Lipids

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Lipids

• Function of Lipids

• Energy storage molecules in animals release 9
  kcal/g versus 4 kcal/g by carbohydrates
• Form membranes (cellular or sub-cellular) that
  separate compartments in cells lipids are
  insoluble in water so help generate membranes
• Serve as chemical messengers steroid hormones
  are primary messengers, while prostaglandins and
  thromboxanes are secondary messengers mediating
  hormonal response

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Lipids

• Classification of Lipids by Structure

• Lipids can be classified into four groups based
  on their structure

1) Simple lipids, e.g., fats 2) Complex
lipids 3) Steroids 4) Prostaglandins,
thromboxanes, and leukotrienes
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Lipids

• Triglycerides

• Animal fats and vegetable oils are triglycerides
• Triesters of glycerol and long chain carboxylic
  acids called fatty acids
• Glycerol component cannot vary, but the fatty
  acid component may vary
• Fatty acids are unbranched carboxylic acids
  having 10 to 20 carbons in even number
• Some fatty acids have double bond(s). Typically
  these double bond(s) are cis

n
A triglyceride

p
Glycerol
m
palmitate (160)
oleate (181)
stearate (180)
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Properties of Triglycerides

• Generally fats from animals are solids at room
  temperature. Most fats from plants and fish are
  liquids. Liquid fats are referred to as oils.
• Presence or absence of unsaturation makes a
  triglyceride solid or liquid

Saturated and trans fatty acids stack well
Unsaturated cis fatty acids do not
. (greater hydrophobic interactions
(lesser hydrophobic
interactions greater order in arrangement
lesser order
in arrangement carries over to triglyceride
structure) carries
over to triglyceride structure)
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Properties of Triglycerides
Saturated Fatty Acids
Tm (OC) 120 CH3(CH2)10COOH lauric
acid 44 140 CH3(CH2)12COOH myristic
acid 58 160 CH3(CH2)14COOH palmitic
acid 63 180 CH3(CH2)16COOH stearic
acid 70 200 CH3(CH2)18COOH arachidic
acid 77 Unsaturated cis Fatty Acids 161 CH3(CH2)
5 CHCH(CH2)7COOH palmitoleic acid
1 181 CH3(CH2)7 CHCH(CH2)7COOH oleic
acid 16 182 CH3(CH2)4 (CHCHCH2)2(CH2)6COOH
linoleic acid -5 183 CH3CH2(CHCHCH2)3(CH2)6COOH
linolenic acid -11 204 CH3(CH2)4
(CHCHCH2)4(CH2)2COOH arachidonic acid -49
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Properties of Triglycerides

• Pure fats and oils are colorless, odorless and
  tasteless.
• Fats are not soluble in water. They are
  hydrophobic molecules and soluble in organic
  solvents
• The presence of double bonds, especially in plant
  fats, makes them susceptible to oxidation.
  Heavily oxidized fats are not edible. These
  generate rancidity.
• The ester group in fats is susceptible to
  hydrolysis releasing acidic groups, e.g., butyric
  acid, which have bad taste and odor.

_
_
_
NaOH




Mixture of fatty acid sodium salt (Soap)
Saponification
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Complex Lipids

• Complex lipids constitute the main component of
  membranes.
• Two main types of complex lipids phospholipids
  and glycolipids. Phospholipids contain a
  phosphate group, while glycolipids contain a
  carbohydrate group
• Phospholipids can be made from glycerol or from
  sphingosine

choline
_
Phosphatidylcholine (lecithin) (A
glycerophospholipid)
A sphingomyelin (A sphingolipid)
sphingosine


_
choline
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Membranes

• Complex lipids containing unsaturated fatty acids
  constitute cell membranes.
• Membranes are made up of lipid bi-layers.

The Fluid Mosaic Model of Cell Membranes
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Steroids

• Steroids constitute the third major class of
  lipids
• Steroids are cyclopentanoperhydrophenanthrenes.
• Cholesterol is the commonest steroid. It is
  present cell membranes. It is also present in
  lipoprotein particles, e.g., LDL, HDL, and others

Cyclopentanoperhydrophenanthrene (Steroid
skeleton)
Cholesterol
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Lipoprotein Particles

• Lipoprotein particles transport cholesterol in
  blood
• Contain a core of hydrophobic lipid molecules
  surrounded by a shell of hydrophilic molecules
  such as proteins and phospholipids
• At least four kinds of particles HDL, LDL, VLDL
  and Chylomicrons

Apolipoprotein
Cholesterol
Phospholipids
Cholesterol ester
A Model of a Lipoprotein Particle
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Prostaglandins, Thromboxanes and Leukotrienes

• Prostaglandins are derived from arachidonic acid
• They have a wide variety of effects of the body
  they can lower or raise blood pressure, cause
  inflammation, induce blood clotting and induce
  labor
• Aspirin and other NSAIDs inhibit the enzyme
  (cyclooxygenase COX) involved in the synthesis of
  prostaglandins

Arachidonic Acid
PGE2
PGF2a
Thromboxane A2
Leukotriene B4