Membrane organization & function

1. Membrane organization & function Lecture 20 Ch 11 pp365 - 386

2. Fantastic Facts • Every cell on Earth has a membrane around it! • No membrane = no cells = no life as we know it! • Two layers of lipids = lipid bilayer • Just 50 atoms thick = which is 5nm • Prevents entry and exit of most molecules, except under controlled conditions - nutrients have to enter & waste has to leave • Has gates to permit certain other molecules to enter; receptors to detect the environment • Can grow and shrink without effect • Piercing of this membrane does not cause it to pop, or tear. It simply reseals!

3. Fantastic Facts • Bacteria has just one membrane - plasma membrane (cell membrane) • Eukaryotes have many internal membranes too - these offer each organelle the ability to concentrate certain chemicals. • ALL natural membranes are made of lipidsandproteins.

4. 11_04_membrane.view.jpg

5. The Lipid Bilayer • Each membrane lipid molecule has two very different properties - a hydrophilic head, and one or two hydrophobic tails • Most abundant lipids = phospholipids - the head is lined to the rest of the molecule through a phosphate group. • PHOSPHATIDYLCHOLINE is the most abundant of all = choline, plus phosphate, plus two long hydrocarbon chains…

6. 11_05_tails.philic.pho.jpg

7. 11_06_Phosphatidylch.jpg

8. Amphipathic nature • A molecule which has two faces - a water loving side and awater hating side. • The water hating parts force water molecules to form cages…

9. 11_09_hydrophobic.jpg

10. The lipid bilayer • The lowest energy structure which is formed by amphipathic molecules…. • …. a lipid bilayer • …Or a sphere with water inside and outside.

11. 11_11_bilayer.in.H20.jpg

12. No Edges • Any tears are quickly repaired by either • The exclusion of the water molecules • The formation of small vesicles • NO FREE edges are permitted by the free energy of the system • Therefore, the only way a large collection of these membranes can exist without edges is as sacs - exactly what the plasma membrane is!

13. No flip-floppers please • A single lipid molecule will rarely flip from one surface of the molecule to the other - no flip-flopping • However, the lipid molecule is free to move within its own layer relatively freely • Spin at RT about 30,000 rpm around themselves!

14. 11_15_Phospho,move.jpg

15. Saturated fats! • The properties of different membranes are dictated by their composition… • The length of the hydrocarbon tail (14 to 24 carbon atoms - the shorter the more fluid) • The number of double bonds (fewer = saturation) - the more saturation the more rigid • Hydrogenated margarines • Cholesterol = more rigid membrane = bad • THE FLUID NATURE OF THE MEMBRANE IS RELATED TO ITS FUNCTION

16. ‘Two faced’ • If you look at the membrane from above, it will have a different composition of both fats and lipids then when viewed from the bottom. It is said to be asymmetrical • FLIPPASES - permit the manufacture of lipid membranes….

17. Different composition on different faces 11_17_asymmetic.dist.jpg

18. Where is membrane made? In place or, Outside the cell or, Inside the cell????

19. ANSWER: It is made as part of the endoplasmic reticulum (ER) system and then moved to the cell surface

20. REMEMBER ME PLEASE! 11_18_Flippases.jpg

21. Membrane Proteins • Most functions of the membrane are performed by membrane proteins • Up to 50% of the mass of the membrane in animal cells can be protein • 50:1 ratio of lipids to proteins • P’s have many functions…

22. 11_20_memb.proteins.jpg Remember these 4 categories of membrane P’s (TARE the membrane)

23. How P’s are associated with the membrane - remember 11_21_proteins.associ.jpg Integral membrane proteins Peripheral membrane proteins

24. How do P’ cross the membrane? Via a helical domain 11_23_helix.cross.LB.jpg Transmembrane proteins are alpha-helixes

25. However, larger structures may involve more complex arrangements too. Here we have a cylinder of 5 helical domains which form a central channel. 11_24_hydrophl.pore.jpg

26. Beta-barrels are formed to make even largee pores, because the sheets cannot be bent too drastically. E.g. Porin proteins of bacteria. 11_25_Porin.proteins.jpg

27. Plasma membrane is flimsy • The membrane itself is not that strong • Nature uses proteins to strengthen it. • These structures composed of a meshwork of fibrous proteins are known as CELL CORTEX • These proteins are complexed to the plasma membrane via other proteins • Great example is the SPECTRIN in the red blood cells of humans…

28. The complex shape of these cells is critical to their function. 11_30_blood.cells.EM.jpg

29. If one were to look from the inside of the cell towards the cell membrane, one would see a complex scaffold of proteins everywhere inside the cytoplasm just below this membrane… 11_31_spectrin_network.jpg The reinforced concrete model = invented by Nature before man

30. Cells’ surface is coated with ‘PAINT’ • The outer surfacelipids of many plasma membranes is complexed with sugars - GLYCOLIPIDS = most cells are sugar coated! • The same is true of the proteins of the outer membrane • Some of these are short sugars - oligosaccharides, they form glycoproteins • Others are longer and form proteoglycans • These ALL constitute the CARBOHYDRATE LAYER (Sugar layer) • Serves a Protective layer • confers functional properties

31. 11_32_sugar_coated.jpg

32. Very important functions Permits: • Cell-cell communications • Cell-cell recognition • Cell behaviour

33. White blood cell rolling brakes… 11_33_neutrophils.jpg

34. Proteins can ‘roam’ around plasma membranes too, just like lipids 11_34_mouse_human_hybid.jpg

35. Proteins are generally free to roam around. Except in circumstances where they are anchored within the cell 11_35_lateral_mobility_plasma_membrane_proteins.jpg

36. TIGHT JUNCTIONS prevent the migration of proteins to other regions of the cell - more about these later. 11_39_protein_restricted_to_domain.jpg