- Lipids are heterogeneous compounds related to fatty acids.
- They are insoluble in water but soluble in nonpolar solvents such as ether, acetone and chloroform.

Biological importance of lipids

1. Lipids act as a source of energy. They are superior to carbohydrate and protein since they yield twice the energy produced by the same weight of carbohydrates or proteins.
2. They are the natural solvent for fat-soluble vitamins
3. They contain essential fatty acids.
4. Lipids in adipose tissue serve as energy store.
5. Lipids have a role in protection and fixation of internal organs as kidneys.
6. Lipids in myelin sheath of nerve fibers serve as electrical insulator.
7. Lipids under the skin serve as thermal insulator.
8. Lipoproteins are essential components in the structure of cell membrane and mitochondria. Also, they are important for lipid transport in the blood.
9. Acetyl CoA derived from fatty acids oxidation is used for biosynthesis of may important compound e.g. steroids.
   

Classification of lipids

Lipids are classified into simple, compound (conjugated) and derived lipids.

I- Simple lipids

They are formed of fatty acids and alcohol. They are further classified according to the type of alcohol present into:
1- Fats and oils
2- Waxes

II- Compound lipids

They are formed of simple lipids and other non-lipid part e.g.:

1. Phospholipids
2. Glycolipids
3. Sulpholipids
4. Lipoproteins

III- Derived lipids

These are substances derived from simple lipids and compound lipids by hydrolysis. They also, include substances related to lipids. Derived lipids include:

1. Fatty acids
2. Glycerol
3. Steroids
4. Isoprenoids
5. prostglandins and leukotriens derived from arachidonic acid.

I- Simple lipids

Simple lipids are esters of fatty acids with various alcohols. The alcohol may be glycerol or other long chain alcohol.

Simple lipids are classified into fats, oils and waxes, according to the type of alcohol they contain.

1- Fats and oils

They are esters of fatty acids with glycerol. They are similar chemically, but they differ in physical properties, as oils are liquid while fats are solid at room temperature.

They are called triglycerides because they are formed of glycerol and 3 fatty acids.

The 3 fatty acids may be the same e.g. palmetic or stearic acids forming tripalmetine or tristearine respectively, or the may be different forming triglycerides with 3 different fatty acids.

2. Waxes

They are esters of acids with long chain monohydric alcohol. They are solid at room temperature
The most important waxes in human body are cholesterol esters, which are present in blood and other tissues.

The following table shows the differences between fats and waxes

Glycerol

It is a trihydric alcohol.
It is colourless and viscid fluid with sweat taste.
It is miscible with water in all proportions.
With strong dehydrating agents, as concentrated sulphuric acid, glycerol can be converted to acrolein that has very irritating odour. This is called acrolein test.

Importance of glycerol:-

1. It is used in pharmaceutical and cosmetic preparations.
2. It is used as explosive in the form of trinitroglycerine.
3. It is used in medicine as a vasodilator agent in coronary heart diseases in the form of nitroglycerine.
   

Fatty acids

These are organic acids, which usually contain an even number of carbon atoms. They are further classified into saturated and unsaturated fatty acids according to absence or presence of double bonds.

A- Saturated fatty acids

- They have no double bonds. They have the general formula CH₃-(CH₂)_n-COOH
- They are further classified, according to the number of carbon atoms, into short chain and long chain fatty acids.

The following table shows the differences between short chain and long chain fatty acids.

The following table shows the formulae of the most common saturated fatty acids.

Common name	Number of carbon atoms	Formula
Acetic acid	2
Butyric acid	4
Caproic acid	6
Palmitic acid	16
Stearic acid	18
Arachidic acid	20

B-Unsaturated fatty acids

They have one or more double bonds.

1. Oleic acid that contains 18 carbon atoms and one double bond
2. Linoleic acid that contains 18 carbon atoms and 2 double bonds
3. Linolenic acid that contains 18 carbon atoms and 3 double bonds
4. Arachidonic acid that contains 20 carbon atoms and 4 double bonds

 

Essential fatty acids

• They are polyunsaturated fatty acids i.e. fatty acids, which contain more than one double bond.
• They include linoleic, linolenic and arachidonic acids.
• They are essential for growth.
• They must be taken in diet because the body cannot synthesize them, as the enzymes that are needed for their synthesis are absent in humans.
• Arachidonic acid, which is one of the essential fatty acids, is important for biosynthesis of prostaglandins.

General properties of fatty acids

1. They usually contain an even number of carbon atoms.
2. They have straight chains.
3. They may be saturated or unsaturated. Unsaturated fatty acids are more reactive than saturated fatty acids.
4. Some fatty acids are hydroxylated e.g. cerebronic acid.
5. Lower fatty acids are soluble in water and this solubility decrease with increasing chain length.
6. Palmitic, stearic and oleic acids make up the bulk of animal depot fat.

