1. Chapter Nineteen
Lipids

2. Lipids
Lipids are:
Biomolecules that contain fatty acids or a steroid nucleus
Soluble in organic solvents, but not in water
Named for the Greek word lipos, which means “fat.”
Extracted from cells using organic (nonpolar) solvents

3. Biological Roles of Lipids
Energy storage
Membrane/barriers
Emulsification
Messengers (hormones, etc.)
Protective coating (waxes)

4. Types of Lipids
Lipids are different from other biomolecules because there is no common structural feature
Lipids with fatty acids (long carbon chain carboxylic acids)
Waxes
Fats and oils (trigycerides)
Phospholipids
Sphingolipids
Lipids without fatty acids
Steroids

5. Lipids cont’d
The structural formulas of these types of lipids illustrate the great structural diversity among lipids.

6. Fatty Acids Long-chain carboxylic acids Insoluble in water
Typically carbon atoms (even number)
Some contain double bonds (unsaturated)
corn oil contains 86% unsaturated fatty acids and % saturated fatty acids

7. Lipids cont’d
The melting point of a fatty acid depends on the length of the carbon chain and on the number of double bonds present in the carbon chain.

8. Fatty Acid Formulas
The formulas for fatty acids are written as condensed and line-bond formulas. For example, caprylic acid with 8 carbon atoms can be written as:
CH3-(CH2)6-COOH
CH3-CH2-CH2-CH2-CH2-CH2-CH2-COOH O ||
CH3-CH2-CH2-CH2-CH2-CH2-CH2-C-OH

9. Saturated and Unsaturated Fatty Acids
Saturated = C–C bonds
Unsaturated = one or more C=C bonds
natural fatty acid double bonds are cis
COOH
Palmitic acid, a saturated acid
Palmitoleic acid, an unsaturated acid

10. Properties of Fatty Acids
Most Contain only single C–C bonds (Saturated)
Closely packed
Strong attractions between chains
High melting points
Melting points increase with chain length
Solids at room temperature

11. Properties of Unsaturated Fatty Acids
Contain one or more double C=C bonds
Nonlinear chains do not allow molecules to pack closely
Few interactions between chains
Lower melting points than saturated
Melting points increase with chain length
And decrease with number of double bonds
Liquids at room temperature

12. Fatty Acid Structures Saturated fatty acids
Fit closely in regular pattern
Unsaturated fatty acids
Cis double bonds

14. Lipids cont’d
Four 18-carbon fatty acids, which differ in the number of double bonds present.

15. Triple esterification reaction between glycerol and three molecules of stearic acid (18:0 acid). Three molecules of water are a by-product of this reaction.
Simple Triacylglycerol

16. Structure of a mixed triacylglycerol in which three different fatty acid residues are present.

17. Representative triacylglycerols from (a) a fat and (b) an oil.

18. Lipids cont’d
Percentages of saturated, monosaturated, and polyunsaturated fatty acids.

19. Lipids cont’d

20. Lipids cont’d

21. Olestra Used in foods as an artificial fat
Sucrose linked by ester bonds to several long-chain fatty chains
Not broken down in the intestinal tract

22. Chemical Properties of Triacylglycerols
The chemical reactions of triacylglycerols are similar to those of alkenes and esters
In hydrogenation, double bonds in unsaturated fatty acids react with hydrogen in the presence of a Ni or Pt catalyst
In hydrolysis, ester bonds are split by water in the presence of an acid, a base, or an enzyme

23. Hydrogenation
Converts double bonds to single bonds

24. Product of Hydrogenation
Hydrogenation converts double bonds in oils to single bonds. The solid products are used to make margarine and other hydrogenated items.

25. Lipids cont’d
Structural equation for the complete hydrogenation of a triacylglycerol with oleic acid and fatty acid residue.

26. Lipids cont’d During hydrogenation, cis double bonds can be converted
to trans double bonds.
Compare the geometry of the
trans 18:2 FA to the 18:0
saturated FA. Both have
similar geometries and can
interact with each other as
though the 18:2 was saturated.
Hydrogenated oils may be
unhealthy, and spreadable
margarine in tubs may be
preferable to stick form.

