Lipids, Membranes & the First Cells

1. Lipids, Membranes & the First Cells Chapter 6

2. Evolution of the Membrane • The evolution of the plasma membrane was a momentous event because it separated life from non-life • Before the cell RNA molecules clung to clay particles, building copies as nucleotides washed over them randomly • Although the debate over when the membrane arose it made the chemical reactions of life more efficient • Separated the inside from the outside of the cell

3. Were Lipids Present during Chemical Evolution? • Like amino acids, nucleic acids & carbohydrates, there is evidence of lipids during chemical evoluton • Simple lipids can be synthesized from H2 and CO2 with mineral catalysts under conditions throught to be present in hydrothermal vent systems • Lipids fell from the sky – meteorites

4. Lipid Characterization • Lipids are defined by • The physical property of not being soluble in water • Hydrocarbons • Hydrophobic • Not strictly characterized by structure • Therefore the structure of lipids varies widely • Saturated vs. unsaturated fat • Triacylgylcerides – adipose tissue energy storage & insulation • Steroids – cholesteral membrane stability & precursor of estrogen and testosterone • Phospholipids – phospholipid bilayer of cells

5. Lipid Characterization

6. Triglycerides • Via condensation reactions one glycerol molecule is connected to three fatty acids • The glycerol and fatty acid are linked by an ester linkage

8. Steroids • Family of lipids that has a four ring structure • Cholesterol is an important membrane component of plasma membranes in some organisms • Others are important hormone signals

9. Phospholipids • Structure • Glycerol linked to phosphate group • Glycerol linked to two fatty acids chains • Phosphate is also bound to a charged or polar head group • For Archaea bacteria • Glycerol linked to phosphate group and isoprenoids units

10. Lipid Classification

11. What is a Sphingolipid Prominent in neurons Function is largely unknown except in a few cases were they are used for recognition sites on cell membranes The carbohydrate moieties of certain sphingolipids define human blood groups

12. Lipids in Membranes • In order to spontaneously form a lipid bilayer lipid must have • Charges and polar bonds in the head region to interact with water • Long fatty acid tails to interact with each other • Amphipathic

13. Lipid Bilayer Formation & Energy liposomes • Lipid structures form spontaneously • No energy input is required • Energy Concepts • Independent phospholipids are unstable in water • Hydrophobic tails disrupt hydrogen bonds • When tails interact with one another reach a lower potential energy state • Formation of these structures clearly decreases entropy • But overall ∆H outweighs ∆S leading to a negative ∆G

14. Artificial Membranes • Researchers produced many types of • vesicles by using many different types of phospholipids • Planar bilayers • Does the substance cross the membrane and how fast does it cross the membrane • Does permeability change when proteins or other molecules are added

15. Selective Permeability of the Bilayers

16. Permeability and Fluidity Higher Short unsaturated phospholipids Higher Temperature Lower Long saturated Cholesterol Low Temperature How Does Lipid Structure Affect Permeability

17. Osmosis Some solutes form hydrogen bonds with water molecules and water associated with these can not pass the membrane

18. Membrane Evolution • Diffusion and osmosis reduce the differences between the chemical compounds between the inside and outside of the membrane • Primary importance of the first lipid bilayers was simply to provide a container for replicating RNA • But how did minerals (PO43- & Mg2+) and other components enter the cell to replicate RNA

19. Membrane Evolution • Jack Szostak and colleagues used fatty acids and other simple amphipathic lipids found during early earth’s conditions • Their experiments showed that ions, ribonucleotides can diffuse across early membranes • These simple vesicle like structures are refered to as protocells • Also, shearing forces such as shaking or wave action cause protocells to divide

20. Obviously the membrane must have gained further proteins Channels highly specific Receptors Ion channels Voltage and ligand gated channels Membrane Evolution

21. IMPs Hydrophobic Regions • Usually α helical structure is found in the transmembrane space • There can be one α helix or several • These α helical structures can also form pores or tubes

22. Other Channel Proteins • Cystic Fibrosis CF • Affects cells that produce mucus, sweat, and digestive juices • Normally these secretions are thin and slipppery and act as lubricants • With CF secretions become sticky and clog passages • The CF transporter it was suggested moves Cl- ions with then allows water to cross as well

