1. Lecture 9 – Chapter 12Membranes

2. Outline • Phospholipids and glycolipids form bimolecular sheets • Membrane fluidity is controlled by fatty acid composition and cholesterol content • Lipids and many membrane proteins diffuse laterally in the membrane • Proteins carry out most membrane processes • A major role of membrane proteins is to function as transporters

3. Essential Questions What are the properties and characteristics of biological membranes that account for their broad influence on cellular processes and transport?

4. Characteristics of membranes: • Sheet-like structures • Composed of lipids and proteins • Membrane lipids are small amphipathic molecules • Proteins serve to mitigate the impermeability of membranes • Membranes are noncovalent assemblies. • Membranes are asymmetric • Membranes are fluid structures.

5. Eukaryotic Membranes • Membranes serve as boundaries that maintain division of labor in the cell. • Membranes are actively involved in cellular processes. Membranes act as permeability barriers and thus establish compartments

7. Phospholipids and glycolipids form lipid bilayers in aqueous solutions

8. Membranes Formation Lipids form ordered structures spontaneously in water • The driving force behind amphipathic lipids to form ordered structures in aqueous solutions is: • Water’s tendency to form H-bonds and share in polar interactions. • The hydrophobic effect which promotes self-association of lipids in water to maximize entropy (by liberating water molecules free)

9. Preparation of Glycine-Containing Liposomes

10. Membrane Composition Reflects their Function Membrane fluidity is controlled by fatty acid composition and cholesterol content The lipid composition of rat liver cell membranes, in weight percent.

11. Membrane Processes Depend on the Fluidity of the Membrane The temperature at which a membrane transitions from being highly ordered to very fluid is called the melting temperature.

12. The packing of fatty acid chains in a membrane Stearate Oleate + Stearate

13. Cholesterol disrupts the tight packing of the fatty acid chains.

14. Organization of Biological Membranes : Asymmetric and Heterogenous Outer leaflet Inner leaflet Asymmetry of membrane structure is functionally important to the membrane • Transverse Asymmetry • Lateral Heterogenity

15. Transverse Asymmetry

16. Oligosaccharides are located on the extracellular surface of plasma membrane

17.  Lipid movement in membranes.

18. Fluorescence Recovery After Photobleaching (FRAP)

19. Proteins Carry Out Most Membrane Processes Integral Peripheral Membranes are Very Crowed Place

20. Peripheral membrane proteins may be associated with the membrane in several different ways as follows:

21. Peripheral membrane proteins may be associated with the membrane in several different ways as follows:

22. Integral Membrane Proteins Associate with the Lipid Bilayer in a Variety of Ways • α helices • β strands • Transmembrane domain • Lipid Anchors The portions of the protein in contact with the nonpolar core of the lipid bilayer are dominated by α-helices or β-sheets because these 2° protein structures neutralize the polarity of the peptide backbone through H-bond formation.

23. Membrane-spanning α helices are a common structural feature of integral membrane proteins. bacteriorhodopsin

24. Other means of embedding integral membrane proteins is by using β strands to form a pore in the membrane Bacterial porin

25. Integral membrane proteins can embed part of the protein into the membrane. Prostaglandin H2 synthase-1

26. Aspirin inhibits cyclooxygenase activity by obstructing the channel.

27. Aspirin inhibits cyclooxygenase activity by obstructing the channel.

28. Integral membrane proteins with a single transmembrane segment Single hydrophobic transmembrane segment outside inside Glycophorin A is an integral membrane protein in the membranes of red blood cells.

29. Membrane Protein Topology Can Be Revealed by Hydropathy Plots glycophorin

30. Integral membrane proteins with multiple transmembrane segment Bacteriorhodopsin (a light-driven transport protein in Halobacterium) has 7 transmembrane α-helical segments.

31. Membrane Protein Topology Can Be Revealed by Hydropathy Plots rhodopsin

32. Transport Across Membranes • 1. Passive Diffusion • Facilitated (Passive) • Diffusion • (channels) • 3. Active Transport • (pumps) • In both cases, the transported species moves in the thermodynamically favored direction (from high to low concentration). • - Hence no energy input required. • The transported species moves in the thermodynamically unfavorable direction (from low to high concentration) ie against concentration gradient. • - Hence energy input required to drive the process.

33. Passive (simple) Diffusion The transported species moves across the membrane in the thermodynamically favorable direction without the help of any specific transport system / protein. The concentration difference across the membrane = [C2] – [C1] is termed as the ‘concentration gradient’.

34. Permeability coefficients of ions and molecules in a lipid bilayer. Lipid Bilayer are Highly Impermeable to Ions and Most Polar Molecules

35. The ability of small molecules to cross a membrane is a function of its hydrophobicity.

36. Facilitated Passive Diffusion (Channels) • Proteins facilitate net movement of solutes only in the thermodynamically favorable direction • These proteins display a measurable affinity and specificity for the transported solute. Consequently rates of facilitated diffusion processes display saturation behavior

37. Facilitating Diffusion is Saturable

38. Membrane Channels Ion Channels are Gated Voltage activated Ligand-activated

39. http://en.wikipedia.org/wiki/Membrane_potential

40. The Action Potential http://www.millerandlevine.com/chapter/35/898-899-rewrite.html

41. The Puffer Fish

42. Potassium Channel

43. The Selective Filter of the Potassium Ion Channel

46. A Model for K+-channel Transport

47. A Model for K+-channel Transport

48. A Model for K+-channel Transport

49. Gating of K+ Channel at pH ≥ 7 at pH < 7 • This K+ channel is gated by intracellular pH. • Closed at neutral pH and above • Open at acidic pH

50. Active Transport • What are the energy sources for active transport ? • ATP hydrolysis (most common) • Hydrolysis of ATP is tightly coupled to the transport process. • Light energy • Energy stored in ion gradients • The active transport process is either Electrogenic or Electroneutral: