1. Chapter 10
Membrane Structure

3. All biological membranes have a common structure – each is a very thin
film of lipid and protein molecules held together by noncovalent interactions

5. Membrane lipids are amphipathic molecules, most of which
The Lipid Bilayer
Membrane lipids are amphipathic molecules, most of which
spontaneously form bilayers

6. The most abundant membrane lipids are the phospholipids
Phospholipids have a polar head group and
two hydrophobic hydrocarbon tails

7. The tails are usually fatty acids, and
they can differ in length. One tail
usually has one or more cis-double
bonds, while the other does not.

8. The shape and amphipathic nature of the
lipid molecules cause them to form bilayers
spontaneouly in aqueous environments.

9. Hydrophilic and hydrophobic molecules interact differently with water

10. molecules spontaneously form bilayers in aqueous environments
Packing arrangements of lipid molecules in an aqueous environment
Lipid molecules spontaneously aggregate to bury their hydrophobic tails in the interior
and expose their hydrophilic heads to water. Being cylindrical, phospholipid
molecules spontaneously form bilayers in aqueous environments

11. the phospholipid molecule
The spontaneous closure of a phospholipid bilayer to form a sealed compartment
The formation of a sealed compartment is fundamental to the creation of a living
cell, and this behavior follows directly from the shape and amphipathic nature of
the phospholipid molecule

12. The lipid bilayer is a two-dimensional fluid
Individual lipid molecules are able to diffuse freely within lipid bilayers.
Demonstrated using synthetic lipid bilayers and
electron spin resonance (ESR) spectroscopy
Liposomes

13. A black membrane

14. Phospholipid mobility

15. The fluidity of a lipid bilayer depends on its composition and its temperature
The membrane becomes more difficult to freeze if the hydrocarbon chains are short
or have double bonds, so that the membrane remains fluid at lower temperatures

16. phospholipids, it often also contains cholesterol and glycolipids
The lipid bilayer of many cell membranes is not composed exclusively of
phospholipids, it often also contains cholesterol and glycolipids

18. Eucaryotic plasma membranes contain large amounts of cholesterol

20. Four major phospholipids predominate in the plasma membrane of mammalian cells

23. Lipid bilayers can form domains of different compositions
Lipid phase separation in artificial lipid bilayers
(A) 1:1 mixture of phosphatidylcholine and sphingomyelin
(B) 1:1:1 mixture of phosphatidylcholine, sphingomyelin, and cholesterol

24. cholesterol, and some membrane proteins
Plasma membrane contains lipid rafts that are enriched in sphingolipids,
cholesterol, and some membrane proteins
Lipid rafts are small specialized areas in membranes where some lipids (primarily
sphingolipids and cholesterol) and proteins are concentrated

25. Lipid droplets are surrounded by a phospholipid monolayer

26. The asymmetry of the lipid bilayer is functionally important
The lipid bilayer of human red blood cells

27. Signaling functions of inositol phospholipids in the cytosolic leaflet of the plasma membrane

28. Glycolipids are found on the surface of all plasma membranes

31. Membrane Proteins

32. Membrane proteins can be associated with the lipid bilayer in various ways

33. Covalent attachment of a protein to the membrane by a
fatty acid chain or a prenyl group

35. in an α-helical conformation
In most transmembrane proteins the polypeptide chain crosses the lipid bilayer
in an α-helical conformation

36. Membrane regions and preferred amino-acid locations
Membrane regions and preferred amino-acid locations. A snapshot of a lipid bilayer membrane and its three major regions. Grey, carbon atoms; red, oxygen; white, hydrogen bound to oxygen; orange, phosphorus. In an a-helix, 20 amino acids (blue) can span the hydrocarbon core, and 10 amino acids (green) can span the interfacial region. Arrows indicate where most of each amino acid (denoted by its three-letter symbol) would be found at equilibrium based on transfer free-energy measurements (Nature, 2005, Vol 433, )

37. A possible scheme for the membrane-insertion decision, as proposed by Hessa et al. A top view of the membrane and the translocon pore that crosses it. Inside the pore is a peptide helix surrounded by water. The pore opens sideways into the membrane, allowing the helix to interact with the membrane lipids. If the peptide is more compatible with lipid than with water, it will transfer into the membrane; otherwise, it will continue to be moved through the pore. The figure is only intended to convey the basic principle, and omits many mechanistic and structural issues.
(Nature, 2005, Vol 433, )

39. Hydropathy plots identify potential α-helical segments in a polypeptide chain

40. Two a helices in the aquaporin water channel, each of which spans only
halfway through the lipid bilayer

41. Converting a single-chain multipass protein into a two-chain multipass protein

42. Steps in the folding of a multipass transmembrane protein

43. hydrogen-binding requirements is for multiple transmembrane strands
An alternative way for peptide bonds in the lipid bilayer to satisfy their
hydrogen-binding requirements is for multiple transmembrane strands
of polypeptide chains to be arranged as β sheet in the form of a closed barrel

44. Many membrane proteins are glycosylated and have intrachain or interchain
disulfide bonds

45. The cell coat, or glycocalyx is the carbohydrate-rich zone on the cell surface
Likely functions –
protect cells against mechanical and chemical damage
keep foreign objects and other cells at a distance

46. A simplified diagram of the cell coat (glycocalyx)

47. Membrane proteins can be solubilized and purified in detergents

49. Structure and function of detergent micelles

50. Solubilizing membrane proteins with a mild detergent

51. functional membrane systems
The use of mild detergents for solubilizing, purifying, and reconstituting
functional membrane systems

52. that contain bacteriorhodopsin molecules
Bacteriorhodopsin is a proton pump that traverses the lipid bilayer as seven α helices
The archaean Halobacterium salinarum showing patches of purple membrane
that contain bacteriorhodopsin molecules

53. The three-dimensional structure of a bacteriorhodopsin molecule

54. Rhodopseudomonas viridis
Membrane proteins often function as large complexes
The three-dimensional structure of the photosynthetic reaction center of the bacterium
Rhodopseudomonas viridis

55. Many membrane proteins diffuse in the plane of the membrane

56. Fluorescence recovery after photobleaching

57. Fluorescence loss in photobleaching

58. Plasma membrane proteins are restricted to particular membrane
domains

59. Three domains in the plasma membrane of a sperm cell

60. Four ways of restricting the lateral mobility of specific plasma
membrane proteins

61. The cytosolic side of plasma membrane proteins can be studied in red blood
cell ghosts

62. The spectrin-based cytoskeleton on the cytosolic side of the human RBC membrane