Transport Across the Cell Membrane

1. Transport Across the Cell Membrane Department of Molecular Biology & Genetics Medical University of Silesia Aleksander L. Sieroń

2. From Cambpellet al.

3. Fluid Mosaic Model • Mosaic due to many proteins embedded in the fluid matrix of lipid bilayer • Two types of Proteins: • 1. Integral - hydrophobic regions span the hydrophobic interior ofmembrane; protein is embedded in membrane • 2. Peripheral- not embedded in bilayer; loosely bound to surface of membrane orintegral protein

4. The structure of a transmembrane protein From Cambpellet al.

5. Functions of the proteins: • transport or carrier proteins • involved in the passage of molecules through the membrane • molecules avoid contact with hydrophobic part and • can enter the membrane through the protein • transport of molecules too large to fit through pores of membrane • Highly specific, • only specific molecules interact with the protein and • then only can that molecule enter the cell.

6. From Cambpellet al.. 1994

7. Functionsof the plasma membrane: • Transport of materials in and out of the cell • Selectively permeable: • only certain substances can enter cell and others can not. • Two types of Transport: • Passive - Does not require Energy • Active - Requires Energy (ATP)

8. Permeability of the Lipid Bilayer • Lipid bilayer of cell membranes serve as a barriers to the passage of most polar molecules • Allows the cell to maintain concentrations of solutes in its cytosol • Highly impermeable to Ions • Easy permeable to hydrophobic (nonpolar) molecules

9. The relative permeability of a synthetic lipid bilayer to different classes of molecules from Alberts et al.

10. Membrane transport proteins • carrier proteins (also called carriers, permeases, or transporters) • channel proteins from Alberts et al., 1994

11. Membrane transport proteins Passive or active transport Passive transport from Alberts et al.

12. Comparison of passive transport down an electrochemical gradient with active transport against an electrochemical gradient from Alberts et al.

13. Small hydrophobic molecules – third type of transport molecules Increase permeability of the cell membrane to specific inorganic ions Synthesized by microorganisms MOBILE ION CARRIERS & CHANNEL FORMERS Only passive transport Ex. Valinomycin Gramidicin A Ionophores from Alberts et al.

14. Mechanism of Ionophore Action http://bio.winona.msus.edu/berg/ANIMTNS/Directry.htm

15. Passive Transport: • Diffusion • Facilitated Diffusion • Osmosis

16. Passive Transport: • Diffusion • molecules move from an area of high concentration to an areaof low concentration • molecules can fit through pores of the membrane • continue to move until equilibrium is attained • Substances move down its concentration gradient • spontaneous, no energy required

17. The diffusion of solutes across membranes From Cambpellet al.

18. From Cambpellet al.

19. Passive transport: • Facilitated Diffusion • passage of molecules from high to low – down its concentration gradient • enter through a carrier protein • molecules are too big to fit through pores of the membrane • spontaneous process

20. From Cambpellet al.

21. Passive Transport: • Facilitated Diffusion • passage of molecules from high to low– down its concentration gradient • enter through a carrier protein • molecules are too big to fit through pores of the membrane • spontaneous process • Highly specific • only certain substances can enter the cell.

22. From Cambpellet al.

23. Passive Diffusion Facilitated Diffusion

24. Passive Transport: • Osmosis • The passive transport of Water • Movement of WATER from high to low concentration • Solution - made of solute and solvent • Solute - substance being dissolved • Solvent -liquid solute is dissolve • If solvent is water: Aqueous solution

25. Passive Transport: • Osmosis • Three terms used to describe aqueous solutions: • Hypertonic • Hypotonic • Isotonic solution with a high solute concentration solution with a low solute concentration solutions with equal solute concentration Water always moves from hypotonic to hypertonic solutions!!!

26. Osmosis From Cambpell et al.

27. Environment: HIGH SOLUTE LOW WATER Cell: LOW SOLUTE HIGH WATER Outcome: WATER WILL MOVE OUT OF THE CELL. THE CELL WILL SHRINK!!! Hypertonic solution

28. Environment: LOW SOLUTE HIGH WATER Cell: HIGH SOLUTE LOW WATER Outcome: WATER WILL MOVE INTO OF THE CELL. THE CELL WILL BURST!!! Hypotonic solution

29. Environment: EQUAL SOLUTE EQUAL WATER Cell: EQUAL SOLUTE EQUAL WATER Outcome: NO CHANGE IN AMOUNT OF WATER. THE CELL WILL STAY THE SAME!!! Isotonic solution

30. The water balance of living cells From Cambpellet al.

31. Animal cells • Hypertonic - shrinks, shriveled • hypotonic - swell, bursts open, lyses • isotonic - normal • Plant Cells • hypertonic - plasmolysis, shrinks • hypotonic - normal (healthiest), turgid • isotonic - limp, no tendency for water to enter plant wilts, flaccid

32. Plasmolysis

33. http://www.ks.uiuc.edu/Research/aquaporins/ Aquaporins • membrane water channels • controlling the water contents of cells • form tetramers in the cell membrane • facilitate the transport of water and, in some cases, other small solutes across the membrane • are completely impermeable to charged molecules • Diseases: • congenital cataracts • nephrogenic diabetes insipidus 2003 Nobel prize in Chemistry

34. http://www.ks.uiuc.edu/Research/aquaporins/ Aquaporins

35. Tajkhorshid, E., Nollert, P., Jensen, M.O., Miercke, L.J., O'Connell, J., Stroud, R.M., and Schulten, K. (2002). Science 296, 525-530

36. ActiveTransport • opposite todiffusion • against the concentration gradient • molecules move from a low to high concentration Requiresenergy

37. Active Transport Using ATP Driven Phosphorylation of the Transport Protein

38. Review: passive and active transport compared From Cambpellet al.

39. Three types of carrier-mediated transport From Albertset al.

40. Mechanism of Symport

41. A hypothetical model showing how a conformational change in a carrier protein could mediate the facilitated diffusion of a solute From Albertset al.

42. The Na+-K+ ATPase • Concentration of K+ 10-20 times higher inside the cell • Concentration of Na+ 10-20 times higher outside the cell • Antiporter • actively pumping Na+ out of the cell • pumping K+ in to the cell

43. The Na+-K+ ATPase From Albertset al.

44. The Na+-K+ ATPase • Concentration of K+ 10-20 times higher inside the cell • Concentration of Na+ 10-20 times higher outside the cell • Antiporter • actively pumping Na+ out of the cell • pumping K+ in to the cell • Is electrogenic

45. A schematic model of the pumping cycle of the Na+-K+ ATPase From Alberts et al.

47. The Na+-K+ ATPase • Function: • regulates cell volume (through the osmotic effects of Na+ gradient) • drives transport of sugars and amino acids into the cell • !!! • Almost one-third of the energy requirement of a typical animal cell is consumed in fueling this pump • !!!

48. Response of a human red blood cell to changes in osmolality (also called tonicity) of the extracellular fluid From Alberts et al.

49. Transport ATPases • membrane-bound enzymes • Hydrolyse the ATP • Active transport The Na+-K+ ATPase SomeCa2+ pumps

50. Membrane-boundenzymesthatsynthesize ATP are transport ATPasesworking in reverse • The plasma membrane of bacteria • thylakoid membrane of chloroplasts • the inner membrane of mitochondria • H+ gradients across these membranes drive the synthesis of ATP from ADP and phosphate • ATP synthase • ATP synthase is responsible for producing nearly all of the ATP in most cells