1. Chapter 10: Membranes Know the terminology:
Phospholipid, phosphoglyceride, sphingolipid, cholesterol, steroids, phosphotide, polar head group, fatty acid, glycerol, glycoprotein, proteoglycan
bilayer, fluidity, homeoviscous adaptation, integral membrane protein, transmembrane domain, peripheral protein, lipid raft, hydropathy plot

2. Membranes allow compartmentation

3. Biological membranes are composed of a:
(i) Lipid bilayer
(ii) Proteins

4. Lipid components of the bilayer
Phospholipids
Phosphoglycerides: glycerol, 2 fatty acids and polar head group
Sphingolipids: sphingosine, 1 fatty acid and polar head group
Other lipids:
Steroids (cholesterol mainly)
Fatty acids: aliphatic chains with carboxylic acid group

5. Protein components of the bilayer
Simple proteins
Glycoproteins:
Proteins with carbohydrate chains
Proteoglycans:
Proteins with glycosaminoglycan chains

6. Phosphoglycerides Composed of: A glycerol backbone (with 3 positions)
2 long chain fatty acids
A polar head group

7. Lipid bilayer

8. Sphingolipids (e.g. sphongomyelin) Composed of: A sphingosine backbone
1 long chain fatty acid
A polar head group

9. Sphingolipids and phosphoglycerides

10. Cholesterol

11. Cholesterol
Cholesterol increases “tightness” of membranes but increases fluidity

12. Membranes are heterogeneous
(1) Inner and outer leaflets are distinct in composition

13. Membranes are heterogeneous
(2) Regions of membranes can be enriched in specific lipids such as cholesterol (lipid rafts)

14. Membranes composed of phospholipids are highly mobile.
Membrane fluidity
Membranes composed of phospholipids are highly mobile.

15. Membrane fluidity depends upon lipid composition
Phospholipid movement depends upon:
Fatty acid chain length
Saturation
Polar head group
Physical conditions
Membrane fluidity also depends upon presence of other macromolecules:
Cholesterol
Glycolipids

16. Homeoviscous adaptation
Environmental conditions (temperature, salt concentration) can alter membrane fluidity
Cells adjust lipid profiles to maintain constant fluidity
Reduced temperature “solidifies” membranes
Cell increase fluidity by:
using shorter fatty acids
introducing double bonds into fatty acids
altering polar head groups

17. Membrane proteins Many membranes are primarily protein
(e.g. mitochondrial inner membrane is about
~80% protein, 20% lipid)
Proteins can be associated with the membrane many different ways:
(1) Integral proteins are embedded within the membrane
(2) Peripheral proteins are only associated with the membrane (via various connections)

18. Topography and membrane proteins

19. Topography and membrane proteins

20. Transmembrane proteins
Many proteins cross completely through the membrane one or more times
Typically an alpha-helix with hydrophobic amino acids (Figs , 10-20)

21. Surface hydrophocity of a-helices
Hydrophobic amino acids-green
Polar-blue,
Charged-red

22. Predicting membrane-spanning domains
Membrane spanning domains can be predicted from primary structures using hydropathy plots.

23. Some integral proteins are b-barrels
Many large pores are composed of beta-sheets arranged into a barrel.

24. Controlling protein location
In a naked cell, proteins are free to move within membranes but often cells restrict movement of proteins.
(1) Some proteins interact with each other (self-assembly) (Fig )

25. Controlling protein location
(2) Others can interact directly with the cytoskeleton (or via linkers)

26. Controlling protein location
(3) Some interact via external domains (e.g. carbohydrate)

27. Controlling protein location
(4) Inter-cellular interactions may prevent movements.