Simple lipids: Esters of fatty acids with importance to the body as the chief alcohols.

  • These are mainly of two types concentrated storage form of energy, besides their role in cellular structure and various other biochemical functions.
  • As such, lipids are a heterogeneous group of compounds and, therefore, it is rather difficult to define them precisely.

Fats and oils (triacylglycerol’s): These are esters of fatty acids with glycerol. The difference between fat and oil is only physical.

  • Thus, oil is a liquid while fat is a solid at room temperature.
  • Lipids may be regarded as organic substances relatively insoluble in water, soluble in organic solvents (alcohol, ether etc.), actually or potentially related to fatty acids and utilized by the living cells.

Waxes: Esters of fatty acids (usually long chain) with alcohols other than glycerol. These alcohols may be aliphatic or alicyclic. Acetyl alcohol is most commonly found in waxes.

  • Unlike the polysaccharides, proteins and nucleic acids, lipids are not polymers. Further, lipids are mostly small molecules.

Complex (or compound) lipids : These are esters of fatty acids with alcohols containing such as phosphate, nitrogenous base, carbohydrate, protein etc. additional groups.


  • They are further divided as follows Lipids are broadly classified (modified from Bloor) into simple, complex, derived and miscellaneous lipids, which are further subdivided into different groups.
  • Phospholipids: They contain phosphoric acid and frequently a nitrogenous base. This is in addition to alcohol and fatty acids.
  • Glycerophospholipids : These phospholipids protect the internal organs, serve as lipids contain glycerol as the alcohol insulating materials and give shape and smooth e.g., lecithin, cephalin. appearance to the body.
  • Sphingophospholipids : Sphingosine is the alcohol in this group of phospholipids e.g., sphingomyelin.


  • Glycolipids: These lipids contain a fatty acid, carbohydrate and nitrogenous base. hydrocarbon side chain.
  • They are the simplest Fatty acids are carboxylic acids with The alcohol is sphingosine, hence they form of lipids. are also called as glycosphingolipids.
  • Glycerol and phosphate are absent e.8- Occurrence cerebrosides, gangliosides.
  • Lipoproteins: Macromolecular complexes Fatty acids mainly occur in the esterified form as major constituents of various lipids.
  • They are of lipids with proteins. also present as free (unesterified) fatty acids.
  • Other complex lipids: Sulfolipids, amino- Fatty acids of animal orgin are much simpler in lipids and lipopolysaccharides are among structure in contrast to those of plant origin which often contain groups such as epoxy, keto,the other complex lipids.

Derived lipids: These are the derivatives hydroxyl and cyclopentane rings. obtained on the hydrolysis of group 1 and group.

  • lipids which possess the characteristics of Even and odd carbon fatty acids lipids.
  • These include glycerol and other alcohols, fatty acids, mono- and diacylgiycerols,  lipids are of even carbons (usually 14C- 20C).
  • The soluble vitamins, steroid hormones, hydro- This is due to the fact that biosynthesis of fatty carbons and ketone bodies.
  • Most of the fatty acids that occur in natural acids mainly occurs with the sequential addition
  • Miscellaneous lipids: These include a of 2 carbon units. Palmitic acid (16C) and large number of compounds possessing the stearic acid (18C) are the most common.
  • Among characteristics of lipids egcarotenoids, the odd chain fatty acids, propionic acid (3 squalene, hydrocarbons such as pentacosane (in and valeric acid (5C) are well known. bees wax), terpenes etc.

Neutral lipids: The lipids which are Saturated and unsaturated uncharged are referred to as neutral lipids.

  • These fatty acids are mono-, di-, and triacylglycerol’s, cholesterol and cholesteryl esters. Saturated fatty acids do not contain double bonds, while unsaturated fatty acids contain one or more double bonds.
  • Both saturated and unsaturated fatty acids almost equally occur in the natural lipids.
  • They are the concentrated fuel reserve of bond are monounsaturated, and those with 2 or 3 saturated molecules.


  • Lipids perform several important functions the body (triacylglycerols). more double bonds are collectively known as
  • Lipids are the constituents of membrane Structure permeability (phospholipids and cholesterol).
  • The polyunsaturated fatty acids (PUFA). membrane Nomenclature of fatty acids and regulate the naming of a fatty acid (systematic name).
  • They serve as a source of tat soluble is based on the hydrocarbon from which it is vitamins (A, D, E and K). derived.
  • Lipids are important as cellular metabolic suffix -anoic (e.g., octanoic acid) while the regulators (steroid hormones and prostaglandins). unsaturated fatty acids end with a suffix -enoic.


  • The fatty acids that cannot be synthesized by the body and, therefore, should be supplied in the diet are known as essential fatty acids (EFA).
  • They are polyunsaturated fatty acids, namely linoleic acid (18 2; 9, 12) and linolenic acid (18 3; 9, 12, 15).
  • Arachidonic acid (20 : 4; 5, 8, 11, 14) becomes essential, if its precursor linoleic acid is not provided in the diet in sufficient amounts.
  • The structures of EFA are given in the Biochemical basis for essentiality: Linoleic acid and linolenic acid are essential since humans lack the enzymes that can introduce double bonds beyond carbons 9 to 10.
  • Functions of EFA: Essential fatty acids are required for the membrane structure and function, transport of cholesterol, formation of lipoproteins, prevention of fatty liver etc.
  • They are also needed for the synthesis of another important group of compounds, eicosanoids.


  • Triacylglycerols (formerly triglycerides) are the esters of glycerol with fatty acids.
  • The fats and oils that are widely distributed in both plants and animals are chemically triacylglycerols.
  • They are insoluble in water and non-polar in character and commonly known as neutral fats.
  • Fats as stored fuel: Triacylglycerols are the most abundant group of lipids that primarily function as fuel reserves of animals.
  • The fat reserve of normal humans (men 20%, women 25% by weight) is sufficient to meet the body’s caloric requirements for 2-3 months.
  • Fats primarily occur in adipose tissue: Adipocytes of adipose tissue-predominantly found in the subcutaneous layer and in the abdominal cavity are specialized for storage of triacylglycerols.
  • The fat is stored in the form of globules dispersed in the entire cytoplasm.
  • Surprisingly, triacylglycerols are not the structural. Simple triacylglycerols contain the same of fatty acid residue at all the three carbons tristearoyl glycerol or tristearin.
  • Mixed triacylglycerols are more com They contain 2 or 3 different types of fatty residues.
  • In general, fatty acid attached to saturated, that attached to C2 is unsaturated molecules while that on C3 can be either. components of biological membranes.


  • Cholesterol, exclusively found in animals, is the most abundant animal sterol.
  • It is widely distributed in all cells and is a major component of cell membranes and lipoproteins.
  • The Cholesterol was first isolated lipoprotein cholesterol literally means ‘solid alcohol from bile.