LIPIDS
LIPIDS - INTRODUCTION
•

•

Lipids are non-polar (hydrophobic) compounds, that is
insoluble in water, but soluble in an or...
LIPIDS


Location in the body
 Membranes
 Blood

and liver (bound to transport proteins)
 Adipocytes (adipose tissue)
FUNCTIONS AND PROPERTIES










Concentrated source of energy (9 kcal/gm)
Energy reserve: stored as triglycerides...
FUNCTIONS AND PROPERTIES







Synthesis of prostaglandins from fatty acids
 Hormone-like compounds that modulates m...
LIPID CLASSIFICATION


Simple: FA’s esterified with glycerol
Fats, Waxes



Complex: same as simple, but with other comp...
FATTY ACIDS
 Contain

an even number of carbon atoms
 Arranged in an unbranched line
 Have a carboxyl group (-COOH) at ...
FATTY ACID CHAIN LENGTH
 Short

chain: 2 to 6 C (volatile fatty acids)
 Medium chain: 8 – 12 C
 Long chain: 14 – 24 C
...
FATTY ACID SATURATION
Saturated - no double bonds
 Unsaturated – contain double bonds


 Monounsaturated

– one double ...
SATURATED FATS
 All

the chemical bonds between the
carbon are single bonds C-C-C No double bonds
 No space for more H ...
MONO-UNSATURATED FATTY ACIDS
Only one double bond
 Therefore, two H atoms can be added
 Liquid at room temperature


 ...
POLY-UNSATURATED FATTY ACIDS
 Two

or more double bonds
 Include omega-3 and omega-6 fatty acids
(essential fatty acids)...
FATTY ACIDS COMMONLY FOUND IN
LIPIDS
Sat. Fatty Acids
Butyric
Palmitic
Stearic
Unsat. Fatty Acids
Oleic
Linoleic
Linolenic...
Some fatty acids and their common names:
14:0 myristic acid;
16:0 palmitic acid;
18:0 stearic acid;
18:1 cis∆9  oleic acid...
FATTY-ACID NOMENCLATURE


Named according to
chain length
 C18

H3C

C
H2

H2
C

C
H2

H2
C

C
H2

H2
C

C
H2

H2
C

C
H...
FATTY-ACID NOMENCLATURE


Named according to the number of
double bonds
 C18:0

H3C

C
H2

H2
C

C
H2

H2
C

C
H2

H2
C
...
FATTY-ACID NOMENCLATURE


Named according to the number of
double bonds
 C18:1

H3C

C
H2

H2
C

C
H2

H2
C

C
H2

H2
C
...
FATTY-ACID NOMENCLATURE


Named according to the number of
double bonds
 C18:2

H3C

C
H2

H2
C

C
H2

H2
C

C
H

H
C

C...
FATTY-ACID NOMENCLATURE


Named according to the number of
double bonds
 C18:3

H3C

C
H2

H
C

C
H

H2
C

C
H

H
C

C
H...
FATTY-ACID NOMENCLATURE


Named according to the
location of the last double bond
system (e.g., omega 3, ω3)
 n–system (...
FATTY-ACID NOMENCLATURE


Named according to the
location of the last double bond
system (e.g., omega 3, ω3)
 n–system (...
FATTY-ACID NOMENCLATURE
H3C

C
H2

H3C

H3C

C
H2

H2
C

C
H2

H2
C

C
H2

H
C

H2
C

C
H2

C
H

C
H2

H2
C

C
H2

H
C

H2...
FATTY ACID NUMBERING SYSTEMS
Delta system: carbon atoms in a fatty acid are
numbered from the carboxyl end
 Omega system:...
OMEGA SYSTEM
Ω-3

Ω-6

Ω-9

C-C-C=C-C-C=C-C-C=C-C-C-C-C-C-C-C-COOH
 Animals

can synthesize a fatty acid with a
double bo...
OMEGA SYSTEM AND
ESSENTIAL FATTY ACIDS
Linoleic acid is an omega-6 fatty acid
 Linolenic and arachidonic acids are omega-...
FATTY-ACID NOMENCLATURE
 Named
 Cis

according to location of H’s

or trans fatty acids

Cis-9-octadecenoic acid
(Oleic ...
FATTY-ACID NOMENCLATURE
H3C

C
H2

H2
C

C
H2

H2
C

C
H2

H2
C

C
H2

H
H2
C
H2
C
H2
C
H

C
3

C 2
H

C 2
H

C 2
H

H
C

...
DESIGNATING POSITION OF BONDS
CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
Numeric (9,12 – 18:2)
 ∆ (18:2 ∆ 9,12)
 n (18:2 n-6)
 ω ...
MELTING POINTS
 Affected
 Longer

