Understanding Membrane Permeability & Diffusion Factors

1. Chapter 4 Movement of substances across the membrane

2. Fluid mosaic model of membrane The fluid nature is due to the lateral movement of the phosopholipid layers or the break and build up nature of these layers at some points. The mosaic appearance is due to the scattering pattern of the protein molecules.

3. Selective permeability of cell membrane Pores of channel proteins allow ions, water soluble molecules of small size to pass through. (Organic or fat soluble molecules cannot pass through these pores.) Phospholipid layers allow fat, fat soluble or organic molecules to pass through. (Ions or water soluble molecules cannot pass across the phospholipid layers.) Carrier molecules transport specific ions or molecules across the membrane. Large molecules are unable to pass through either the channel protein, phospholipid or by the carrier molecules.

4. Factors affecting membrane permeability 1. Temperature: If substances diffuse across the cell membrane, the permeability increases with temperature. But very high temperature destroys the cell membrane so that it will be freely permeable. 2. Organic chemicals such as ether, chloroform and alcohol dissolve the bimolecular layer of phospholipids. They break the cell membrane and hence it becomes freely permeable. 3. Charges of protein channels OR transmembrane potential can affect the rate of diffusion of cations/ anions. 4. Solubility of fat soluble molecules in the phospholipid layers: The higher its solubility, the easier it can pass across the membrane.

5. Diffusion Movement of fluid particles along concentration gradient. Does it need metabolic energy? Does it need enzyme? Does it need carrier protein? Does it take place in living cell only? All answers above: No! No! No! No!

6. + + + + + High conc. of NaCl membrane Low conc. of NaCl - - - - - Factors affecting the rate of diffusion • Concentration gradient between two regions; • Distance and surface area of diffusion; • Size and nature of molecules; • Temperature; • Nature of the barrier???? Compare the rate of downward diffusion of Na+ and Cl- across the membrane.

7. Facilitated diffusion Along a concentration gradient Across the cell membrane Requires specific channels or carrier molecules Does not require the expenditure of energy Example: absorption of glucose from plasma into the red blood cell by facilitated diffusion.

8. Osmosis The net movement of water molecules along the water potential gradient through a selectively permeable membrane.

9. Water potential It is the measure of the free energy of water molecules. It is related to the motion of the water molecules. Pure water at standard atmospheric pressure and 25oC has its water potential arbitrarily set at zero. The water potential of a solution is the difference in free energy between the water molecules of the solution and the water molecules of pure water at the standard conditions.

10. Water potential has two components. They are solute potential and pressure potential. Water potential = Solute potential + pressure potential Solute potential is due to the dissolved substances. Solute attracts water molecules and make them less mobile. Therefore the solute potential is always negative in value. Pressure potential is due to the hydrostatic pressure other than the standard atmospheric pressure. Hydrostatic pressure usually renders the water molecules more mobile. Therefore the pressure potential is usually positive in value.

11. Plant cell A Solute potential is – 2 atm Pressure potential is + 1 atm Plant cell B Solute potential is – 3 atm Pressure potential is +1atm What is the direction of the net movement of water?

12. Plant cell A Solute potential is – 2 atm Pressure potential is + 2 atm Plant cell B Solute potential is – 2 atm Pressure potential is +1atm What is the direction of the net movement of water?

13. Xylem vessel Solute potential is –1atm Pressure potential is –2atm Cortex cell of root Solute potential is –1.5 atm Pressure potential is +1atm What is the direction of the net movement of water?

14. Plant cell A is added to a beaker of distilled water. Plant cell A Solute potential = -2 atm Pressure potential = + 1 atm Distilled water What is the direction of the net movement of water?

15. Plant cell A is added to a beaker of distilled water. Plant cell A Solute potential = -2 atm Pressure potential = + 1 atm Distilled water What will the final water potential of plant cell be? Its solute potential is getting less negative; its pressure potential is getting more positive. Finally, the water potential will approach zero. The cell will be fully turgid. No net water movement occurs.

16. Plant cell A is added to a beaker of sugar solution with solute potential of – 3 atm. Plant cell A Solute potential = -2 atm Pressure potential = + 1 atm Sugar solution What is the direction of the net movement of water?

