What are lipids? Write a detailed account of lipids and their importance.

 Answer: 

LIPIDS:

 The lipids are a heterogeneous group of compounds related to fatty acids. They are insoluble in water but soluble in organic solvents such as ether, alcohol, chloroform and benzene. Like carbohydrates, fats have received a lot of bad publicity. It is true that eating an excess of fried foods and other “fatty” foods leads to weight gain. However, fats do have important functions. Many vitamins are fat soluble, and fats serve as a long-term storage form of fatty acids: a source of energy. 

 Importance of Lipids 

Lipids play very important roles in living organisms. 

Structural components of cell: 

Lipids, as hydrophobic compounds, are components of cellular membranes.

1. Source of energy: 

Lipids are also used to store energy. Because of highe proportion of C-H bonds and very low proportion of oxygen, lipids store double the amount of energy as compared to the same amount of carbohydrate. 

2. Insulating agent: 

Some lipids provide insulation, e.g. beneath the skin against atmospheric heat and cold and also provide water protection material. 

3. Protection:

 Waxes, in the exoskeleton of insects, and cutin, an additional protective layer on the cuticle of epidermis of some plant organs e.g. leaves, fruits, seeds etc., are some of the main examples

 4. Steroids & Carotenoides: 

Lipids are components of many sex hormones, vitamin D, cholesterol, carotenes, etc. 

5. Solvent: 

Lipids serve as a solvent for fat-soluble vitamins, sex hormones and related hormones, etc.

 6. Carcinogenic:

Higher fat contents of food cause slower movements of feces through the bowels. These bacteria present in food convert the undigested fats into cancer causing compounds.




CLASSIFICATION OF LIPIDS:

 Lipids have been classified into following groups. 

1.Waxes 2.Phospholipids 3. Acylglycerols

 4. Sphingolipids 5. Glycolipids 6. Terpenoid 

1. Acylglycerols:

Acylglycerols are composed of glycerol and fatty acids. The most widely spread acyl glycerol is triacyl glycerol, also called triglycerides or neutral lipids. Chemically, acylglycerols can be defined as esters of fatty acids and alcohol.

Esters:

 An ester is the compound produced as the result of a chemical reaction of an alcohol with an acid and a water molecule is released as shown below: 

C2H5OH+HOOCCH3 + C2H5OCOCH3 +H20

Fatty Acids: 

Fatty acids are one of the most important components of triglycerides. Their general formula is R.COOH, where R is hydrogen or an alkyl group. Fatty acids contain even numbers (4-30) of carbon atoms in straight chain attached with hydrogen and having an acidic group COOH (carboxylic group). In minerals the fatty acids are straight chains, while in plants these may be branched or ringed. 

Types of Fatty Acids: 

There are two main types of fatty acids. 

1. Saturated Fatty Acids

2. Unsaturated Fatty Acids 

1. Saturated Fatty Acids:

 They contain no double bond. They melt at higher temperature than unsaturated fatty acids. Lipids containing such fatty acids are called saturated lipids.

 Fats:

Lipids containing saturated fatty acids are solids e.g. butter. Animal fats are solid at room temperature.

Example :

Acetic acid (C2)

Butyric acid(C4).

Palmatic acid(C16).

Trans Fats:

In the food industry, oils are artificially hydrogenated to make them semi-solid and of a consistency desirable for many processed food products. Simply speaking, hydrogen gas is bubbled through oils to solidify them. During this hydrogenation process, double bonds of the cis– conformation within the hydrocarbon chain could also be converted to double bonds within the trans– conformation.

Margarine, some types of peanut butter, and shortening are examples of artificially hydrogenated trans fats. Recent studies have shown that a rise in trans fats within the human diet may cause a rise in levels of low-density lipoproteins (LDL), or “bad” cholesterol, which in turn may cause plaque deposition within the arteries, leading to heart condition . Many fast food restaurants have recently banned the use of trans fats, and food labels are required to display the trans fat content.

Omega Fatty Acids:

The molecular structures of alpha-linolenic acid, an omega-3 fatty acid is shown. Alpha-linolenic acid has three double bonds located eight, eleven, and fourteen residues from the acetyl group. It has a hooked shape.

Solubility and melting points of fatty acids: 

Solubility of fatty acids to organic solvents and their melting points increase with increasing number of carbon atoms in chain. Palmatic acid (C16) is much more soluble in organic solvent than butyric acid (C4). The melting point of palmatic acid is 70C as against -8C for butyric acid.

 2.Unsaturated Fatty Acids: 

They contain one or more double bonds (C=C); the number of double bond is up to 6. They melt at much lower temperature than the saturated fatty acids, Lipids containing such fatty acids are called unsaturated lipids. Oils: Fats containing unsaturated fatty acids are usually liquid at room temperature and are said to be oils. Most of the plant fats are liquids. 

Example:

Oleic acid(C18).

Fats and oils are lighter than water and have a specific gravity of about 0.8. They are not crystalline but some can be crystallized under specific conditions.

 2. Waxes 

Chemical Composition:

Chemically, waxes are mixtures of long chain alkanes (with odd number of carbons ranging from C25 to C35) and alcohols, ketones and esters of long chain fatty acids.

 Occurrence:

Waxes are widespread as protective coatings on fruits and leaves. Some insects also secrete wax.

 Importance: 

Waxes protect plants from water loss and abrasive damage. They also provide water barrier for insects, birds and animals such as sheep. 

3. Phospholipids: 

Phospholipids are derivatives of phosphatidic acid, which are composed of glycerol, fatty acids and phosphoric acid. 

Nitrogenous bases such as choline, ethanolamine and serine are important components of phospholipids. 

Functions:

They are widespread in bacteria, animal and plant cells and are frequently associated with membranes. Phosphatidylcholine is one of the common phospholipids. It is composed of glycerol, 2 fatty acids (on Cl and C2), and a phosphoric acid on C3 of glycerol. In phospholipid, a nitrogenous base (e.g. choline) is attached to phosphoric acid in phosphatidic acid.

4. Terpenoids:

 Terpenoids are a very large and important group of compounds, which are made up of simple repeating units, isoprenoid units. This unit by condensation in different ways gives rise to compounds such as rubber, carotenoids, steroids, terpenes etc.



Pre-medical ist year biology notes