Fatty acid:
C12:0
Melting point: 44°C

by chain length
chain = higher melting temp
C14...
CHAIN LENGTH
 In

most fats with a mixture of fatty acids,
the chain length of the majority of fatty
acids will determine...
MELTING POINTS
 Affected
 More

Fatty acid:
Melting point:

by number of double bonds

saturated = higher melting temp
C...
ESSENTIAL FATTY ACIDS
 Must

be in diet

 Tissues

can not synthesize
 Linoleic acid (18:2)


Omega-6-FA

 Linolenic
...
STURCTURE OF ESSENTIAL FA’S
FUNCTIONS OF ESSENTIAL FATTY
ACIDS
A component of the phospholipids in cell membranes
 Precursor for prostaglandins: arac...
ESSENTIAL FATTY ACIDS


Deficiency of essential fatty acid intakes:
 Growth

retardation
 Problems with reproduction
 ...
TRIGLYCERIDES – SIMPLE LIPIDS
Most common structure in dietary lipids
 Composed of one glycerol molecule and three fatty
...
MOST COMMON FATTY ACIDS
IN DI- AND TRIGLYCERIDES
Fatty acid

Carbon:Double bonds

Double bonds

Myristic

14:0

Palmitic

...
COMPLEX LIPIDS - PHOSPHOLIPIDS


Two primary types:
 Glycerophosphatides

Core structure is glycerol
 Part of cell memb...
PHOSPHOLIPIDS


Phospholipid sources:
 Liver,

egg yolk,
 Soybeans, wheat germ
 Peanuts
GLYCEROPHOSPHOLIPIDS
Glycerophospholipids
(phosphoglycerides), are common
constituents of cellular
membranes.
They have a ...
PHOSPHATIDATE
O
O
R1

C

H2C
O

O

CH
H2C

C

R2

O
O

phosphatidate

P

O−

O−

In phosphatidate:
 fatty acids are ester...
O
O
R1

C

H2C
O

O

CH
H2C

C

R2

O
O

glycerophospholipid

P

O

X

O−

In most glycerophospholipids (phosphoglycerides...
O
O
R1

C

H2C
O

O

CH
H2C

C

R2

O
O

P

O

O−
OH

phosphatidylinositol

H
OH
H
OH

OH
H
H

H
H

OH

Phosphatidylinosit...
O
O
R1

C

H2C
O

O

CH
H2C

C

R2

O
O

P

CH3
O

CH2

O−

CH2

+
N CH3
CH3

phosphatidylcholine
Phosphatidylcholine,(Lec...
Each
glycerophospholipid
includes




a polar region:
glycerol, carbonyl O
of fatty acids, Pi, & the
polar head group (X...
Sphingolipids are derivatives of the

lipid sphingosine, which has a long hydrocarbon
tail, and a polar domain that includ...
OH
H
C

CH

H3N+

H2C
The amino group of sphingosine can form an amide
bond with a fatty acid carboxyl, to yield a
ceramid...
−
Sphingomyelin has
a
phosphocholine or
phosphethanolamine head
group.
Sphingomyelins are common
constituent of plasma
mem...
GLYCOLIPIDS

CH2OH
OH

A cerebroside is a
sphingolipid (ceramide) with a
monosaccharide such as
glucose or galactose as po...
LIPOPROTEINS
Cholesterol, an
important constituent of cell
membranes, has a rigid ring
system and a short branched
hydrocarbon tail.

H...
HO

Cholesterol

Cholesterol
in membrane

Cholesterol inserts into bilayer membranes
with its hydroxyl group oriented towa...
Amphipathic lipids in association with
water form complexes in which polar
regions are in contact with water and
hydrophob...
Membrane fluidity:
The interior of a lipid bilayer is normally
highly fluid.

liquid crystal

crystal

In the liquid cryst...
Interaction with the relatively rigid cholesterol decreases
the mobility of hydrocarbon tails of phospholipids.

