LIPID CHEMISTRY
LIPID OF BIOLOGICAL IMPORTANCE
Definition:
Lipids are organic compounds, which have
the following common properties:
1- They are esters of fatty acids or
substances associated with them in nature.
2- Most of them are insoluble in water but
soluble in fat solvents (non polar solvents)
e.g.: benzene, chloroform, acetone and ether.
Classification:
Lipids are classified into three main groups:
Simple Lipids
Esters of fatty
acids with various
types of alcohols
Compound lipids
Esters of fatty
acids with various
types of alcohols
in addition to a
prosthetic group
Derived Lipids
They are
produced by
hydrolysis of
the first two
groups
Simple lipids:
NUTERAL
FAT
Esters of three
fatty acids with
glycerol.= TAG
WAXES
Esters of fatty acids
with higher
molecular weight
monohydric
alcohols.
Complex lipids (Compound or
Conjugated lipids)
Phospholipids: Lipids
containing, in addition
to fatty acids and an
alcohol, a phosphoric
acid residue.
Glycolipids
(glycosphingoLipids:)
containing a fatty acid,
sphingosine,
and
carbohydrate.
Proteolipids: fatty acid,
alcohol and protein
Fatty Acids
• Fatty acids that occur in natural fats
are usually monocarboxylic acids
containing an even number of
carbon atoms. The chain may be
saturated (containing no double
bonds) or unsaturated (containing
one or more double bonds).
I- Saturated Fatty Acids (SFA):
• They contain no double bonds.
• They are either short chain (from C2 to C10)
or long chain (from C12 to C24)
• All have the following general formula:
•
CH₃- (CH₂)n- COOH
• Where n = Total number of carbons – 2
I- Saturated Fatty Acids (SFA):
•
•
•
•
A- Short chain fatty acids include:
Acetic acid (C2)
Butyric acid (C4)
B- Long chain fatty acid, the most common
include mainly:
• Palmitic acid (C16)
• Stearic acid (C18)
• Arachidic acid (C20)
II- Unsaturated Fatty Acids (USFA):
• They contain one or more double bonds.
• Unsaturated fatty acids are classified
according to the number of double bonds in
their chains into two main groups:
• 1- Monoethenoid: one double bond:
• Palmitoleic acid
• Oleic acid
II- Unsaturated Fatty Acids (USFA):
• Polyethenoid
• They have more than one double bond in their
structure, termed polyunsaturated fatty acids
(PUFA).
• Linoleic acid
• Linolenic acid
• Arachidonic acid
Nutritional classification of fatty acids:
• a)- Essential Fatty Acids:
• They are not synthesized in our body, so it is essential to
take them in diet. They include Linolenic and Linoleic
acids.
• Deficiency of essential fatty acids produces:
• Fatty liver and sterility in adults.
• Impaired growth and dermatitis in infants.
• Sources of PUFA: They are present mainly in fish and
vegetable oils e.g.: maize, cottonseed, linseed, olive, sun
flower and soya been oils.
• b)- Non Essential Fatty Acids:
• They include all other fatty acids because they are formed
in our body in good amounts mainly from carbohydrates. It
is not essential to take them in diets.
•
Physical Properties of Fatty Acids
1. Solubility in water:
fatty acids are insoluble in water but soluble in
fat solvents.
2. Physical state at room temperature:
The saturated fatty acids are solid at room
temperature
Unsaturated long chain fatty acids are liquids
due to the presence of cis double bonds.
Chemical Reactions of Fatty Acids
•
•
•
•
•
1. Ester Formation:
2. Salt formation:
3. Reduction:
4. Rancidity:
It is the development of bad odor and taste
(bad flavor) of fats and oils.
• Rancidity is due to exposure of fats or oils to
high temperature, oxygen, moisture or
humidity, light and metals (act as catalysts).
I- Simple Lipids
• They are esters of fatty acids with alcohols,
according to the types of alcohols there are
two main sub-groups:
• 1. Neutral fats or Triacylglycerol (TAG): They
are esters of three fatty acids with glycerol.
• 2. Waxes: They are esters of one fatty acid
with long chain monohydroxyalcohol higher
than glycerol.
•
I- Simple Lipids
Sphingosine containing lipids
Phospho
choline
FA
• Sphingosine
Ceramide
Sphingomyline
Importance of glycolipids:
• They are found mainly in the myelin
sheath and cell membrane of RBCs.
They act as cell membrane receptors
for hormones and external stimuli
also they provide recognition
properties.
III- Derived Lipids
• They are produced by hydrolysis of either
simple or conjugated lipids.
• They include the following :
• 1. Fatty acids.
• 2. Alcohols.
• 3. Steroids.
• 4. Carotenoids.
• 5. Fat soluble vitamins: as vitamins A, D, E &
K.
•
Steroids
Classification of steroids
•Steroid hormones.
•Sterols.
•Bile acids.
•
Cholesterol
• It is the most important animal sterol. It is
present either free (non esterified) or
esterified with fatty acid to form cholesteryl
ester. Free cholesterol contains 27 carbon
atoms.
• Distribution of cholesterol: It is widely
distributed in all tissues but higher
concentrations are present in brain, nervous
tissue, liver, adrenals, gonads, skin and
adipose tissue.
Cholesterol
• Precursor of cholesterol: It is formed
from active acetate (acetyl-CoA).
• Blood level of cholesterol: Normally it is
present in the plasma in concentrations
ranging from 100 to 200 mg/dL (30% as
free cholesterol and 70% as cholesteryl
esters).
•.
Importance and derivatives of
cholesterol:
• It is important constituent of cell membranes.
• It is converted into bile acids and bile salts in
the liver.
• It is the precursor of all steroid hormones.
• It can be oxidized in the liver into 7-dehydro
cholesterol which can be converted under the
skin into vitamin D3 by ultra violet
Bile Acids
• 1. Primary bile acids: they are formed in the liver
from cholesterol and they include:
• - Cholic acid
• - Chenodeoxy cholic acid
• 2- Secondary bile acids: they are formed of
primary bile acid in large intestine.
• - Deoxycholic acid
• - Lithocholic acid
Bile salts:
• They are formed by conjugation of cholic
acid with glycine (80%) or taurine (20%)
then they are excreted by liver in bile as
sodium
glycocholate
or
sodium
taurocholate. Bile salts pass to the
intestine where they are reabsorbed and
return back to the liver to be excreted
again in bile (entero-hepatic circulation).
•
Importance of bile salts:
• Conversion of cholesterol to bile salt is
an important mechanism for removal of
excess cholesterol from blood.
• They are good emulsifying factors
important for digestion and absorption of
fats.
• They
prevent
precipitation
of
cholesterol in the bile as cholesterol
stones.
• They stimulate liver cells to secrete
more bile (choleretic effect).
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