Lipids

Lipids are the the esters of fatty acids and glycerol made of C, H and O. However, the number of of oxygen is less as compared to carbohydrate.

They are insoluble in water but soluble in organic solvent like acetone, ether alcohol etc. They need large amount of oxygen for the the oxidation to release energy.  The energy density yield of  lipid  is 37KJ/gm. The caloric value of 1gm of fat is 9.45kcal.

Fatty Acid 

Fatty acids are the organic acids having hydrocarbon chain that in end carboxylic acid. They are carboxylic acid having long hydrophobic chain usually from 4 to 30.
General formula : -RCOOH

where R= Hydrocarbon chains and                      COOH= Carboxylic acid

Types of fatty acids

On the basis of bond formed between the carbon chains of fatty acids are of two types they are:

• Unsaturated fatty acids


• 
  Saturated fatty acids
  

 Unsaturated fatty acids

• They are the fatty acids having one or more double bond in carbon chain.


• They have low melting point


• Such fatty acids are abundant in nature


• For example: Oleic acid (one double bond), Linoleic acid (two double bond), Linolenic acid (three double bonds)

Saturated fatty acids

• They do not possess double bond in carbon chain.


•  They have high melting point.


• They are abundant in fatty acids.


• For example:


• Stearic acid CH₃ (CH₂)₁₆COOH


• Palmitic acid CH₃(CH₂)₁₄COOH


• Arachidic acid CH₃(CH₂)₁₈COOH

Types of lipids 

On the basis of composition lipids categorized into tree types:

• Simple lipids


• Conjugated lipids


• Derived lipids 

Simple Lipids 

Fig: Triglyceride

They are esters of fatty acids and alcohol only. There are no additional compounds. On hydrolysis they yield fatty acid and alcohol.

They are broadly classified into  Neutral or true fat, Oils and Wax

I) Neural or True Fat

These are esters of fatty acid and glycerol and are called glycerides each molecule of glycerol can react with maximum three molecules of fatty acids depending upon the number of fatty acid they are of three types

Monoglyceride: Consists of one molecule of glycerol and one molecule of fatty acid.

Diglyceride: Consists one molecule of glycerol and two molecules of fatty acid.

Triglyceride: Consists one molecule of glycerol and three molecules of fatty acid.

Triglyceride is the most common type of lipid in nature. It is formed by condensation of one molecule of glycerol with three molecules of fatty acid and removal of three molecules of water.

II)  Oil 

• They are fats which are liquid at room temperature.


• They have low melting point.


• They are rich in unsaturated fatty acids.


• Example: Mustard oil, Sunflower oil, Coconut oil etc.

III) Waxes

• They are formed of fatty acids and alcohol other than glycerol.


• They contain both saturated and unsaturated fatty acids.


• They are solid at room temperature.


• They are found in cuticle of plants which help to reduce transpiration.


• For example: hair wax, feather wax, bee hive wax 

Conjugated Lipids

Phospholipid: They are composed of lipid and phosphoric acid. They are the most important Lipids from biological point of view. They form plasma membrane and other endomembranes of a nerve tissue. 

Glycolipid: They are composed of lipids and carbohydrates. They are found in membranes and are important constituents of white matter and nerve tissue.

Lipoprotein: They are composed of lipid and carbohydrates. Various types of lipoproteins are found in blood plasma.

Derived Lipids 

They include hydrolyzed products of simple and complex Lipids. For example: Steroids, Terpenes, Carotenoids etc. 

Steroids 

Fig: Cholesterol

Steroids are the derived lipid composed of fused hydrocarbon rings  and long hydrocarbon chains. Eg: Cholesterol, Diosgenin

Importance of Steroids :

Cholesterol is a precursor molecule for many sex hormones like progesterone, testosterone and adrenocorticotropic hormones like corticol.

• Cholesterol on irradiation by ultraviolet rays form Vitamin D necessary for growth of bones.


• Diosgenin is used in manufacture of anti fertility pills.


• 
  Cholesterol is said to create abnormal thickening of walls of arteries. It can raise blood pressure and hence lead to circulatory problems.