Classification of fatty acids : Saturated and Unsaturated fatty acids

 

     What are fatty acids?

•   Fatty acids are carboxylic acids with long hydrocarbon chain. They are simplest form of lipids. General formula :  R-(CH₂)_n- COOH.

• Fatty acids usually consist of 12-20 molecules of C. They are divided into 2 portions- Hydrophobic tail & Hydrophilic head. Although they contain polar carboxylic group in their structure, however they're insoluble in water. Because of their hydrophobic portion is much bigger than the hydrophilic portion.

Ø Physical  Properties  of Fatty acids

 1. Solubility  in  water :

•       Fatty acids containing  up to  6 carbons are completely soluble in water.  
•       Longer  chain fatty  acids are  insoluble  in water  but  soluble in fat solvents.  

2.  Physical  state  at  room  temperature :  

•     The  lower  members  of  saturated fatty acids are liquid  at  room temperature  & volatile. They  have irritant  odour  & bad taste.  
•       Those higher  than  10  carbons are  solids.  
•       Unsaturated long chain fatty acids are  liquids  due to  the  presence of cis  double bonds 

Ø   Nomenclature of Fatty Acids

Carbon atoms are numbered from the carboxyl carbon (carbon No. 1). The Carbon atoms adjacent to the carboxyl carbon are numbered as 2, 3…etc & are also known as α, β…etc, carbons respectively. 

The terminal methyl(-CH3) group is known as the ω(omega) carbon which is used to indicate the site of a double bond. For example: ω9 indicates a double bond on the 9th C counting from the terminal methyl group. 

Ø Classification of fatty acids

1. Saturated fatty acids

• They contain only single bonds in their structure. They are generally solids in room T & have high boiling point &  melting point.

• They are pack together in a tightly regular pattern, which allow a strong attraction to occur between C chains. Since there are single bonds & no double bonds, there are enough H atoms in it, i.e. this fat is saturated with H atoms. As a result, the Vander Waals force between molecules is very robust & stable. This makes the fats remain solid at room T .

•  Increased  intake of  saturated  fatty  acids  may  lead  to increase in  plasma cholesterol  levels  & incidence  of heart diseases.

• Sources : Animal foods such as meat, poultry, full-fat dairy products.

2. Unsaturated fatty acids

• They contain 1 or more double bonds in their structure. They are mostly liquids at room T & have lower boiling & melting point.
• They are pack together in a irregular pattern, which allow a low attraction to occur between C chains. Since there is presence of double bonds, the amount of H in the fa chain gets reduced. Furthermore, the carbon atoms in unsaturated fatty acids don’t form a straight chain-the chain gets bent. This results in the fa chain being weak & unstable. As a result of these two thigs, the Vander walls force between molecules isn’t that robust or strong. Hence Unsaturated fatty acids remain liquid at room temperature.

• They may be -

1. Mono-unsaturated : Containing one double bond
2. Poly-unsaturated :  Containing 2 or more double bonds

Unsaturated fatty acids can occur in 2 distinct structural configurations –

1. Cis isomers:  The H atoms attached to the C double bond are on the same side
2. Trans isomers:  The H atoms attached to the C double bond are on different side

  1. Cis-fatty acids

• Most  of  the double bonds  present  in unsaturated fatty acids  are  of  the  cis -type & they  are liquid at  room  Temperature.   A trans- bond creates a straight chain, whereas a cis-bond results in a chain that is bent. This disrupt the vandar waals forces by preventing the tails from packing close to one another. So, they can't bunch tightly together. The bend helps the fat stay liquid rather than solid.
• Sources :  Vegetable oils such as olive, corn, soybean, peanut
• They are important  for  synthesis of  phospholipids.   2-  Formation of  eicosanoids  such  as prostaglandins, prostacyclins,  thromboxanes,  leukotrienes  &  lipoxins.
• Intake of cis-fatty acids decrease the incidence of plasma cholesterol & high blood pressure.

 2. Trans-fatty acids

• A trans bond creates a straight chain & molecules in chain are tightly pack together. So, unsaturated fatty acids  containing  trans-double bonds are solid  at  room  Temperature.  Their boiling point & melting point is relatively higher than cis-fatty acids.
• Sources :  Fast foods, backed foods, packaging foods, frozen foods, chips, cookies, candies, beverages etc.   
• Studies have shown that trans fats may act similarly to saturated fats. Trans fats are well known to cause bad cholesterol, heart disease, & obesity.

Nutritional  Classification  of Fatty  Acids

1. Essential fatty acids

• Essential fatty acids defined as the fatty acids that our body can't synthesize & they should be applied through diet.
• Linoleic Acid-  C17H31COOH (omega-6-fatty acids) & Linolenic Acid- C17H29COOH (omega-3-fatty acids) are 2 important Essential fatty acids for our body. They are found in vegetables oils like cottonseed, castor, olive or peanut oil.
• Arachidinic acid is considered as semi-essential fatty acid since it can be synthesized from linoleic acid.

• Significance of essential fatty acids :

1. Components of cell membrane
2. Required for brain growth & development
3. Precursors of eicosanoids
4. Play an important role in vision

• Deficiency of Essential fatty acids  may lead to dermatitis, weight loss, growth retardation etc.

2. Non -Essential  Fatty  Acids

• They include all other fatty acids because they are formed in our body in good amounts mainly from carbohydrates.  It is not essential to take them in our regular diet.

See also -  Difference between fats and oils

See also-     What's cholesterol | It's sources, classification, significance etc