Saturday, June 15, 2024

How Are Lipids defined, classified, and structured?


Fats are biomolecules containing carbon, hydrogen, and oxygen in simple lipids, however, in compound lipids they contain nitrogen and phosphorous in addition. In general, fats are used in daily routine but are more burnt during starvation or in the absence of carbohydrates in food.

Fats play the role of a source of energy as and when required, they are like the storehouse of energy. The fats in common terms refer to simple lipids as another synonym. They are biofuels and provide more energy i.e., 9Kcal/g as compared to 4Kcal/g provided by carbs.

Fats are basically of two types: a) Dietary fats which when taken with food undergo digestion and absorption and get stored as b) Body fat.

Lipids are organic substances that are relatively insoluble in water, soluble in organic solvents (alcohol, ether, etc), actually or potentially related to fatty acids, and utilized by living cells. Unlike polysaccharides, proteins, and nucleic acids, lipids are not polymers.


Fats are present in plants and animals in nature. In plants, they are in the form of nuts and seeds, while in animals they are in subcutaneous tissues, fatty tissues, bone marrow, etc. In food items, fats are found in milk, eggs, meat, liver, fish oil, and various oils of seeds.


  • Fats are the storehouse of energy are stored as body fat in the adipose tissues (under the skin)
  • Fats act as an excellent insulator and maintain the normal temperature of about 37°C in the human body. It remains unaffected by the temperature variations of ±15-20°C in the surroundings.
  • Lipids are constituents of the cell membrane and mitochondria in the form of lipoproteins and phospholipids. They regulate membrane permeability.
  • They act as protective fatty tissue around kidneys and other internal organs which are sensitive to shock and get damaged.
  • Fats are the building blocks of biologically active material like cholesterol and its derivatives like bile acid, vitamin D3, and sex hormones.
  • The body requires essential fatty acids in the control of bad cholesterol and these fatty acids and produced from fats.
  • Fats serve as the solvent for ingesting the fat-soluble vitamins- A, D, E, and K.
  • Fats have a major role in the entire nervous system and its protection.

Classification of Lipids

They are broadly classified into:

  • Simple Lipids: Waxes; Fats
  • Compound Lipids: Phospholipids; Glycolipids; Sulpholipids; Sphingomyelin; Lipoprotein
  • Derived Lipids: Fatty acids; Cholesterol; Others
  • Miscellaneous Lipids
  • Neutral Lipids

Simple Lipids

These are esters of fatty acids with alcohols. Simple lipids are mainly of two types:

Waxes– These are simple fats. Esters of fatty acids (long-chain usually) with alcohols other than glycerol.  These alcohols may be aliphatic or alicyclic. Cetyl alcohol is most commonly found in waxes. Waxes are used in the preparation of candles, lubricants, cosmetics, ointments, polishes etc.

Fats and Oils– Fats or glyceride esters are made up of glycerol and higher fatty acids. Oil and fat differ only physically- oil is liquid at room temperature, whereas fat is solid.

Compound Lipids

Compound lipids are esters of fatty acids with alcohols, having additional groups such as phosphate, nitrogenous base, carbohydrate, protein, etc. These are sub-divided as follows:

Phospholipids– the constituents of phospholipids are glycerol, fatty acids, phosphoric acid, and amino alcohol.

Glycerophospholipids: These phospholipids contain glycerol as the alcohol e.g., lecithin, cephalin.

Lecithin, an important phospholipid, is necessary for the transport of other lipids particularly in the liver. It is made up of glycerol, fatty acids, phosphoric acid, and amino alcohol. It is also called trimethyl aminoethanol or choline (HO-CH2- CH2-N(CH3)3).

Cephaline is important for the clotting of blood and is also a source of phosphoric acid required for new tissues. Constituents of cephalin- glycerol, fatty acids, phosphoric acid, and aminoethanol (NH2CH2CH2OH)

Sphingophospholipids: Sphingosine is the alcohol in this group of phospholipids e.g., sphingomyelin.

Glycolipids– these lipids have a fatty acid, carbohydrate, and nitrogenous base. They have no phosphoric acid and glycerol. The alcohol is sphingosine; thus, they are also called glycosphingolipids. Example- cerebrosides, gangliosides.

Lipoproteins– these are lipids attached to proteins. Since lipids are immiscible in plasma, they move through blood in the form of lipoprotein.

Lipids present in lipoproteins are- lecithin, cephalin, fatty acids, cholesterol, glycerides.

They are of 4 types: Chylomicron, VLDL, LDL, and HDL.

Sulpholipids– also glycolipids and they contain sulfates of galactose residue.

Sphingomyelin– also called sphingophospholipids. These are formed in the brain and nerve tissues. Constituents are fatty acid, phosphoric acid, choline, and sphingosine.

Derived Lipids

Derived lipids are those that are obtained from simple and compound lipids by hydrolysis. These include glycerol and other alcohols, fatty acids, mono- and diacylglycerols, lipid (fat) soluble vitamins, steroid hormones, hydrocarbons, and ketone bodies.

Miscellaneous Lipids

These include a large number of compounds that have the characteristics of lipids e.g., carotenoids, squalene, hydrocarbons such as pentacosane (in beeswax), terpenes, etc.

Neutral Lipids

The uncharged lipids are referred to as neutral lipids. These are mono-, di-, and triacylglycerols, cholesterol, and cholesteryl esters.


  • Any fat or oil is immiscible in water. It is sticky/greasy to touch and leaves a mark as a stain on the paper.
  • Exposure to air, on long storage, and on heating, the fats give a bad odor called rancidity. This is due to oxidation hydrolysis taking place on the double bond resulting in the formation of aldehyde or ketone which produces an unpleasant smell. This can be prevented by adding anti-oxidants like vitamin C or E.
  • All oils undergo hydrogenation in the presence of a nickel catalyst to give fats. This is a characteristic reaction of any oil.
  • Saponification i.e., treating a fat or oil with sodium hydroxide (NaOH) to hydrolyze it to give glycerol and soap.

Saponification number– The weight of potassium hydroxide (KOH) to saponify 1mg of fat or oil. This value indicates the molecular weight of the fat and is inversely proportional to the molecular weight. Saponification number of:

  • human fat- 195
  • butter- 220
  • coconut oil- 260.

Lodine number- of a fat is defined as number of grams of iodine (I2) taken by 100g of fat. It is an indication of unsaturation. Since iodine gets added on the double bond, higher the iodine number greater is the unsaturation. Iodine number of:

  • human fat- 65
  • butter- 27
  • coconut oil- 8
  • sunflower oil- 130
  • groundnut oil- 90

References –

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