ISC 11>Content>Unit-4>Biomolecules :LIPIDS
Lipids – classification, structure and functions of fats and oils.
CLASS PRESENTATION
COMPOSITION
- Lipids form a group of organic compounds which are widely distributed in living organisms.
- Made up of carbon, hydrogen and oxygen but the proportion of oxygen is much less than 2:1.
- Most lipids are esters formed by condensation of alcohols and fatty acids.
PROPERTIES
- Insoluble in water.
- Soluble in organic solvents like chloroform, alcohol, acetone etc.
CLASSIFICATION (Bloor)
A. SIMPLE LIPIDS : 1. NEUTRAL FATS OR TRUE FATS
These are esters of fatty acids with various alcohols.
Composition
Fat molecule is composed of 1 molecule of glycerol and 1-3 molecules of fatty acids.
Fat molecule is composed of 1 molecule of glycerol and 1-3 molecules of fatty acids.
- Glycerol
2. Fatty acids
- Fatty acids are carboxylic acid with hydrocarbon side chains. They are the simplest form of lipids and they are water soluble. They exist in the body either as free acids or fatty acyl esters such as triacylglycerol. The fatty acids are released from these lipids on hydrolysis by lipases.
- Fatty acids may be divided into (1) saturated fatty acids and (2) unsaturated fatty acids.
A. Saturated fatty acids :
B. Unsaturated fatty acids :
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Essential fatty acid (EFA)
The fatty acids that cannot be synthesised by the body and therefore should be supplied in the diet are known as essential fatty acids.
Chemically they are polyunsaturated fatty acids (PUFA), namely linoleic acid, linolenic acid and arachidonic acid.
The fatty acids that cannot be synthesised by the body and therefore should be supplied in the diet are known as essential fatty acids.
Chemically they are polyunsaturated fatty acids (PUFA), namely linoleic acid, linolenic acid and arachidonic acid.
Structure of triglycerides or simple fat
Triglycerides are simple lipids in which glycerol backbone is esterified with three fatty acids.
Triglycerides are simple lipids in which glycerol backbone is esterified with three fatty acids.
Simple (Pure) and mixed fats
- If the three hydroxyl groups are esterified with same type of fatty acid then the lipid is called as simple glyceride. (tristearin)
- If the three hydroxyl groups are esterified with different type of fatty acids, the lipid is called as mixed glyceride. (butter)
A. SIMPLE LIPIDS : 2. WAX
Waxes Serve as Energy Stores and Water Repellents
Biological waxes are esters of long-chain (C14 to C36) saturated and unsaturated fatty acids with long-chain (C16 to C30) alcohols.
Their melting points (60 to 100 C) are generally higher than those of triacylglycerides.
Biological waxes are esters of long-chain (C14 to C36) saturated and unsaturated fatty acids with long-chain (C16 to C30) alcohols.
Their melting points (60 to 100 C) are generally higher than those of triacylglycerides.
- In plankton, the free-floating microorganisms at the bottom of the food chain for marine animals, waxes are the chief storage form of metabolic fuel.
- Waxes also serve a diversity of other functions related to their water-repellent properties and their firm consistency.
- Certain skin glands of vertebrates secrete waxes to protect hair and skin and keep it pliable, lubricated, and waterproof.
- Birds, particularly waterfowl, secrete waxes from their preen glands to keep their feathers water-repellent.
- The shiny leaves of holly, rhododendrons, poison ivy, and many tropical plants are coated with a thick layer of waxes, which prevents excessive evaporation of water and protects against parasites.
- Plant waxes-coats leaves and stem
- Bee wax- abdominal glands of bees
- Lanolin- sheep’s wool
- Cerumen- ceruminous glands in ear
- Suberin- bark of trees
- Sebum- sebaceous gland
- In the pharmaceutical, cosmetic, and other industries.
- Lanolin (from lamb’s wool), beeswax , carnauba wax (from a Brazilian palm tree), and wax extracted from spermaceti oil (from sperm whale) are widely used in the manufacture of lotions, ointments, and polishes.
B. CONJUGATED LIPIDS :
These are esters of fatty acids with alcohol, but it contains extra groups in addition to alcohol and a fatty acid. on the basis of this additional group conjugated lipids can be further classified as follows:
1. PHOSPHOLIPIDS
Phosphoglycerides, are membrane lipids in which two fatty acids are attached in ester linkage to the first and second carbons of glycerol,
and a highly polar or charged group is attached through a phosphodiester linkage to the third carbon.
and a highly polar or charged group is attached through a phosphodiester linkage to the third carbon.
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2. GLYCOLIPIDS
Complex lipids with one or more simple sugars, with long chain fatty acids or alcohols.
Examples:
a) Cerebrosides : White matter of brain cells and myelin sheath of nerve.
b) Gangliosides : Grey matter of the brain.
Examples:
a) Cerebrosides : White matter of brain cells and myelin sheath of nerve.
b) Gangliosides : Grey matter of the brain.
3. LIPOPROTEINS
- Phospholipids containing lipids and proteins in their molecules.
- Many enzymes, transporters, structural proteins,antigens, adhesins, and toxins are lipoproteins.
- Occur in milk and egg yolk.
- Constituents of cell membranes, cutin and suberin are lipoproteins.
- They are classified based on their density:
- Chylomicron (largest; lowest in density due to high lipid/protein ratio; highest in triacylglycerols as % of weight)
- VLDL (very low density lipoprotein; 2nd highest in triacylglycerols as % of weight)
- IDL (intermediate density lipoprotein)
- LDL (low density lipoprotein, highest in cholesteryl esters as % of weight)
- HDL (high density lipoprotein, highest in density due to high protein/lipid ratio).
C. DERIVED LIPIDS :
This class includes a large variety of highly lipid-like compounds or compounds derived from the intermediates of lipid metabolism.
These compounds can be classified into two main groups or categories.
A) Terpenes (Isoprenoids)
B) Icosanoids
These compounds can be classified into two main groups or categories.
A) Terpenes (Isoprenoids)
B) Icosanoids
A) Terpenes or Isoprenoids:
- This is a large class of derived lipids formed from combinations of two or more units of a common precursor molecule, named isoprene.
- Isoprene is a five carbon compound derived from condensation of acetyl co-enzyme A molecule.
- Isoprene units can be linked in terpenes to form a variety of straight chain or cyclic molecules.
- Terpenes include steroids , sterols, fat soluble vitamins, bile salts, pigment etc.
Steroids:
- Steroids are not formed from fatty acids but have some lipid like characters.
- They posses 17 carbon nucleus of four fused hydrocarbon rings.
- Various steroids differ in the number and position of double bonds between carbon atoms and in the side group linked to the ring.
- Examples: Steroids include sterols, like cholesterol, ergosterol, bile salts, sex hormones and pigments.
B. Icosanoids
These are of following three types.
1) Prostaglandins :
1) Prostaglandins :
- These are hormones-like compounds. Prostaglandins are hydroxy derivatives of
20 carbon polyunsaturated fatty acids.
- Found in human seminal fluid, uterus, stomach lungs etc.
- Inflammation
- Allergic reaction
- Blood clotting
- Smooth muscle contraction
- They are formed in the blood platelets and are associated with blood clotting.
- These are secreted by leucocytes and include contraction of muscle in the lining of air passages to the lungs.
- Their overproduction causes asthmatic attacks.
FUNCTIONS OF LIPIDS
- Food material
- Food reserves
- Structural components
- Heat insulation
- Hormone synthesis
- Solvent (Vitamin A, D ,E & K)