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UNIT 1

LIPIDS

• The word lipid is derived from a Greek word “lipos” which means Fat.
• Lipids are the heterogeneous group of substances.
• “Lipids are organic compounds that contain hydrogen, carbon, and oxygen atoms, which forms the framework for the structure and function of living cells.”
• Lipids are condensation product of fatty acids and glycerol in 3: 1 proportion.
• They are polymers of fatty acids and glycerol.
• They are energy rich compounds.
• They differ from polysaccharides, nucleic acids and proteins in lacking well defined monomeric units.
• These organic compounds are nonpolar molecules, which are soluble only in nonpolar solvents and insoluble in water because water is a polar molecule like ether, chloroform, benzene.

INTRODUCTION

INRODUCTION (contd.)

• Lipids are a heterogeneous group of compounds, mainly composed of hydrocarbon chains.
• Lipids are significant in biological systems as they form a mechanical barrier dividing a cell from the external environment known as the cell membrane.
• The majority of plant lipids act as energy stores as plant membrane components, surface lipids such as waxes and cutin. Lipids are found in relatively small amounts in temperate grasses and generally decrease as the plant matures
• Lipids are abundant in the endosperms of seeds.
• In the human body, these molecules can be synthesized in the liver and are found in oil, butter, whole milk, cheese, fried foods and also in some red meats.

GENERAL STRUCTURE

• The most common building blocks of lipids are fatty acids.
• A fatty acid is a compound consisting of a hydrocarbon chain and a carboxyl group (−COOH) at one end.
• Typically, fatty acids are bound to a glycerol molecule which consists of three hydroxyl groups (-OH), each bound to a carbon atom.
• If a fatty acid contains only single bonds, it is called a saturated fatty acid. This name is given because the fatty acid has the maximum number of hydrogens in its structure so, saturated with hydrogens.
• This type of fatty acid will be solid at room temperature e.g.Butter.
• If a double bond exists in the fatty acid structure, it's called unsaturated. This type of fatty acid will be liquid at room temperature. Olive oil is an example. If a fatty acid has more than one double bond, it is called a polyunsaturated fatty acid.

GENERAL STRUCTURE

SATURATED FATTY ACIDS

• All the carbon atoms are long chain carboxylic acid linked together by single bonds.
• They have even number of carbon atoms up to 12-24
• The general formula is C_nH_2nCOOH
• They have maximum possible number of hydrogen atoms.
• They end with the suffix “anoic” e. g. Actanoic acid, Octadecanoic acid (Stearic acid), Hexadecanoic acid (Palmitic acid), Butanoic acid.
• They are all straight chain of carbon atoms.
• They are commonly found in natural sources like coconut oil, palm oil, groundnut oil.

UNSATURATED FATTY ACIDS

• They have one or more double bonds in their structure.
• These double bonds may occur after carbon number 9, 12, 15, 18…etc.
• Unsaturated fatty acid with more than 1 double bond are called as polyunsaturated fatty acids.
• They have odd or even number of carbon atoms.
• Increase in double bonds lowers the melting point.
• Generally they have lower melting point.
• The unsaturated fatty acids end with a suffix –enoic.
• They are abundantly found in both animals and plants than saturated fatty acids.
• Plant lipids have a large proportion of unsaturated fatty acids such as oleic acid, linoleic acid and linolenic acid
• Depending upon number of double bonds the unsaturated fatty acids are called monoconic, diconic, menoic etc.
• They are oily and greasy.

PHYSICAL PROPERTIES OF LIPIDS

1. Solubility: Lipids are soluble in non-polar solvent (ether, chloroform, benzene etc.) but insoluble in water.
2. Consistency: They are colorless, odorless and tasteless. Being lighter than water they have a specific gravity of 0.86. The consistency of lipids depends upon the presence of saturated and/or unsaturated fatty acids.

CHEMICAL PROPERTIES OF LIPIDS

1. Hydrolysis: Hydrolysis is the breakdown of a substance by the addition of water. Fats and oils are hydrolyzed by moisture to yield glycerol and 3 fatty acids.
2. Emulsification: When fats or oils are rubbed with water , the large molecules of lipids braked into smaller ones forming the emulsion and the process is called emulsification.

3. Rancidity: When lipids are exposed to atmosphere (heat, light, air, moisture) for more than 30 days, an unusual and undesirable odour is developed. Such types of lipids are called rancid lipids and the phenomenon is termed as Rancidity. Rancidity is of two types: (a) Hydrolytic and (b) Oxidative

a. Hydrolytic Rancidity: This type of phenomenon occurs due to liberation of volatile acids during the hydrolysis of fats.

b. Oxidative rancidity: When a part of unsaturated fatty acid oxidized to form aldehyde, ketone, the phenomenon is termed as oxidative rancidity.

Hydrolysis of fat by alkali is called saponification. The products are glycerol and the alkali salts of the fatty acids, which are called soaps. Acid hydrolysis of fat yields free fats and glycerol.

4. SAPONIFICATION

Hydrogenation of unsaturated fats in the presence of a catalyst (nickel) is referred to as “hardening”. It is commercially important as a method of converting these liquid fats, usually of plant origin into solid fats as margarine, vegetable ghee, and so on.

5. HYDROGENATION

CLASSIFICATION OF LIPIDS

Lipids are broadly classified into:

1. Simple Lipids
2. Complex Lipids
3. Derived Lipids
4. Miscellaneous Lipids

which are further subdivided into different groups

1. Simple lipids

• These are Esters of fatty acids with alcohols.
• These are also called as Homolipids and triglycerides .
• On hydrolysis they produce 3 molecules of fatty acids and 1 molecule of glycerol.

These are mainly of two types:

(a) Fats and oils (triacylglycerols) : These are esters of fatty acids with glycerol. Thus, oil is a liquid while fat is a solid at room temperature.

(b) Waxes : Esters of fatty acids (usually long chain) with alcohols other than glycerol. These alcohols may be aliphatic or alicyclic. Cetyl alcohol is most commonly found in waxes. Waxes are used in the preparation of candles, lubricants, cosmetics, ointments, polishes etc.

2. Complex (or compound) lipids

• These are esters of fatty acids with alcohols containing additional groups such as phosphate, nitrogenous base, carbohydrate, protein etc.
• Also called as Heterolipids.

They are further divided as follows:

(a) Phospholipids : They contain phosphoric acid and frequently a nitrogenous base in addition to alcohol and fatty acids. They occur in all membranes.

(i) Glycerophospholipids : These phospholipids contain glycerol as the alcohol e.g., lecithin, cephalin.

(ii) Sphingophospholipids : Sphingosine is the alcohol in this group of phospholipids e.g., sphingomyelin.

(b) Glycolipids : These lipids contain a fatty acid, carbohydrate and nitrogenous base. The alcohol is sphingosine, hence they are also called as glycosphingolipids. e.g., cerebrosides, gangliosides.

(c) Lipoproteins : Macromolecular complexes of lipids with proteins. They are component of Cell Membranes.

(d) Other complex lipids : Sulfolipids, aminolipids and lipopolysaccharides are among the other complex lipids.

2. Complex (or compound) lipids

3. Derived lipids

• These are the hydrolysis products of lipids.
• These include glycerol and other alcohols, fatty acids, mono-and diacylglycerols, lipid (fat) soluble vitamins, steroids, hormones, hydrocarbons and ketone bodies.

4. Miscellaneous lipids

These include a large number of compounds possessing the characteristics of lipids e.g., carotenoids, squalene, hydrocarbons such as pentacosane (in bees wax), terpenes etc.

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