

22. 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 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 C 2 /C 1 + zF V membrane ΔG = 2.303 RT log C 2 /C 1 + zF V membrane = 2.303 x 8.314 x 310 log 0.1/5x10 -3 + 1x 96.5 x 0.04 = 2.303 x 8.314 x 310 log 0.1/5x10 -3 + 1x 96.5 x 0.04

23. Characteristics of active transport 1- It depends on a source of metabolic energy to pump a solute against a gradient of concentration. 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. 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. When we add fluoride which inhibits glycolysis, the intracellular conc of K + will decrease and Na + will rise.

25. 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. 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. 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.

26. 4-Active transport have a specific directionality 4-Active transport have a specific directionality K + is pumbed only inward K + is pumbed only inward Na + is pumbed outword Na + 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 kidney is poisoned by phlorizin. -active transport of glucose in the kidney is poisoned by phlorizin. - Active transport of Na out of RBCs is inhibited by the toxic ouabain. - Active transport of Na out of RBCs is inhibited by the toxic ouabain.