Cholester...
Two strategies by which phase changes of
membrane lipids are avoided:


Cholesterol is abundant in membranes, such as pla...
ERGOSTEROL
Occurs in plant
 Found as structural constituents of membrane in
yeast and fungi.
 Important precursor of vit...
7.8. lipids
7.8. lipids
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7.8. lipids

  1. 1. LIPIDS
  2. 2. LIPIDS - INTRODUCTION • • Lipids are non-polar (hydrophobic) compounds, that is insoluble in water, but soluble in an organic solvent (e.g., ether, benzene, acetone, chloroform) “lipid” is synonymous with “fat”, but also includes phospholipids, sterols, etc. • Most membrane lipids are amphipathic, having a non-polar end and a polar end. • Chemical structure: glycerol + fatty acids
  3. 3. LIPIDS  Location in the body  Membranes  Blood and liver (bound to transport proteins)  Adipocytes (adipose tissue)
  4. 4. FUNCTIONS AND PROPERTIES       Concentrated source of energy (9 kcal/gm) Energy reserve: stored as triglycerides in adipose tissues Provide insulation to the body from cold  Maintain body temperature Mechanical insulation  Protects vital organs Electrical insulation  Protects nerves, help conduct electro-chemical impulses (myelin sheath) Formation of cell membranes  Phospholipids, a type of fat necessary for the synthesis of every cell membrane (also glycoproteins and glycolipids)
  5. 5. FUNCTIONS AND PROPERTIES     Synthesis of prostaglandins from fatty acids  Hormone-like compounds that modulates many body processes  Immune system, nervous systems, and GI secretions  Regulatory functions: lower BP, blood clotting, uterine contractions Help transport fat soluble vitamins Palatability and aroma  Flavor and taste The satiety value – help control appetite  Fullness; fats are digested slower
  6. 6. LIPID CLASSIFICATION  Simple: FA’s esterified with glycerol Fats, Waxes  Complex: same as simple, but with other compounds also attached  Phospholipids (+ P)  Glycolipids (+ CHO)  Lipoproteins (+ fat)  Derived: substances from the above derived by hydrolysis  Steroids  Prostaglandins
  7. 7. FATTY ACIDS  Contain an even number of carbon atoms  Arranged in an unbranched line  Have a carboxyl group (-COOH) at one end  Have a methyl group (CH3) at the other end • A 16-C fatty acid: CH3(CH2)14 -COONon-polar polar
  8. 8. FATTY ACID CHAIN LENGTH  Short chain: 2 to 6 C (volatile fatty acids)  Medium chain: 8 – 12 C  Long chain: 14 – 24 C  As chain length increases, melting point increases  Fatty acids synthesized by plants and animals have an even number of carbons  Mostly long chain  16C to 18C fatty acids are most prevalent
  9. 9. FATTY ACID SATURATION Saturated - no double bonds  Unsaturated – contain double bonds   Monounsaturated – one double bond  Polyunsaturated - >1 double bond  The double bond is a point of unsaturation  As number of double bonds increases, melting point decreases
  10. 10. SATURATED FATS  All the chemical bonds between the carbon are single bonds C-C-C No double bonds  No space for more H atoms; fully “saturated”  Solid at room temperature  Butter, shortening, lard, coconut oil, palm oil, and fully hydrogenated vegetable oils  Poultry skin, whole milk
  11. 11. MONO-UNSATURATED FATTY ACIDS Only one double bond  Therefore, two H atoms can be added  Liquid at room temperature   Olive oil, canola oil, peanut oil  Other sources: avocado, almonds, cashews, pecans and sesame seeds (tahini paste)
  12. 12. POLY-UNSATURATED FATTY ACIDS  Two or more double bonds  Include omega-3 and omega-6 fatty acids (essential fatty acids)  Linolenic acid: omega 3 fatty acid  Linoleic acid: omega 6 fatty acid  Richest sources of poly-unsaturated fatty acids include:  Vegetable  oils Corn, sunflower, safflower, cotton seed oils
  13. 13. FATTY ACIDS COMMONLY FOUND IN LIPIDS Sat. Fatty Acids Butyric Palmitic Stearic Unsat. Fatty Acids Oleic Linoleic Linolenic Formula C4H8O2 C16H22O2 C18H36O2 Formula C18H34O2 C18H32O2 C18H30O2 Melting Point ( oC) Liquid 63 70 Melting Point ( oC) Liquid Liquid Liquid
