Introduction to lipids
Contents
– Introduction to lipids
– Classification of lipids
Objective
• At the end of this lecture, student will be able to
– Explain lipids
– Classify lipids
– Describe the properties of lipids
– Discuss the tests to check purity of fats and oils
Lipids
Definition:
Lipids can be regarded as organic substances relatively insoluble in water,
soluble in organic solvent, actually or potentially related to fatty acids and
utilized by living cells
Functions
of lipids
• They are the fuel reserve of the body (triacylglycerols)
• Lipids are the constituents of membrane structure and
regulate the membrane permeability (phospholipids and cholesterol)
• They serve as a source of fat soluble vitamins (A, D, E
and K)
• Lipids are important as cellular metabolic regulators
(steroid hormones and prostaglandins
• Lipids protect the internal organs, serve as insulating
materials and give shape and smooth appearance to the body
Classification
Lipids are broadly classified in to simple, complex, erived
and miscellaneous lipids
1. Simple lipids:
Esters of fatty acid with alcohol. 2 types,
I. Fats and oils:
Esters of fatty acid with glycerols. The difference is fats are solid and oils
is liquid by physical state
II. Waxes: Esters
of fatty acid with alcohol other than glycerols.
2. Complex lipids:
these are esters of fatty acid with alcohols containing additional groups such
as phosphate, nitrogen base, proteins, carbohydrates etc. further divided into
I. Phospholipids:
In addition to fatty acid and alcohol, they contains phosphoric acid and
nitrogen base – 2 types
a)
Glycerophospholipids: They contains glycerol as alcohol e.g. Lecithin,
Cephalin
b)
Sphingophospholipids: They contains spingosine as alcohol e.g.
Sphingomyelin
II. Glycolipids:
These lipids contain a fatty acid, carbohydrate & nitrogen base. The
alcohol is spingosine, hence, they also called as glycospingolipids e.g.
Cerebrosides, gangliosides
III. Lipoproteins:
complex of lipids with proteins
IV. Other complex
lipids: Sulfolipids, aminolipids etc
3. Derived lipids:
these are the depravities obtained on the hydrolysis of group I and group II
lipids which possess characteristic of lipids, They includes mono and diacyl
glycerols
4. Miscellaneous:
Large number of compounds possess the characteristics of lipids e.g.
Carotenoids, Terpenes etc
5. Neutral lipids:
Lipids which are uncharged are referred as neutral lipids. e.g. Mono, di and
tri-glycerols, cholesterol and cholesterol ester
Fatty acid – carboxylic acid with hydrocarbon side chain and
simplest form of lipids
Classification
based on saturation
• Saturated fatty acids do not contain double bonds, while
unsaturated fatty acids contain one or more double bonds
• Both saturated and unsaturated fatty acids almost equally
occur in the natural lipids
• Fatty acids with one double bond are monounsaturated and
those with 2 or more double bonds are collectively known as polyunsaturated
fatty acids (PUFA)
• Saturated fatty acids end with a suffix -anoic (e.g.,
octanoic acid) while unsaturated fatty acids end with a suffix- enoic (e.g., octadecanoic acid)
Classification
based on essential and non-essential fatty acid
• The essential fatty acid are Linoleic acid and Linolenic
acid and other are non-essential fatty acid
Properties
of Triacylglycerols
• Triacylglycerols (formerly triglycerides) are the esters
of glycerol with fatty acids
1. Hydrolysis: It undergo stepwise enzymatic hydrolysis to
finally liberate free fatty acids and glycerol, catalysed by lipases
2. Saponification:
The hydrolysis of triacylglycerols by alkali to produce glycerol and soaps is
known as saoonification
3. Rancidity:
Rancidity is the term used to represent the deterioration of fats and oils
resulting in unpleasant taste. Fats
containing unsaturated fatty acids are more susceptible to rancidity. Rancidity
occurs when fats and oils are exposed to air, moisture, light, bacteria etc
4. Antioxidants:
The substances which can prevent the occurrence of oxidative rancidity are
known as antioxidants. Trace amounts of antioxidants such as tocopherols
(vitamin E), etc are added to commercial preparations of fats and oils to
prevent rancidity
5. Lipid peroxidation
in vivo: In the living cells, lipids undergo oxidation to produce peroxides
and free radicals which can damage the tissue
Tests to
check purity of fats and oils
• Adulteration of fats and oils is increasing day by day.
Several tests are employed in the laboratory to check the purity of fats and
oils
lodine number:
• Defined as the grams of iodine absorbed by 100g of fat or
oil.
• Lower is the iodine number, less is the degree of
unsaturation
• Determination of iodine number will help to know the
degree of adulteration of a given oil
Saponification
number:
• Defined as the mg of KOH required to hydrolyse (saponify)
one gram of fat or oiL
• Saponification number is a measure of the average
molecular size of the fatty acids present
• The value is higher for fats containing short chain fatty
acids
• Human fat: 195-200
• Butter: 230-240
• Coconut oil: 250-260
Acid number:
• Defined as the number of mg of KOH required to completely
neutralize free fatty acids present in one gram fat or oil.
• In normal circumstances, refined oils should be free from
any free fatty acids.
• Oils, on decomoposition-due to chemical or bacterial
contamination-yield free fatty acids. Therefore, oils with increased acid
number are unsafe for human consumption
Summary
• Lipids can be regarded as organic substances relatively
insoluble in water, soluble in organic solvent, actually or potentially related
to fatty acids and utilized by living cells
• They are the fuel reserve of the body
• Lipids are broadly classified into simple, complex,
derived and miscellaneous lipids
• The essential fatty acid are Linoleic acid and Linolenic
acid and other are non-essential fatty acid
• Iodine number, saponification number and acid number are
used to check the purity of oils and fats