27. Hydrolysis
Triglycerides split into glycerol and three fatty acids (H+ or enzyme catalyst)

28. Oxidation
Carbon-carbon double bonds in unsaturated fatty acids can be oxidized to form both short chain aldehydes and carboxylic acids
Oils go “rancid”
They often have bad odors

29. Saponification and Soap
Hydrolysis with a strong base
Triglycerides split into glycerol and the salts of fatty acids
The salts of fatty acids are “soaps”
KOH gives softer soaps

30. Saponification

31. Lipids cont’d

32. Phospholipids
A lipid that contains one or more fatty acids, a phosphate group, a platform molecule to which the fatty acids and the phosphate group are attached, and an alcohol attached to the phosphate group

33. Glycerolphospholipids
The most abundant lipids in cell membranes
Composed of glycerol, two fatty acids, phosphate, and an amino alcohol
Has polar and nonpolar regions

34. Lipids cont’d
(a) structural formula and (b) molecular model showing the “head and two tails” structure of a phosphatidyl-choline molecule.

35. Lipids cont’d
Molecular models for (a) sphingosine and (b) sphingophosopholipid.

36. Sphingophospholipids
Sphingophospholipids are phospholipids that have an amide bond between a fatty acid and sphingosine, an 18-carbon alcohol

37. Sphingoglycolipids Another type of membrane protein
Glycosphingolipids contain monosaccharides bonded to the –OH of sphingosine by a glycosidic bond
Cerebrosides contain only one monosaccharide
Gangliosides are similar to cerebrosides, but contain two or more monosaccharides

38. Lipids cont’d

39. Lipids cont’d
Structural formula and model for the cholesterol molecule.

40. Lipids cont’d
A severely occluded artery.
Howard Socurek / Medichrome

41. Lipoproteins Combine lipids with proteins and phospholipids
Are soluble in water because the surface consists of polar lipids
LDL = low density (more fat than protein)
HDL = high density (more protein, relatively)

42. Lipids cont’d

43. Lipids cont’d
Cross section of a lipid bilayer.

44. Lipids cont’d
Space-filling model of a section of a lipid bilayer.

45. Lipids cont’d
The kinks associated with cis double bonds in fatty acid chains prevent tight packing of the lipid molecules in a lipid bilayer.

46. Lipids cont’d
Cholesterol molecules fit between fatty acid chains in a lipid bilayer.

47. Cell Membranes
Separate cellular contents from the external environment
Consist of a lipid bilayer made of two rows of phospholipids
Have an inner portion made of the nonpolar tails of phospholipids with the polar heads at the outer and inner surfaces

48. Fluid Mosaic Model of Cell Membranes
The lipid bilayer contains proteins, carbohydrates and cholesterol
The unsaturated fatty acids make cell membranes fluid-like rather than rigid
Proteins and carbohydrates on the outer surface communicate with hormones and neurotransmitters

49. Lipids cont’d
Proteins are important structural components of cell membranes.

50. Transport through Cell Membranes
Diffusion (passive transport)
Moves particlse from a higher to a lower concentration
Facilitated transport
Uses protein channels to increase the rate of diffusion
Active Transport
Moves ions against a concentration gradient

51. Three Mechanisms

53. G. Siegel, B Agranoff, R. W. Albers, and P. Molinoff editiors
G. Siegel, B Agranoff, R.W. Albers, and P. Molinoff editiors. In Basic Neurochemistry 4 th Ed., Raven Press, Ltd. 1989, p. 57.

54. Bile Salts Synthesized from cholesterol and stored in the gall bladder
Emulsify fats and oils to give a greater surface area for lipid digesting enzymes

55. Bile Acids and Salts
Line-angle structural formulas for cholesterol, cholic acid, and two deoxycholic acids.

56. Lipids cont’d
A large percentage of gallstones are almost pure crystallized cholesterol.
C. James Webb / Phototake

57. Steroid Hormones Produced from cholesterol
Include sex hormones such as androgens (testosterone) in males and estrogens (estradiol) in females
Anabolic steroids are derivatives of testosterone
Also include aldosterone (controls electrolytes and water balance by kidneys) and cortisone (increases blood glucose levels and stimulates the synthesis of glycogen in the liver)

58. Lipids cont’d
Selected sex hormones and synthetic compounds that have similar actions.

59. Lipids cont’d

60. Lipids cont’d
Relationship of the structures of various eicosanoids to their precursor, arachidonic acid.

61. Lipids cont’d
A biological wax has a structure with a small, weakly polar “head” and two long, nonpolar “tails”.

62. Lipids cont’d
Plant leaves often have a biological wax coating to prevent excessive loss of water.
© Kevin Schaefer / Peter Arnold, Inc.