6. Biomolecules - part 02 - Lipids

 


6. Biomolecules - part 02 - Lipids


B. Lipids :

  • These are group of substances with greasy consistency with long hydrocarbon chain containing carbon, hydrogen and oxygen.
  • In lipids, hydrogen to oxygen ratio is greater than 2:1 (in carbohydrates it is always 2:1).
  • Lipid is a broader term used for fatty acids and their derivatives. 
  • They are soluble in organic solvents (non-polar solvents). 
  • Fatty acids are organic acids which are composed of hydrocarbon chain ending in carboxyl group (-COOH). 
  • They can be saturated fatty acids with no double bonds between the carbon atoms of the hydrocarbon chain. 
  • Palmitic and stearic acids found in all animal and plant fats are examples of saturated fatty acids.
  • Unsaturated Fatty Acids are with one or more double bonds between the carbon atoms of the hydrocarbon chain. 
  • Oleic acid found in nearly all fats and linoleic acid found in many seed oils are examples of unsaturated fatty acids.
  • These fatty acids are basic molecules which form different kinds of lipids. 
Classifiacatiuon of Lipids - 

  1. simple lipids
  2. compound lipids
  3. derived lipids.

1. Simple Lipids :

  •  These are esters of fatty acids with various alcohols. 
  •  Simple lipids are- 
  1. Fats and 
  2. waxes 
Fats :-
  • Fats are esters of fatty acids with glycerol (CH2OH-CHOH-CH2OH).
  • Triglycerides are three molecules of fatty acids and one molecule of glycerol. 
  • Generally, unsaturated fats are liquid at room temperature and are called oils. Unsaturated fatty acids are hydrogenated to produce fats e.g. Vanaspati ghee.
  • Fats are a nutritional source with high calorific value. 
  • Fats act as reserved food materials. 
  • In plants it is stored in seeds to nourish embryo during germination. 
  • In animals fat is stored in the adipocytes of the adipose tissue. 
  • Fats deposited in subcutaneous tissue act as an insulator and minimise loss of body heat. 
  • Fats deposited around the internal organs act as cushions to absorb mechanical shocks.
Waxes :-
  • Wax  is another example of simple lipid.
  • They are esters of long chain fatty acids with long chain alcohols. 
  • They are most abundant in the bloodthe gonads and the sebaceous glands of the skin. 
  • Waxes are not as readily hydrolysed as fats. 
  • They are solid at ordinary temperature.
  • Waxes form water insoluble coating on hair and skin in animals, waxes form an outer coating on stems, leaves and fruits.



2. Compound lipids : 

  • These are ester of fatty acids containing other groups like phosphate (Phospholipids), sugar (glycolipids), etc. 
  • They contain a molecule of glycerol, two molecules of fatty acids and a phosphate group or simple sugar.
  • Some phospholipids such as lecithin also have a nitrogenous compound attached to the phosphate group. 
  • Phospholipids have both hydrophilic polar groups (phosphate and nitrogenous group) and hydrophobic non-polar groups (hydrocarbon chains of fatty acids).
  • Phospholipids contribute in the formation of cell membrane. 
  • Glycolipids contain glycerol, fatty acids, simple sugars such as galactose and nitrogenous base. They are also called cerebrosides
  • Large amounts of them have been found in the brain white matter and myelin sheath.


3. Sterols - derived lipids : 

  • They are derived lipids. 
  • They are composed of fused hydrocarbon rings (steroid nucleus) and a long hydrocarbon side chain.
  • One of the most common sterol is cholesterol.
  • It is widely distributed in all cells of the animal body, but particularly in nervous tissue.
  • Cholesterol exists either free or as cholesterol ester.
  • Adrenocorticoids, sex hormones (progesterone, testosterone) and vitamin D are synthesised from cholesterol. 
  • Cholesterolis not found in plants. 
  • In plants, sterols existchiefly as Phytosterols
  • Yam Plant (Dioscorea) produces a steroid compound called diosgenin.
  • It is used in the manufacture of antifertility pills. i.e. birth control pills.

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