  14. 14. Some fatty acids and their common names: 14:0 myristic acid; 16:0 palmitic acid; 18:0 stearic acid; 18:1 cis∆9  oleic acid 18:2 cis∆9,12  linoleic acid 18:3 cis∆9,12,15  linolenic acid 20:4 cis∆5,8,11,14  arachidonic acid 20:5 cis∆5,8,11,14,17  eicosapentaenoic acid (an omega-3)
  15. 15. FATTY-ACID NOMENCLATURE  Named according to chain length  C18 H3C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C O C OH
  16. 16. FATTY-ACID NOMENCLATURE  Named according to the number of double bonds  C18:0 H3C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C Common name: Common name: Stearic acid Stearic acid C H2 H2 C C H2 H2 C O C OH
  17. 17. FATTY-ACID NOMENCLATURE  Named according to the number of double bonds  C18:1 H3C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H C C H H2 C C H2 Common name: Common name: Oleic acid Oleic acid H2 C C H2 H2 C C H2 H2 C O C OH
  18. 18. FATTY-ACID NOMENCLATURE  Named according to the number of double bonds  C18:2 H3C C H2 H2 C C H2 H2 C C H H C C H2 H C C H H2 C C H2 H2 C Common name: Common name: Linoleic acid Linoleic acid C H2 H2 C C H2 H2 C O C OH
  19. 19. FATTY-ACID NOMENCLATURE  Named according to the number of double bonds  C18:3 H3C C H2 H C C H H2 C C H H C C H2 H C C H H2 C C H2 H2 C Common name: Common name: Linolenic acid Linolenic acid C H2 H2 C C H2 H2 C O C OH
  20. 20. FATTY-ACID NOMENCLATURE  Named according to the location of the last double bond system (e.g., omega 3, ω3)  n–system (e.g., n–3)  Omega
  21. 21. FATTY-ACID NOMENCLATURE  Named according to the location of the last double bond system (e.g., omega 3, ω3)  n–system (e.g., n–3)  Omega  Count from the methyl end H3C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C O C OH
  22. 22. FATTY-ACID NOMENCLATURE H3C C H2 H3C H3C C H2 H2 C C H2 H2 C C H2 H C H2 C C H2 C H C H2 H2 C C H2 H C H2 C C H C H2 H2 C C H2 H2 C Omega 9 or n–9 fatty acid H2 C C H H C C H2 H C C H H2 C C H2 H2 C C H2 H2 C Omega 6 or n–6 fatty acid H2 C C H H C C H2 H C C H H2 C C H2 H2 C C H2 Omega 3 or n–3 fatty acid C H2 C H2 H2 C H2 C O C OH H2 C C H2 O C OH H2 C O C OH
  23. 23. FATTY ACID NUMBERING SYSTEMS Delta system: carbon atoms in a fatty acid are numbered from the carboxyl end  Omega system: carbon atoms are numbered from the methyl end 
  24. 24. OMEGA SYSTEM Ω-3 Ω-6 Ω-9 C-C-C=C-C-C=C-C-C=C-C-C-C-C-C-C-C-COOH  Animals can synthesize a fatty acid with a double bond in the omega 9 position but not at either 3 or 6 positions  Omega-3 and omega-6 fatty acids must be derived from diet  Cold water fish accumulate high levels of omega 3 fatty acids from their diet
  25. 25. OMEGA SYSTEM AND ESSENTIAL FATTY ACIDS Linoleic acid is an omega-6 fatty acid  Linolenic and arachidonic acids are omega-3 fatty acids   Linoleic and linoleic acids are essential fatty acids  Arachidonic acid can be synthesized from them, so not essential
  26. 26. FATTY-ACID NOMENCLATURE  Named  Cis according to location of H’s or trans fatty acids Cis-9-octadecenoic acid (Oleic acid) Trans-9-octadecenoic acid (Elaidic acid)
  27. 27. FATTY-ACID NOMENCLATURE H3C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H H2 C H2 C H2 C H C 3 C 2 H C 2 H C 2 H H C H C C C 2 H C H C H2 C H2 H2 C H2 C C H2 C H2 H2 C H2 C C H2 C H2 H2 C H2 C O C H2 C O C H2 C OH OH
  28. 28. DESIGNATING POSITION OF BONDS CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH Numeric (9,12 – 18:2)  ∆ (18:2 ∆ 9,12)  n (18:2 n-6)  ω (18:2 ω-6)  γ 4 β 3 α 2 O C 1 O− fatty acid with a cis-∆9 double bond
  29. 29. MELTING POINTS  Affected  Longer Fatty acid: C12:0 Melting point: 44°C by chain length chain = higher melting temp C14:0 58°C C16:0 63°C C18:0 72°C C20:0 77°C
  30. 30. CHAIN LENGTH  In most fats with a mixture of fatty acids, the chain length of the majority of fatty acids will determine the “hardness” of the fat  <10 carbons = liquid  Between 10 and 20 carbons = ???  >20 carbons = solid Acetic Acid (2 C) Vinegar liquid Stearic Acid (18 C) Beef Tallow Solid Arachidic Acid (20 C) Butter Solid
