1. Chapter 9 - Lipids and Membranes
Lipids are essential components of all living organisms
Lipids are water insoluble organic compounds
They are hydrophobic (nonpolar) or amphipathic (containing both nonpolar and polar regions)

2. Fig 9.1 Structural relationships of major lipid classes

3. Nomenclature of fatty acids
Fatty acids - R-COOH (R=hydrocarbon chain) are components of triacylglycerols, glycerophospholipids, sphingolipids
Fatty acids differ from one another in:
(1) Length of the hydrocarbon tails
(2) Degree of unsaturation (double bond)
(3) Position of the double bonds in the chain
Nomenclature of fatty acids
Most fatty acids have 12 to 20 carbons
Most chains have an even number of carbons
IUPAC nomenclature: carboxyl carbon is C-1
Common nomenclature: a,b,g,d,e etc. from C-1
Carbon farthest from carboxyl is w

4. Prentice Hall c2002
Chapter 9

5. Structure and nomenclature of fatty acids
Saturated - no C-C double bonds
Unsaturated - at least one C-C double bond
Monounsaturated - only one C-C double bond
Polyunsaturated - two or more C-C double bonds
Double bonds in fatty acids
Double bonds are generally cis
Position of double bonds indicated by Dn, where n indicates lower numbered carbon of each pair
Shorthand notation example: 20:4D5,8,11,14
(total # carbons : # double bonds, D double bond positions)
Prentice Hall c2002
Chapter 9

6. Fig. 9.3 Structures of three C18 fatty acids
(a) Stearate (octadecanoate)
(b) Oleate (cis-D9-octadecenoate)
(c) Linolenate (all-cis-D9,12,15-octadecatrienoate)
The cis double bonds produce kinks in the tails of unsaturated fatty acids
Fig Structures of three C18 fatty acids

8. Fig 9.5 Structure of a triacylglycerol
Triacylglycerols
Fatty acids are stored as neutral lipids, triaclyglycerols (TGs) Fats have 2-3 times the energy of proteins or carbohydrates
TGs are 3 fatty acyl residues esterified to glycerol
TGs are hydrophobic, stored in fat cells (adipocytes)
Fig 9.5 Structure of a triacylglycerol

11. Fig 9.9 Phospholipases hydrolyze phospholipids

12. Fig 9.10 Structure of an ethanolamine plasmalogen
Plasmalogens - C-1 hydrocarbon substituent attached by a vinyl ether linkage (not ester linkage)

13. Sphingolipids
Sphingolipids - sphingosine is the backbone abundant in central nervous system tissues
Ceramides - fatty acyl group linked to C-2 of sphingosine by an amide bond
Sphingomyelins - phosphocholine attached to C-1 of ceramide
Cerebrosides - glycosphingolipids with one monosaccharide residue attached via a glycosidic linkage to C-1 of ceramide
Galactosylcerebrosides - a single b-D-galactose as a polar head group
Gangliosides - contain oligosaccharide chains with N-acetyl-neuraminic acid (NeuNAc) attached to a ceramide
Prentice Hall c2002
Chapter 9

14. Fig 9.11
(a) Sphingosine (b) Ceramides
(c) Sphingomyelin

15. Fig 9.12
Structure of a galactocerebroside

16. Fig 9.13 Ganglioside GM2 (NeuNAc in blue)

17. Steroids
Classified as isoprenoids - related to 5-carbon isoprene (found in membranes of eukaryotes)
Steroids contain four fused ring systems: 3-six carbon rings (A,B,C) and a 5-carbon D ring
Ring system is nearly planar
Substituents point either down (a) or up (b)
Fig 9.14
Isoprene
Prentice Hall c2002
Chapter 9
Fig 9.15

18. Fig 9.15 Structures of several steroids

19. Fig 9.15 Structures of several steroids

20. Cholesterol
Cholesterol modulates the fluidity of mammalian cell membranes
It is also a precursor of the steroid hormones and bile salts
It is a sterol (has hydroxyl group at C-3)
The fused ring system makes cholesterol less flexible than most other lipids

21. Cholesterol esters Fig 9.17 Cholesteryl ester
Cholesterol is converted to cholesteryl esters for cell storage or transport in blood
Fatty acid is esterified to C-3 OH of cholesterol
Cholesterol esters are very water insoluble and must be complexed with phospholipids or amphipathic proteins for transport
Fig Cholesteryl ester

22. Fig 9.18 Myricyl palmitate, a wax
Waxes
Waxes are nonpolar esters of long-chain fatty acids and long chain monohydroxylic alcohols
Waxes are very water insoluble and high melting
They are widely distributed in nature as protective waterproof coatings on leaves, fruits, animal skin, fur, feathers and exoskeletons
Fig Myricyl palmitate, a wax

23. Eicosanoids
Eicosanoids are oxygenated derivatives of C20 polyunsaturated fatty acids (e.g. arachidonic acid)
Prostaglandin E2 - can cause constriction of blood vessels
Thromboxane A2 - involved in blood clot formation
Leukotriene D4 - mediator of smooth-muscle contraction and bronchial constriction seen in asthmatics
Aspirin alleviates pain, fever, and inflammation by inhibiting cyclooxygenase (COX), an enzyme critical for the synthesis of prostaglandins. (NSAID family of compounds)

