CHAPTER 7 MEMBRANE STUCTURE AND FUNCTION

1. CHAPTER 7 MEMBRANE STUCTURE AND FUNCTION

2. Study guide #20-27

3. Diffusion • 2nd Law of Thermodynamics Governs biological systems! Universe tends to disorder! Diffusion – movement from high>low concentration of THAT SUBSTANCE! diffusion

4. Diffusion of 2 Solutes Each substance diffuses down its own concentration gradient, independent of concentration gradient of any other substance.

5. Diffusion • PASSIVE TRANSPORT • No energy required • So – How does that happen? Brownian Motion

6. Osmosis is the diffusion of water across a membrane • Water is very important to life, so we talk about water separately • Diffusion of water from high concentration of water to low concentration of water • across a semi-permeable membrane

7. hypotonic hypertonic Concentration of water • Direction of osmosis is determined by comparing total solute concentrations • Hypertonic - more solute, less water • Hypotonic - less solute, more water • Isotonic - equal solute, equal water water net movement of water

8. Osmosis • 250 times the volume of cell/second! • Osmosis = NET movement of water across a selectively permeable membrane driven by a difference in solute concentration on either side of the memrane. • “Free” water moves • Less solute = more free water • Water flows from low solute [ ] to high solute [ ] • Until equilibrium Slide 2 osmosis

9. Osmosis

10. What determines when and what direction water will move? 3 molecules of albumin 15 molecules of glucose 66,000 mw 180 mw Water?

11. Isotonic? Hypertonic? Hypotonic? A & B are isotonic A & B are hypertonic to C C is hypotonic to A and B

12. Osmotic Pressure • Pressure generated by diffusion of water across a membrane • When pressure is equal water flow will stop • Called “hydrostatic pressure” – water-stopping pressure • Osmolarity- [ ] in terms of # of particles in a volume of liquid • 1 osmolar soln = 1 M of osmoltically active particles per liter.

13. Red Blood Cells in NaCl solutions 100mOs 500 mOs hypotonic isotonic hypertonic

14. Osmosis problems Hydrostatic generator Osmosis problems

15. End Diffusion Osmosis Cell membrane

16. The Cell Membrane

17. Overview • Cell membrane separates living cell from nonliving surroundings • thin barrier = 8nm thick • Controls traffic in & out of the cell • selectively permeable • allows some substances to cross more easily than others • hydrophobic vs hydrophilic • Made of phospholipids, proteins & other macromolecules

18. Phospholipids Phosphate • Fatty acid tails • hydrophobic • Phosphate group head • hydrophilic • Arranged as a bilayer Fatty acid Aaaah, one of thosestructure–function examples

19. Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads

21. How do we know? Two generations of membrane models 1972; Singer, Nicholson Dispersion model Hydrophyllic regions in aqueous Hydrophobic in hydrophobic PL FLUID MOSAIC MODEL! Freeze Fracture evidence Permanent model???? phospholipid bilayer -1920’s; 1930-60’s; models from EM views; ??-Not as hydrophyllic as pure PL’s? D & D - Hydrophyllic Protein sandwich ??- all membranes identical??? ??-amphipathic proteins? Solubile in H2O hydrophobic region in aqueous?

22. More than lipids… • In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer It’s like a fluid…It’s like a mosaic… It’s the Fluid Mosaic Model!

23. Evidence for the drifting of membrane proteins Other Evidence: microsurgery on cells

24. Membranes are fluid- like salad oil! • held in place weak hydrophobic interactions • PL’S drift laterally • rarely flip-flop between layers • larger proteins move slower • some proteins guided by cytoskeleton “motors” • some proteins anchored by cytoskeleton Fluid Membrane

25. Glycoprotein Glycolipid Transmembrane proteins Peripheral protein Filaments ofcytoskeleton Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Extracellular fluid Phospholipids Cholesterol Cytoplasm

26. The fluidity of membranes Maintains and increases/decreases fluidity

27. Fluidity influenced by temperature • ·cool - more solid- PL’s closely packed • ·if rich in unsaturated fatty acids - more fluid than those rich in saturated fatty acids - kinks prevent tight packing • ·cholesterol steroid- wedged between PL’s of animal cells • warm -limits mvmnt. of PL’s, reduces fluidity • cool - maintains fluidity, prevents tight packing-EX: salmon

28. Must be fluidto work w/ enzymes & be permeable • Cells alter lipid makeup to adjust for temp. changes • EX: cold organisms ( winter wheat, salmon, bears) increase % of unsat PL’s in autumn • - prevents solidifying membranes