  31. 31. MELTING POINTS  Affected  More Fatty acid: Melting point: by number of double bonds saturated = higher melting temp C18:0 72°C C18:1 16°C C18:2 –5°C C18:3 –11°C
  32. 32. ESSENTIAL FATTY ACIDS  Must be in diet  Tissues can not synthesize  Linoleic acid (18:2)  Omega-6-FA  Linolenic  acid (18:3) Omega-3-FA  Arachidonic (20:4) Not in plants!  Can be synthesized from C18:2 (except in cat) 
  33. 33. STURCTURE OF ESSENTIAL FA’S
  34. 34. FUNCTIONS OF ESSENTIAL FATTY ACIDS A component of the phospholipids in cell membranes  Precursor for prostaglandins: arachidonic acid  Important metabolic regulator   Contraction of smooth muscle  Aggregation of platelets
  35. 35. ESSENTIAL FATTY ACIDS  Deficiency of essential fatty acid intakes:  Growth retardation  Problems with reproduction  Skin lesions  Kidney and liver disorders
  36. 36. TRIGLYCERIDES – SIMPLE LIPIDS Most common structure in dietary lipids  Composed of one glycerol molecule and three fatty acids connected by an ester bond (bond between an alcohol and and organic acid)   Fatty acids may be same or mixed Fatty Acid Fatty Acid Glycerol Fatty Acid
  37. 37. MOST COMMON FATTY ACIDS IN DI- AND TRIGLYCERIDES Fatty acid Carbon:Double bonds Double bonds Myristic 14:0 Palmitic 16:0 Palmitoleic 16:1 Stearic 18:0 Oleic 18:1 Cis-9 Linoleic 18:2 Cis-9,12 Linolenic 18:3 Cis-9,12,15 Arachidonic 20:4 Cis-5,8,11,14 Eicosapentaenoic 20:5 Cis-5,8,11,14,17 Docosahexaenoic 22:6 Cis-4,7,10,13,16,19 CH3(CH2)nCOOH Cis-9
  38. 38. COMPLEX LIPIDS - PHOSPHOLIPIDS  Two primary types:  Glycerophosphatides Core structure is glycerol  Part of cell membranes, chylomicrons, lipoproteins   Sphingophosphatides Core structure is sphingosine  Part of sphingomyelin 
  39. 39. PHOSPHOLIPIDS  Phospholipid sources:  Liver, egg yolk,  Soybeans, wheat germ  Peanuts
  40. 40. GLYCEROPHOSPHOLIPIDS Glycerophospholipids (phosphoglycerides), are common constituents of cellular membranes. They have a glycerol backbone. Hydroxyls at C1 & C2 are esterified to fatty acids. An ester forms when a hydroxyl reacts with a carboxylic acid, with loss of H2O. CH2OH H C OH CH2OH glycerol Formation of an ester: O R'OH + HO-C-R" O R'-O-C-R'' + H2O
  41. 41. PHOSPHATIDATE O O R1 C H2C O O CH H2C C R2 O O phosphatidate P O− O− In phosphatidate:  fatty acids are esterified to hydroxyls on C1 & C2  the C3 hydroxyl is esterified to P . i
  42. 42. O O R1 C H2C O O CH H2C C R2 O O glycerophospholipid P O X O− In most glycerophospholipids (phosphoglycerides), Pi is in turn esterified to OH of a polar head group (X): e.g., serine, choline, ethanolamine, glycerol, or inositol. The 2 fatty acids tend to be non-identical. They may differ in length and/or the presence/absence of double bonds.
  43. 43. O O R1 C H2C O O CH H2C C R2 O O P O O− OH phosphatidylinositol H OH H OH OH H H H H OH Phosphatidylinositol, with inositol as polar head group, is one glycerophospholipid. In addition to being a membrane lipid, phosphatidylinositol has roles in cell signaling.
  44. 44. O O R1 C H2C O O CH H2C C R2 O O P CH3 O CH2 O− CH2 + N CH3 CH3 phosphatidylcholine Phosphatidylcholine,(Lecithin) with choline as polar head group, is another glycerophospholipid. It is a common membrane lipid.
  45. 45. Each glycerophospholipid includes   a polar region: glycerol, carbonyl O of fatty acids, Pi, & the polar head group (X) non-polar hydrocarbon tails of fatty acids (R1, R2). O O R1 C H2C O O CH H2C C R2 O O glycerophospholipid P O O− X
  46. 46. Sphingolipids are derivatives of the lipid sphingosine, which has a long hydrocarbon tail, and a polar domain that includes an amino group. OH OH CH H3N+ H2C H C CH HC O − O P O (CH2 )12 − O H2C OH H C CH H3N+ CH HC (CH2 )12 sphingosine-1-P CH3 sphingosine CH3 Sphingosine may be reversibly phosphorylated to produce the signal molecule sphingosine-1phosphate. Other derivatives of sphingosine are commonly found as constituents of biological membranes.