17. Plant cell A is added to a beaker of sugar solution with solute potential of – 3 atm. Plant cell A Solute potential = -2 atm Pressure potential = + 1 atm Sugar solution What will the final water potential of plant cell be? Its solute potential is getting more negative; its pressure potential is getting less positive. Eventually, the solute potential becomes –3atm; its pressure potential becomes zero. In other words, its water potential becomes –3atm. The cell is fully plasmolysed. No more net movement of water occurs.

18. Incipient plasmolysis is the condition at which the protoplast just detaches from the cell wall. Full plasmolysis is the condition at which the protoplast detaches from the cell wall. No more exosmosis occurs. Plant cell = Protoplast + cell wall

19. It is placed in a hypertonic soln It is placed in distilled water Condition of ordinary plant cell

20. Sketch a graph to show the change in water potential, solute potential and pressure potential of the sucrose solution with time. (Assume that the dialysis tubing is not permeable to sucrose molecule.) Length of the liquid column time

21. Sketch a graph to show the change in water potential, solute potential and pressure potential of the sucrose solution with time. (Assume that the dialysis tubing is not permeable to sucrose molecule.) positive Pressure potential time 0 negative Water potential

22. Sketch a graph to show the change in water potential, solute potential and pressure potential of the sucrose solution with time. (Assume that the dialysis tubing is not permeable to sucrose molecule.) Pressure potential positive time 0 Water potential negative Solute potential

23. A plant cell is added to a sucrose solution. Plant cell: Solute potential: - 3 units Pressure potential: +1 unit Sucrose solution: Solute potential: - 5 units Sketch a graph to show the change in water potential, solute potential and pressure potential of the plant cell with time.

24. Active transport It can transport the fluid particles across the cell membrane against the concentration gradient. It needs energy supply. The fluid particles are transported by carrier proteins which are specific in function. The active transport is uni-directional. e.g. epithelial cell of ileum villus epithelial cell of proximal and distal tubule of kidney nephron.

25. Active Transport

26. Active Transport

27. Active Transport

28. Active Transport

29. Active Transport

30. Can active transport carries molecules along concentration gradient? Yes!

31. Active Transport along concentration gradient

32. Active Transport along concentration gradient

33. Active Transport along concentration gradient

34. Active Transport along concentration gradient

35. Active Transport along concentration gradient

36. Active Transport along concentration gradient

37. Phagocytosis and pinocytosis They are bulk transport (i.e. swallowing or drinking) of particles into a cell . They are active process involved expenditure of energy. The particles transported into a cell can be large particles. Some of them can be seen under light microscope. Intracellular digestion is common accompanying pinocytosis and phagocytosis. Therefore they are associated with the activities of lysozomes.

38. Phagocytosis It is the bulk transport of solid particles into a cell. It is cell “eating” e.g. Amoeba ingests food particles Mammalian phagocytes ingest pathogens Kupfer cell of mammalian liver ingests aged erythrocytes.

39. Phagocytosis When a phagocytic cell approaches a solid particle, its plasma membrane pushes out to form pseudopodia to enclose the solid. The tips of the pseudopodia then fuse, forming a phagocytic vesicle enclosing the solid food. Lysozomes then fuse with it to form a vesicle in which intracellular digestion of the solid is carried out. The end product of digestion is absorbed into cytoplasm by active transport and diffusion.

40. Pinocytosis It is bulk transport of liquid into a cell. It is cell “drinking”. Examples: Epithelial cell of ileum villus incorporate the digestive end products from the ileum lumen. Epithelial cell of nephron proximal tubule reabsorb the fluid from the glomerular filtrate.

41. Pinocytosis Plasma membrane of such cell invaginates forming a pinocytic channel. The channel is then cut off from the plasma membrane to give a pinocytic vesicle. Lysozome approaches and carries out intracellular digestion as mentioned in phagocytosis.

42. Exopinocytosis and Exophagocytosis They are reverse process of pinocytosis and phagocytosis respectively. Exopinocytosis is related to secretion of a cell. Exophagocytosis is related to the elimination of the undigested solid from a cell after phagocytosis and intracellular digestion.

43. Phagocytosis, exophagocytosis, pinocytosis and exopinocytosis are good examples to show the fluid nature of cell membrane. The fluidity is shown by breaking and immediate rebuilding of the cell membrane.

44. End of Chapter