Fats are more concentrated form of storage of energy than carbohydrates. They are stored in adipose tissue, where it also serves as a thermal insulator in the subcutaneous tissues and around certain organs. Although the term lipid is sometimes used as a synonym for fat, it is in fact a subgroup of lipids called triglycerides and should not be confused with the term fatty acid.

Chemical composition

Fat is a complex molecule constituting a mixture of fatty acids and alcohol, generally glycerol. Fats contain more carbon, hydrogen and less oxygen as compared to carbohydrates. When oxidized , it gives 9.1 calories per gram fat which are approx. 2 1/4 times more than those supplied by one gram of carbohydrate.

Lipids can be broadly defined as any fat-soluble (hydrophobic), naturally-occurring molecules. The lipids are a heterogeneous group of compounds,including fats, oils, steroids, waxes, and related compounds.The term is more-specifically used to refer to fatty-acids and their derivatives (including tri-, di-, and monoglycerides and phospholipids) as well as other fat-soluble sterol-containing metabolites such as cholesterol. Lipids serve many functions in living organisms including nutrients, energy storage, structural components of cell membranes, and important signaling molecules. They are important dietary constituents not only because of their high energy value but also because of the fat-soluble vitamins and the essential fatty acids contained in the fat of natural foods.

Combinations of lipid and protein (lipoproteins) are important cellular constituents, occurring both in the cell membrane and in the mitochondria, and serving also as the means of transporting lipids in the blood.

Classification of fats (Lipids)

  1. Simple lipids- are neutral fats. These are chemically made up of triglycerides. Triglycerides contain a glycerol base with three fatty acids.
    • C3H5(OH)3 + 3 Molecules of fatty acids -> Neutral fat These neutral fats make up 98-99% of food and body fats.
  2. Compound lipids- These are chemically made up of simple lipids containing phosphorus, carbohydrate or protein. Such compounds are known as phospholipids, glycolipids and lipoproteins respectively.
    • Glycerol+ 3 fatty acids -> Fat (triglyceride)+ 3 water molecules
  3. Derived lipids- They are fat like substances produced from fats and fatty compounds, e.g, glycerol and fatty acids.
    • Glycerol- makes upto 10 % of fat. It is a water soluble base of Triglycerides or neutral fats.
    • Fatty acids-They are part of the basic structural unit of fats and there are two types of fatty acids : saturated or unsaturated fatty acids. examples of fatty acids are oleic acid, linolenic acid, linoleic acid and stearic acid etc.
  4. Unsaponifiable lipids- These include steroids ( fat related substances containing sterols, e.g cholestrol) , terpenoids, etc.

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