Physical properties of fatty acids

1. They are colourless, odourless and tasteless.
2. Solubility in water
   a)- Short chain fatty acids are soluble in water. The solubility decreases with the increase in chain length.
   b)- Long chain fatty acids are insoluble in water but soluble in nonpolar solvents.
3. Melting point
   It depends on the length of the chain of fatty acid and the degree of unsaturation.
   a)- Short chain and unsaturated fatty acids have lower melting point. They are liquid at room temperature.
   b)- Long chain saturated fatty acids have higher melting point. They are solid at room temperature.
4. Optical activity
   Fatty acids that contain double bond can be present in cis and trans stereoisomeric forms.
   Cis configuration means that the groups around the double bond are on the same side of the bond
   Trans configuration means that the groups around the double bond are on the opposite sides of the bond.
   
   For example, oleic acid, which is a cis form, and its isomer eliadic acid, which is a trans form
   
   
   

Chemical properties of fatty acids :-

1. Salt formation (Reaction with alkalie)
   Being acids, fatty acids react with alkalie to form salts
   Salts of fatty acids are called soaps. Sodium and potassium soaps are soluble in water so they are called soft soaps.
   Calcium and magnesium soaps are insoluble in water so they are called hard soaps.
2. Ester formation (Reaction with alcohols)
   Fatty acids react with alcohol to form esters. For example, fatty acids react with glycerol to form mono, di and triglycerides.
   Also, fatty acids react with long chain monohydric alcohols to give waxes. The most common wax in the body is cholesterol esters.
3. Hydrogenation
   Addition of hydrogen to unsaturated fatty acids at the double bond changes it to the corresponding saturated fatty acids
4. Halogenation
   Halogens, as chlorine (Cl), fluorine (F) and iodine (I), can be added to unsaturated fatty acids at the double bond forming halogenated fatty acid.
5. Reduction
   Fatty acids can be reduced to fatty aldehydes, fatty alcohols, and hydrocarbons.
6. Oxidation
   Unsaturated fatty acids are oxidized by oxygen when exposed to air forming lipid peroxides, fatty aldehydes, ketones and short chain fatty acids
   Oxidation of unsaturated fatty acid with dilute solution of potassium permanganate gives dihydroxy fatty acid and then cleaves the fatty acid at the double bond.

Physical properties of fats and oils

1. They are colourless, odourless and tasteless.
   The presence of any colour, odour or taste in fat is due to addition of foreign substances.
2. Specific gravity
   Specific gravity of fats and oils is less than that of water, so they float on the surface of water
3. Solubility
   Fats and oils are insoluble in water. They are soluble in fat solvents (nonpolar solvents) as benzene, ether, alcohol and chloroform.
4. Melting point
   - Oils have a low melting point while fats have a higher melting point.
   - Oils are liquid at room temperature because they contain high proportions of unsaturated fatty acids, while fats are solid at room temperature, as they do not contain unsaturated fatty acids.
   - The consistency of fat at room temperature gives an idea of its saturation. If it contains high amount of unsaturated fatty acids, it is liquid at room temperature, but if it does not contain unsaturated fatty acids, it is solid at room temperature.
   

1. Acrolein test
   Fats and oils contain glycerol so; when they are dehydrated by concentrated sulphuric acid they give very irritant and pungent odour due to formation of acrolein.
2. Hydrolysis
   Hydrolysis means breakdown of substance by addition of water. Fats and oils can be hydrolyzed by superheated steam, or by lipase enzyme to glycerol and 3 fatty acids.
3. Saponification (Action of alkalie)
   Alkalie react with fats or oils giving glycerol and soap (salts of fatty acids). Sodium and potassium soaps are soluble in water so they are called soft soaps, while calcium and magnesium soaps are insoluble in water so they are called hard soaps
4. Hydrogenation
   Addition of hydrogen to fats or oils depends on the presence of unsaturated fatty acids changing them to saturated fatty acids.
   Hydrogen is usually added at high temperature in the presence of nickel as a catalyst.
   This reaction changes oil, which is liquid at room temperature, to fat, which is solid at room temperature. This is the basis of margarine preparation from oils.
5. Halogenation
   Halogenation means addition of halogen as iodine (I) fluorine (F) and chlorine (Cl) to fat.
   It depends on the presence of unsaturated fatty acids. The halogen is added at the double bonds that are present in the unsaturated fatty acids.
6. Oxidation
   The unsaturated fatty acids present in fats and oils react with oxygen when exposed to air forming lipid peroxides, fatty aldehydes, ketones and short chain fatty acids.
   

Rancidity is a condition in which fat attains a bad taste and disagreeable odour.

Types of rancidity
There are 2 types of rancidity:

1. Hydrolytic rancidity
   Fats are hydrolyzed in presence of moisture and warm temperature and also by bacterial enzymes into glycerol and fatty acids.
2. Oxidative rancidity
   It occurs by oxidation of unsaturated fatty acids present in fats and oils forming lipid peroxides, fatty aldehydes, ketones and short chain fatty acids

Predisposing factors of rancidity

Rancidity is predisposed by:

1. Light
2. Moisture
3. Warm temperature.

Effects of rancidity

Rancidity leads to:

1. Fats and oils attain bad taste.
2. Fats and oils attain disagreeable odour.
3. Production of toxic compounds as lipid peroxides, aldehydes and ketones.

Prevention of rancidity

Rancidity can be prevented by:

1. Addition of antioxidants to fats and oils specialty the natural antioxidant tochopherol (vitamin E).
2. Also, avoid exposure of fats to light, moisture and high temperature.