29. Membrane fat composition varies • Fat composition affects flexibility • membrane must be fluid & flexible • about as fluid as thick salad oil • % unsaturated fatty acids in phospholipids • keep membrane less viscous • cold-adapted organisms, like winter wheat • increase % in autumn • cholesterol in membrane

30. Why areproteins the perfect molecule to build structures in the cell membrane?

31. Classes of amino acids What do these amino acids have in common? nonpolar & hydrophobic

32. Classes of amino acids What do these amino acids have in common? I like thepolar onesthe best! polar & hydrophilic

33. Membrane Proteins • Proteins determine membrane’s specific functions • cell membrane & organelle membranes each have unique collections of proteins • Membrane proteins: • peripheral proteins • loosely bound to surface of membrane • cell surface identity marker (antigens) • integral proteins • penetrate lipid bilayer, usually across whole membrane • transmembrane protein • transport proteins • channels, permeases (pumps)

34. 2 major types of membrane proteins. • Peripheral proteins - not embedded; bound to inner/outer surface; may be connected to integral proteins • Integral proteins - penetrate bilayer; • - Transmembrane protein • hydrophobic regions of nonpolar aa’s in contact with bilayer core; often alpha helices • hydrophilic regions of aa’s in contact w/ environment – beta pleated • - Unilateral – partially through; coupled

35. Protein’s domain anchor molecule Polar areas of protein • Within membrane • nonpolar amino acids • hydrophobic • anchors protein into membrane • On outer surfaces of membrane • polar amino acids • hydrophilic • extend into extracellular fluid & into cytosol Nonpolar areas of protein

36. Porin monomer H+ Retinal chromophore b-pleated sheets NH2 Bacterial outer membrane Nonpolar (hydrophobic) a-helices in the cell membrane COOH Cytoplasm H+ Examples water channel in bacteria proton pump channel in photosynthetic bacteria function through conformational change = shape change

37. Many Functions of Membrane Proteins Outside Plasma membrane Inside Transporter Enzymeactivity Cell surfacereceptor Uniport Symport Antiport Cell adhesion Cell surface identity marker Attachment to thecytoskeleton

38. Membrane carbohydrates • Play a key role in cell-cell recognition • ability of a cell to distinguish one cell from another • antigens • important in organ & tissue development • basis for rejection of foreign cells by immune system

39. Some proteins reinforce shape of cell • cytoplasmic side, • some connect to cytoskeleton • exterior side, • some attach to fibers of ECM • ECM = extracellular matrix

40. Sidedness of the plasma membrane What makes the endomembrane system work?

41. Membranes are bifacial and sided • may differ in lipid composition • proteins have a direction orientation • outer surface has carbo’s • asymmetry begins w/ synthesis in ER • proteins in plasma membrane provide a variety of major cell functions

42. The detailed structure of an animal cell’s plasma membrane, in cross section

43. Cell-cell recognition = distinguish one type of neighboring cell from another • ·important in cell sorting • organization into tissues and organs during development • ·basis for rejection of foreign cells by immune system • key in on surface molecules, often carbs

44. Carbo’s - usually branched oligosaccharides = < 15 monomers • may be covalently bonded to • lipids= glycolipids • proteins = glycoproteins • external OS’s vary from species to species, individual to individual, and even from cell type to cell type w/in same individual - identification • ·marks each cell type as distinct • blood types (A, B, AB, O)-RBCs • Glycocalyx of animal cells – fuzzy!

45. End membranes

46. Movement across the Cell Membrane

47. Diffusion • 2nd Law of Thermodynamicsgoverns biological systems • universe tends towards disorder (entropy) • Diffusion • movement from highlow concentration

48. Diffusion • Move from HIGH to LOW concentration • “passive transport” • no energy needed movement of water diffusion osmosis

49. Diffusion across cell membrane • Cell membrane is the boundary between inside & outside… • separates cell from its environment NO! Can it be an impenetrable boundary? OUT waste ammonia salts CO2 H2O products IN food carbohydrates sugars, proteins amino acids lipids salts, O2,H2O OUT IN cell needs materials in & products or waste out

50. inside cell outside cell Diffusion through phospholipid bilayer • What molecules can get through directly? • fats & other lipids • What molecules can NOT get through directly? • polar molecules • H2O • ions • salts, ammonia • large molecules • starches, proteins lipid salt NH3 sugar aa H2O