24. Fig 9.19 Arachidonic acid and eicosanoid derivatives

25. Lipid vitamins Vitamins A,D,E, and K are isoprenoid derivatives
Vitamin E

26. Biological Membranes Are Composed of Lipid Bilayers and Proteins
Biological membranes define the external boundaries of cells and separate cellular compartments
A biological membrane consists of proteins embedded in or associated with a lipid bilayer
Prentice Hall c2002
Chapter 9

27. Several important functions of membranes
Some membranes contain protein pumps for ions or small molecules
Some membranes generate proton gradients for ATP production
Membrane receptors respond to extracellular signals and communicate them to the cell interior
Lipid Bilayers
Lipid bilayers are the structural basis for all biological membranes
Noncovalent interactions among lipid molecules make them flexible and self-sealing
Polar head groups contact aqueous medium
Nonpolar tails point toward the interior

28. Fig 9.21 Membrane lipid and bilayer

29. Fluid Mosaic Model of Biological Membranes
Fluid mosaic model - membrane proteins and lipids can rapidly diffuse laterally or rotate within the bilayer (proteins “float” in a lipid-bilayer sea)
Membranes: ~25-50% lipid and 50-75% proteins
Lipids include phospholipids, glycosphingolipids, cholesterol (in some eukaryotes)
Compositions of biological membranes vary considerably among species and cell types

30. Fig 9.22 Structure of a typical eukaryotic plasma membrane

31. Lipid Bilayers and Membranes Are Dynamic Structures
Fig (a) Lateral diffusion is very rapid (b) Transverse diffusion (flip-flop) is very slow

32. Fig 9. 25 Freeze-fracture electron microscopy,
Fig Freeze-fracture electron microscopy, distribution of membrane proteins

33. Fig 9.22 Structure of a typical eukaryotic plasma membrane

34. Three Classes of Membrane Proteins
(1) Integral membrane proteins
Contain hydrophobic regions embedded in the lipid bilayer
Usually span the bilayer completely
(2) Peripheral membrane proteins
Associated with membrane through charge-charge or hydrogen bonding interactions to integral proteins or membrane lipids
More readily dissociated from membranes than covalently bound proteins
Change in pH or ionic strength often releases these proteins
(3) Lipid-anchored membrane proteins
Tethered to membrane through a covalent bond to a lipid

35. Fig 9.22 Structure of a typical eukaryotic plasma membrane

36. Membrane Transport Three types of integral membrane protein transport:
(1) Channels and pores
(2) Passive transporters
(3) Active transporters
Prentice Hall c2002
Chapter 9

37. Table 9.3 Characteristics of membrane transport

38. Pores and Channels
Pores and channels are transmembrane proteins with a central passage for ions and small molecules
Solutes of appropriate size, charge, and molecular structure can diffuse down a concentration gradient
Process requires no energy
Central passage allows molecules and ions of certain size, charge and geometry to transverse the membrane.
Figure 9.30

39. Types of passive transport systems
Passive transport (facilitated diffusion) does not require an energy source
Protein binds solutes and transports them down a concentration gradient
Types of passive transport systems
Uniport - transporter carries only a single type of solute
Some transporters carry out cotransport of two solutes, either in the same direction (symport) or in opposite directions (antiport)

40. Fig 9.31
Types of passive transport (a) Uniport (b) Symport (c) Antiport

41. Fig 9.32 Kinetics of passive transport
Initial rate of transport increases until a maximum is reached (site is saturated)

42. Types of active transport
Transport requires energy to move a solute up its concentration gradient
Transport of charged molecules or ions may result in a charge gradient across the membrane
Types of active transport
Primary active transport is powered by a direct source of energy as ATP, light or electron transport
Secondary active transport is driven by an ion concentration gradient
Prentice Hall c2002
Chapter 9

43. Fig 9.34 Secondary active transport in E. coli
Oxidation of Sred generates a transmembrane proton gradient
Movement of H+ down its gradient drives lactose transport (lactose permease)

44. Fig 9.35 Secondary active transport in animals: Na+-K+ ATPase
Na+ gradient (Na+-K+ATPase) drives glucose transport

45. Endocytosis and Exocytosis
Cells import/export molecules too large to be transported via pores, channels or proteins by:
Endocytosis - macromolecules are engulfed by plasma membrane and brought into the cell inside a lipid vesicle
Exocytosis - materials to be excreted from the cell are enclosed in vesicles that fuse with the plasma membrane

46. Transduction of Extracellular Signals
Specific receptors in plasma membranes respond to external chemicals (ligands) that cannot cross the membrane: hormones, neurotransmitters, growth factors
Signal is passed through membrane protein transducer to a membrane-bound effector enzyme
Effector enzyme generates a second messenger which diffuses to intracellular target

47. Fig 9.37 General mechanism of signal transduction across a membrane

48. Fig 9.43
Summary of the adenyl cyclase signaling pathway