The document discusses cell membranes and their structure and composition. It notes that membranes are made of a bilayer of polar lipids, including sphingomyelin, glycerolipids, and cholesterol. It describes how the length and saturation of fatty acid chains and the presence of cholesterol affect membrane fluidity. Homeothermic organisms regulate membrane fluidity through adaptations like changing fatty acid chain length and saturation. The document also discusses asymmetries in membrane composition and lipid movement across membranes. Integral membrane proteins span or are embedded in the bilayer, and are anchored via interactions with specific lipid groups.

1. Molecular Cell Biology Fifth Edition Chapter 5: Biomembranes and Cell Architecture Copyright © 2004 by W. H. Freeman & Company Harvey Lodish • Arnold Berk • Paul Matsudaira • Chris A. Kaiser • Monty Krieger • Matthew P. Scott • Lawrence Zipursky • James Darnell

2. Cellula Animale Cellula Vegetale

5. Struttura a bilayer di membrane biologiche

6. 2 strati polari

17. RAFTS di sfingomielina all’ Atomic Force Microscope

18. Eritrociti

19. Assoni circondati da mielina (strati multipli di membrane modificate)

21. 3 Classi di Lipidi di Membrana

22. Sfingomielina Glicolipid glucoisil cerebroside

26. Il colesterolo aumenta “ la robustezza” delle membrane

27. Factors that affect membrane fluidity: Length of the fatty acyl chains Degree of saturation of the fatty acyl chains Presence of cholesterol The rigid cholesterol ring system interferes with close packing of phospholipid fatty acid tails and thus inhibits transition from liquid crystal to crystalline state upon temperature decrease. At the same time, the rigid cholesterol makes the membrane somewhat less fluid.

28. The effect of fatty acid composition on membrane fluidity Two aspects: the length and the degree of un-saturation Membrane lipids often contain one saturated and on unsaturated fatty acids The effect of sterols on membrane fluidity Sterols – cholesterol and phytosterols. Sterols account for up to 50% of total membrane lipid on a molar basis The cholesterol molecules form hydrogen bonds with the oxygen atom on its neighboring phospholipid molecule The paradoxical effect of cholesterol : decreasing membrane fluidity at high temperature (due to the rigid “rings”) and increasing it at low temperature (by preventing regular arrangement of neighboring hydrocarbon tails) However, sterols decrease the permeability of a lipid bilayer, by plugging in the spaces between phospholipid molecules Most organisms can regulate membrane fluidity Poikelotherms (bacteria, fungus, plant, amphibians etc.) vs. homeotherms (warm-blooded animal) Homeoviscous adaptation – keeping the viscosity of the membrane about the same despite changes in temperature, by removing the terminal carbons to shorten the fatty acid molecules or introducing unsaturated bonds into the existing saturated fatty acid molecules (by e.g. E. coli desaturasae). Lipids of 18 carbon 70 o C -11 o C 76 o C 32 o C

29. Forme gel e fluide dei P-lipidi di membrana

30. Effetto della composizione lipidica sullo spessore e sulla curvatura

31. Il colesterolo ha un effetto lipid-ordering solo con PC

34. Asimmetria delle Membrane La maggior parte dei lipdi sono distribuiti in maiera non uguale nei 2 foglietti del bilayer L’ asimmetria include differenze nei tipi di lipidi presenti e nel grado di saturazione degli acidi grassi. L’ asimmetria si forma durinte la biogenesi delle membrane. Membrana Esterna : glicolipidi per le reazioni di signaling e riconoscimento Membrana Interna : phosphatidylethanolamine, phosphatidylinositol, e phosphatidylserine per la trasmissione di segnali all’interno della cellula. Movimenti dei lipidi nelle membrane: flip-flop , o diffusione trasversa non avviene spontane amente Rotaz ione Diffusione Laterale Il movimento flip-flop dei phospholipidi è raro, ma viene condotto dalle flippasi. Quindi, la traslocazione di phospholipidi nelle membrabne biologiche è più frequente che in bilayer artificiali di lipidi

35. wobbling

36. Specificità delle P-Lipasi

37. Classi di proteine che interagiscno con le membrane

39. Glicoforina Proteina di membrana tipica a singolo passaggio

40. Ancoraggio di proteine di membrana con legami con gruppi specifici

41. α-helix is the major structural element of membrane-bound proteins

43. Membrane lipids: the “mosaic” part of the model Freezing fracturing: quick frozen (in liquid nitrogen) and then subjected to a sharp blow of a diamond knife. E face (exoplasmic sicde) and P face (protoplasmic side) Membrane contain integral, peripheral, and lipid-anchored proteins Integral membrane proteins: contain ~20 aa hydrophobic transmembrane segment and hard to isolate or extract and need detergent treatment. Intergral monotopic proteins Transmembrane protein: singlepass multipass multi-subunit Glycophorin of erythrocyte Bacteriorhodopsin The hydropathy plot

46. Membrane-spanning proteins

49. Lipids are responsible for compartmentalization of cell metabolites Proteins carry out most of the biochemical reactions taking place in a cell