Lipids Definition, Properties, Structure, Classification, and Functions

What are Lipids?

Lipids are a significant source of energy for the body besides their various other biochemical function and their function in cellular structure. Lipids are a heterogeneous group of water-insoluble (hydrophobic) organic particles. Lipids consist of fats, oils, steroids, waxes, and associated substances.

” Lipids might be defined as organic substances insoluble in water but soluble in organic solvents like chloroform, ether, and benzene. They are esters of fats with alcohol esters and are utilizable by the living organism.”

Properties of Lipids
  • Lipids might be either liquids or non-crystalline solids at room temperature.
  • Pure fats and oils are colorless, odor-free, and unsavory.
  • They are energy-rich natural molecules.
  • Insoluble in water.
  • Soluble in organic solvents like alcohol, chloroform, acetone, benzene, and so on.
  • No ionic charges
  • Solid triglycerols (Fats) have high percentages of saturated fatty acids.
  • Liquid triglycerols (Oils) have high proportions of unsaturated fats.
1.Hydrolysis of triglycerols

Triglycerols like any other esters react with water to form their carboxylic acid and alcohol– a process known as hydrolysis.


Triacylglycerols might be hydrolyzed by numerous treatments, the most common of which makes use of alkali or enzymes called lipases. Alkaline hydrolysis is called saponification due to the fact that one of the products of the hydrolysis is a soap, typically sodium or potassium salts of fats.


The carbon-carbon double bonds in unsaturated fatty acids can be hydrogenated by reacting with hydrogen to produce saturated fatty acids.


Unsaturated fats, whether they are totally free or combined as esters in fats and oils, react with halogens by addition at the double bond(s). The reaction leads to the decolorization of the halogen solution.

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The term rancid is applied to any fat or oil that establishes an unpleasant smell. Hydrolysis and oxidation reactions are accountable for triggering rancidity. Oxidative rancidity happens in triacylglycerols containing unsaturated fats.

Structure of Lipids

Lipids are made of the elements Carbon, Hydrogen, and Oxygen, however have a much lower percentage of water than other molecules such as carbohydrates. Unlike polysaccharides and proteins, lipids are not polymers– they lack a repeating monomeric system.

They are made from 2 molecules: Glycerol and Fatty Acids. A glycerol molecule is made up of three carbon atoms with a hydroxyl group attached to it and hydrogen atoms occupying the remaining positions. Fatty acids consist of an acid group at one end of the particle and a hydrocarbon chain, which is typically signified by the letter ‘R’. They may be saturated or unsaturated.

A fat is saturated if every possible bond is made with a Hydrogen atom, such that there exist no C= C bonds. Unsaturated fatty acids, on the other hand, do consist of C= C bonds. Monounsaturated fats have one C= C bond, and polyunsaturated have more than one C= C bond.


Structure of Triglycerides

Triglycerides are lipids including one glycerol molecule bonded with 3 fatty acid molecules. The bonds between the particles are covalent and are called Ester bonds. They are formed during a condensation reaction. The charges are equally dispersed around the molecule so hydrogen bonds do not form with water molecules making them insoluble in water.


Classification of Lipids

There are various approaches to categorizing lipids. The most commonly used category of lipids is modified from Bloor as follows:

  1. Simple lipids
  2. Complex or compound lipids
  3. Derived lipids
Simple Lipids

These are esters of fatty acids with different alcohols. Depending upon the type of alcohols, these are subclassified as:

  1. Neutral fats or triacylglycerol or triglycerides
  2. Waxes
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Neutral fats or triacylglycerol or triglycerides

These are esters of fats with alcohol glycerol, e.g., tripalmitin. Due to the fact that they are uncharged, they are termed as neutral fat. The fat we eat is mostly triglycerides. Fat in the liquid state is called an oil, e.g., vegetable oils like groundnut oil, mustard oil, corn oil, and so on.


Waxes are widespread as protective finishing’s on fruits and leaves. Some insects likewise produce wax. Chemically, waxes are mixtures of long-chain alkanes (with an odd number of carbons atoms ranging from C25 to C35) and alcohols, ketones, and esters of long-chain fatty acids. Waxes safeguard plants from water loss and abrasive damage. They likewise supply water barrier for bugs, birds, and animals such as sheep.

Complex or Compound Lipids

These are esters of fats, with alcohol including additional (prosthetic) groups. These are subclassified according to the type of prosthetic group present in the lipid as follows:

  1. Phospholipids
  2. Glycolipids
  3. Lipoproteins.

Phospholipids are derivatives of phosphatidic acid which are made up of glycerol, fatty acids, and phosphoric acid. Nitrogenous bases such as choline, ethanolamine, and serine are essential elements of phospholipids. They are widespread in bacteria, animal, and plant cells and are frequently connected with membranes. Phosphatidylcholine is one of the common phospholipids.



Lipids containing fat, alcohol sphingosine, and extra residue are carbohydrates with nitrogen bases. They do not include a phosphate group. These sugar including sphingolipids are likewise called glycosphingolipids.

  • Cerebrosides
  • Gangliosides

Lipoproteins are formed by the combination of a lipid with a prosthetic group protein, e.g., serum lipoproteins like:

  • Chylomicrons
  • Very low-density lipoprotein (VLDL)
  • Low-density lipoprotein (LDL)
  • High-density lipoprotein (HDL)
Derived Lipids

Derived lipids consist of the products acquired after the hydrolysis of simple and compound lipids which possess the qualities of lipids, e.g.

  • Fats
  • Steroids
  • Cholesterol
  • Lipid soluble vitamins and hormones
  • Ketone bodies
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Functions of Lipids

Lipids function as:

  • Storage form of energy: The fats and oils are utilized almost universally as stored forms of energy in living organisms.
  • Structural Lipids: Lipids are significant structural parts of membranes, e.g., phospholipids, glycolipids, and sterols.
  • Cholesterol, a sterol, is a precursor of lots of steroid hormones, vitamin D, and is likewise a crucial component of the plasma membrane.
  • Lipid serves as a thermal insulator in the subcutaneous tissues and around certain organs.
  • Nonpolar lipids serve as electrical insulators in neurons.
  • Lipids are very important dietary constituents because of the fat-soluble vitamins and essential fatty acids which exist in the fat of healthy foods.
  • Lipids assist in the absorption of fat-soluble vitamins (A, D, E, and K). They act as a solvent for the transport of fat-soluble vitamins.

Lipids are nonpolar organic molecules that are insoluble in water but soluble in nonpolar organic solvents like ether, alcohol and chloroform. These are heterogeneous group of compounds related to fatty acids.

Lipids are the important constituents of cell membranes. The most common lipids in animals are fats. The hydrophobic nature of lipids makes them best suited to be a structural component of cell membrane.

Lipids store high energy content. It can be said that lipids store doubles the amount of energy that carbohydrates do. This is due to high proportion of C-H bonds and low proportion of oxygen.

They serve as insulating layer e.g. waxes in exoskeleton of insects and cutin which is an additional protective layer on the cuticle of epidermis of some parts of plants like seeds, leaves and fruits.