23. In addition to the concentration gradient there is also an electro chemcial gradient

24. Gramicidin – Ion Channel • Ions carry charge movement of ions produces and electric current Can carry H+, K+ and Na+ Ions travel down this pore

25. Other Channel Proteins • Researchers did use artificial membranes and the isolated GLUT-1 transporter and found transport rates were the same as in living cells

26. All Types of Transport

27. Evolution of Membrane Models • Today cell membranes are characterized by what is known as a fluid mosaic model • Over 100 years of research was performed before this model • Historical Perspectives

28. History & Membranes

29. water soluble Charles Overton, 1890’s • Question • What is the composition of the cell’s membrane? • Experiment • Added both water soluble (hydrophilic) and lipid soluble (hydrophobic) substances to cells to determine if those substances could enter cell • Conclusion • Cells have a lipid ‘coat’ on their surface Cell lipid soluble

30. Gorter and Grendel, 1925 • Question: • What is the arrangement of lipids in the plasma membrane • Experiment • Measure surface area of RBC • Measure surface area of lipid monolayer • Compare areas • Why use RBCs Langmuir trough

31. Gorter and Grendel, Results Results: Conclusions: Surface area of monolayer = 2x the SA of RBCs, therefore lipid is oriented as a bilayer

32. Gorter and Grendel, Conclusion • Lipids are two layers thick in the membrane. • bilayer • Proposed that lipids were in bilayer with polar groups toward the aqueous compartments and non-polar fatty acid parts toward the center of the bilayer • Phospholipids can rotate, diffuse and flip in the lipid sea

33. Davson and Danielli, 1935 • A role for proteins • Surface tension

34. Davson and Danielli • 1st model • Evidence • surface tension of oil droplets is high • surface tension of cell membranes is low • using starfish eggs • Surface tension of oil droplets coated with protein is low

35. Davson and Danielli • 2nd model • Realized that there was a problem with the Davson/Danielli 1st model • Transport • Model was revised to allow for pores

36. Protein Coat Lipid Robertson, 1959 This was believed to confirm the Davson/Danielli model of the plasma membrane structure.

37. Singer and Nicholson, 1972 • Disproved Robertson, Davson, & Danielli • Lipid bilayer kept • Protein coat lost • This model had two important components • Fluid • all components are free to diffuse in the plane of the membrane • Mosaic • heterogeneity in the membrane – proteins and lipids interspersed • AND, because of fluidity, randomly distributed

38. Evidence for Fluid Mosaic • Mixing of fluorescently tagged proteins on hybrid cells • Frye & Edidin, 1970 • Florescence recovery after photobleaching • FRAP • Mosaic • Freeze fracture

39. Fluorophore Frye and Edidin, 1970 • Journal of cell science • Used fluorescently labeled antibodies

40. + Membrane proteins Mouse cell Mixed proteins after 1 hour Human cell Hybrid cell Frye and Edidin, 1970 • Explanations • Proteins are free to diffuse in the membrane • Newly synthesized membrane proteins are inserted into the membrane • Process is ATP dependent

41. Frye and Edidin (1970) Note: Experiment done at 37°C

42. Frye and Edidin, 1970 • Is protein synthesis of new membrane proteins responsible for intermixing • Add protein synthesis inhibitor cyclohexamide • Mixing still occurred • Is intermixing an ATP dependent process • Block ATP production with DNP, cyanide • Mixing still occurred • Conclusion • Mixing is due to fluidity

43. Fluorescence Recovery After Photobleaching • FRAF • Measures lateral diffusion of molecules (lipids/proteins) in cell membranes • Method allows us to look at populations of molecules • Information obtained addresses whether components are, in fact, free to diffuse

44. 1 2 3 • Measure Recovery • All labeled components are free to diffuse • Slow diffusion • A fraction of the population is not mobile • Anchored proteins • Conclusion • Most but not all components are free to diffuse

45. Mosaic Evidence • Scanning electron micrographs showed pits and mounds studding the inner surfaces of the bilayer

46. Cell Membrane

47. Lipids & Disease • Membrane lipids undergo constant metabolic turnover • Breakdown is performed by hydrolytic enzymes in lysosomes • Impaired degradation by a defect enzyme leads to the accumulation of partial breakdown products • Brain, liver & spleen • Genetic basis for Niemann-Pick disease and Tay-Sachs disease • Metal retardation, paralysis, blindness, early death