1. BC368 Biochemistry of the Cell II
Biological Membranes
Chapter 11: Part 1
February 10, 2015

2. Plasma Membrane
“Possibly the decisive step [in the origin of life] was the formation of the first cell, in which chain molecules were enclosed by a semi-permeable membrane which kept them together but let their food in.”
J. B. S. Haldane, 1954

3. Plasma Membrane

4. Plasma Membrane Membrane is composed of: Lipids Phospholipids Sterols
B. Proteins
Integral
Peripheral
C. Carbohydrates
Glycolipids
Glycoproteins

5. Plasma Membrane
Variable components in different membrane types

6. Membrane Lipids Amphiphilic lipids
Glycolipid
sphingosine
Amphiphilic lipids
Major types: phospholipids, glycolipids, sterols
glycerophospholipid
sphingophospholipid

7. Phospholipids
Two classes: glycerophospholipids (aka phosphoglycerides) and sphingophospholipids
Fig 10-7

8. Phospholipids
Two classes: glycerophospholipids (aka phosphoglycerides) and sphingophospholipids

9. Membrane Lipids: 1A. Glycerophospholipids
Two fatty acids; phosphate and polar “head group” on glycerol.
Vary in the FA’s and head group.

10. Membrane Lipids: 1B. Sphingophospholipids
Named for the enigmatic Sphinx
Common in nerve and brain cell membranes

11. Membrane Lipids: 1B. Sphingophospholipids
Named for the enigmatic Sphinx
Sphingosine replaces glycerol, so only 1 FA tail
note amide
linkage

12. Membrane Lipids: 1B. Sphingophospholipids
Example: sphingomyelin
Head group = phosphocholine or phosphoethanolamine

13. Glycolipids
Two classes: glycosphingolipids and galactolipids
Fig 10-7

14. Membrane Lipids: 2A. Glycosphingolipids
Sphingolipids with carbohydrate head group; common on cell surfaces
Ganglioside
Sugar
Examples: cerebrosides and gangliosides
Glucose or galactose

15. Membrane Lipids: 2B. Galactolipids
Diglycerides with galatose groups
Common in plant (thylakoid) membranes

16. Membrane Lipids: 3. Sterols
Cholesterol and cholesterol-like compounds

17. Lipid Components of Membranes
Lipid composition varies across different membranes.
Fig 11-2

18. Lipid Components of Membranes
Lipid composition varies across the two leaflets of the same membrane.

19. Turnover of Membrane Lipids
Fig 10-16

20. Defects in Membrane Turnover
Deposits of gangliosides in Tay Sachs brain

21. Lipid Aggregates
Lipids spontaneously aggregate in water as a result of the Hydrophobic Effect.

22. Lipid Aggregates
Amphiphilic lipids form structures that solvate their head groups and keep their hydrophobic tails away from water.
Above the critical micelle concentration, single-tailed lipids form micelles.
Fig 11-4

23. Lipid Aggregates
Fig 11-4
Double-tailed lipids form bilayers, the basis of cell membranes.
Bilayers can form vesicles enclosing an aqueous cavity (liposomes).
Fig 11-4

24. Membrane Proteins
Integral proteins (includes lipid-linked): need detergents to remove
Peripheral proteins: removed by salt, pH changes
Amphitropic proteins: sometimes attached, sometimes not

25. Single Transmembrane Segment Proteins
Usually alpha-helical, ~20-25 residues, mostly nonpolar.
Example: glycophorin of the erythrocyte.
Fig 11-8

26. Multiple Transmembrane Segment Proteins
7 alpha-helix motif is very common.
Example: bacteriorhodopsin
Fig 11-10

27. Beta Barrel Transmembrane Proteins
Multiple transmembrane segments form β sheets that line a cylinder.
Example: porins.

28. Lipid-Linked Membrane Proteins
Attached lipid provides a hydrophobic anchor.
Fig
An important lipid anchor is GPI (glycosylated phosphatidylinositol.

29. Membrane Carbohydrates
On exoplasmic face only

30. Membrane Carbohydrates
On exoplasmic face only
An example is the blood group antigens
glycosphingolipids

31. Membrane Dynamics
At its transition temperature (TM), the bilayer goes from an ordered crystalline state to an a disordered fluid one.
Fig 11-16

32. Membrane Dynamics
Phospholipids in a bilayer have free lateral diffusion.
Fig 11-17

33. Membrane Dynamics
Phospholipids in a bilayer have restricted movement between the two faces.
Fig 11-17

34. Membrane Dynamics
Flippases, floppases, and scramblases catalyze movement between the two faces.

35. Fluid Mosaic

36. Fluorescent Recovery After Photobleaching
Fluorescent tag is attached to a membrane component (lipid, protein, or carbohydrate).
Fluorescence is bleached with a laser.
Recovery is monitored over time.

37. Fluorescent Recovery After Photobleaching
FRAP Movie

38. Protein Mobility in the Membrane
Some membrane proteins have restricted movement.
May be anchored to internal structures (e.g., glycophorin is tethered to spectrin).
Fig

39. Protein Mobility in the Membrane
Lipid rafts are membrane microdomains enriched in sphingolipids, cholesterol, and certain lipid-linked proteins.
Thicker and less fluid than neighboring domains.
Fig

40. Protein Mobility in the Membrane
Lipid rafts are membrane microdomains enriched in sphingolipids, cholesterol, and certain lipid-linked proteins.
Thicker and less fluid than neighboring domains.
Lipid Rafts

41. Nature Reviews Molecular Cell Biology 4, 414-418 (May 2003)
Domains of gel/fluid lipid segregation in a model membrane vesicle, which is a mixture of fluid dilaurylphosphatidylcholine phospholipids with short, disordered chains and gel dipalmitoylphosphatidylcholine phospholipids with long, ordered chains. A red fluorescent lipid analogue (DiIC18) partitions into the more ordered lipids, whereas a green fluorescent lipid analogue (BODIY PC) partitions into domains of more fluid lipids. These domains in a model membrane are much larger than the domains of cell membranes.