  47. 47. OH H C CH H3N+ H2C The amino group of sphingosine can form an amide bond with a fatty acid carboxyl, to yield a ceramide. OH CH HC (CH2 )12 OH OH O CH C R ceramide CH3 CH NH H2C H C sphingosine HC (CH2 )12 CH3 In the more complex sphingolipids, a polar “head group" is esterified to the terminal hydroxyl of the sphingosine moiety of the ceramide. 
  48. 48. − Sphingomyelin has a phosphocholine or phosphethanolamine head group. Sphingomyelins are common constituent of plasma membranes phosphocholine sphingosine fatty acid Sphingomyelin Sphingomyelin, with a phosphocholine head group, is similar in size and shape to the glycerophospholipid phosphatidyl choline.
  49. 49. GLYCOLIPIDS CH2OH OH A cerebroside is a sphingolipid (ceramide) with a monosaccharide such as glucose or galactose as polar head group. A ganglioside is a ceramide with a polar O H OH H H H OH O H OH O cerebroside with β-galactose head group CH NH H2C H C CH C R HC (CH2 )12 head group that is a complex oligosaccharide, including the acidic sugar derivative sialic acid. Cerebrosides and gangliosides, collectively called glycosphingolipids, are commonly found in the outer leaflet of the plasma membrane bilayer, with their sugar chains extending out from the cell surface. CH3
  50. 50. LIPOPROTEINS
  51. 51. Cholesterol, an important constituent of cell membranes, has a rigid ring system and a short branched hydrocarbon tail. HO Cholesterol Cholesterol is largely hydrophobic. But it has one polar group, a hydroxyl, making it amphipathic. PDB 1N83 cholesterol
  52. 52. HO Cholesterol Cholesterol in membrane Cholesterol inserts into bilayer membranes with its hydroxyl group oriented toward the aqueous phase & its hydrophobic ring system adjacent to fatty acid chains of phospholipids. The OH group of cholesterol forms hydrogen bonds with polar phospholipid head groups.
  53. 53. Amphipathic lipids in association with water form complexes in which polar regions are in contact with water and hydrophobic regions away from water. Depending on the lipid, possible molecular arrangements:  Various micelle structures. E.g., a spherical micelle is a stable configuration for amphipathic lipids with a conical shape, such as fatty acids.  A bilayer. This is the most stable configuration for amphipathic lipids with a cylindrical shape, such as phospholipids.
  54. 54. Membrane fluidity: The interior of a lipid bilayer is normally highly fluid. liquid crystal crystal In the liquid crystal state, hydrocarbon chains of phospholipids are disordered and in constant motion. At lower temperature, a membrane containing a single phospholipid type undergoes transition to a crystalline state in which fatty acid tails are fully extended, packing is highly ordered, & van der Waals interactions between adjacent chains are maximal. Kinks in fatty acid chains, due to cis double bonds, interfere with packing in the crystalline state, and lower the phase transition temperature.
  55. 55. Interaction with the relatively rigid cholesterol decreases the mobility of hydrocarbon tails of phospholipids. Cholesterol in membrane But the presence of cholesterol in a phospholipid membrane interferes with close packing of fatty acid tails in the crystalline state, and thus inhibits transition to the crystal state. Phospholipid membranes with a high concentration of cholesterol have a fluidity intermediate between the liquid crystal and crystal states.
  56. 56. Two strategies by which phase changes of membrane lipids are avoided:  Cholesterol is abundant in membranes, such as plasma membranes, that include many lipids with long-chain saturated fatty acids. In the absence of cholesterol, such membranes would crystallize at physiological temperatures.  The inner mitochondrial membrane lacks cholesterol, but includes many phospholipids whose fatty acids have one or more double bonds, which lower the melting point to below physiological temperature.
  57. 57. ERGOSTEROL Occurs in plant  Found as structural constituents of membrane in yeast and fungi.  Important precursor of vitamin D 
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