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Lipids- Properties, Structure, Classification and Functions
- Lipids are a heterogeneous group of organic compounds that are insoluble in water and soluble in non-polar organic solvents.
- They naturally occur in most plants, animals, microorganisms and are used as cell membrane components, energy storage molecules, insulation, and hormones.
Properties of Lipids
- Lipids may be either liquids or non-crystalline solids at room temperature.
- Pure fats and oils are colourless, odorless, and tasteless.
- They are energy rich organic molecules
- Insoluble in water
- Soluble in organic solvents like alcohol, chloroform, acetone, benzene etc.
- No ionic charges
- Solid triglycerols (Fats) have high proportions of saturated fatty acids.
- Liquid triglycerols (Oils) have high proportions of unsaturated fatty acids.
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 may be hydrolysed by several procedures, the most common of which utilizes alkali or enzymes called lipases. Alkaline hydrolysis is termed saponification because one of the products of the hydrolysis is a soap, generally sodium or potassium salts of fatty acids.
The carbon-carbon double bonds in unsaturated fatty acids can be hydrogenated by reacting with hydrogen to produce saturated fatty acids.
Unsaturated fatty acids, whether they are free or combined as esters in fats and oils, react with halogens by addition at the double bond(s). The reaction results in the decolourisation of the halogen solution.
The term rancid is applied to any fat or oil that develops a disagreeable odour. Hydrolysis and oxidation reactions are responsible for causing rancidity. Oxidative rancidity occurs in triacylglycerols containing unsaturated fatty acids.
Structure of Lipids
- Lipids are made of the elements Carbon, Hydrogen and Oxygen, but have a much lower proportion of water than other molecules such as carbohydrates.
- Unlike polysaccharides and proteins, lipids are not polymers—they lack a repeating momomeric unit.
- They are made from two molecules: Glycerol and Fatty Acids.
- A glycerol molecule is made up from 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 molecule and a hydrocarbon chain, which is usually denoted by the letter ‘R’.
- They may be saturated or unsaturated.
- A fatty acid is saturated if every possible bond is made with a Hydrogen atom, such that there exist no C=C bonds.
- Saturated fatty acids on the other hand do contain C=C bonds. Monounsaturated fatty acids have one C=C bond, and polyunsaturated have more than one C=C bond.
Structure of Triglycerides
- Triglycerides are lipids consisting of one glycerol molecule bonded with three fatty acid molecules.
- The bonds between the molecules are covalent and are called Ester bonds.
- They are formed during a condensation reaction.
- The charges are evenly distributed around the molecule so hydrogen bonds to not form with water molecules making them insoluble in water.
Classification of Lipids
Lipids can be classified according to their hydrolysis products and according to similarities in their molecular structures. Three major subclasses are recognized:
- Simple lipids
(a) Fats and oils which yield fatty acids and glycerol upon hydrolysis.
(b) Waxes, which yield fatty acids and long-chain alcohols upon hydrolysis.
Fats and Oils
- Both types of compounds are called triacylglycerols because they are esters composed of three fatty acids joined to glycerol, a trihydroxy alcohol.
- The difference is on the basis of their physical states at room temperature. It is customary to call a lipid a fat if it is solid at 25°C, and an oil if it is a liquid at the same temperature.
- These differences in melting points reflect differences in the degree of unsaturation of the constituent fatty acids.
- A wax is an ester of a long-chain alcohol (usually mono-hydroxy) and a fatty acid.
- The acids and alcohols normally found in waxes have chains of the order of 12-34 carbon atoms in length.
- Compound lipids
(a) Phospholipids, which yield fatty acids, glycerol, amino alcohol sphingosine, phosphoric acid and a nitrogen-containing alcohol upon hydrolysis.
They may be glycerophospholipids or sphingophospholipid depending upon the alcohol group present (glycerol or sphingosine).
(b) Glycolipids, which yield fatty acids, sphingosine or glycerol, and a carbohydrate upon hydrolysis.
They may also be glyceroglycolipids or sphingoglycolipid depending upon the alcohol group present (glycerol or sphingosine).
- Derived lipids:
Hydrolysis product of simple and compound lipids is called derived lipids. They include fatty acid, glycerol, sphingosine and steroid derivatives.
Steroid derivatives are phenanthrene structure that are quite different from lipids made up of fatty acids.
Functions of Lipids
It is established that lipids play extremely important roles in the normal functions of a cell. Not only do lipids serve as highly reduced storage forms of energy, but they also play an intimate role in the structure of cell membrane and organellar membranes. Lipids perform many functions, such as:
- Energy Storage
- Making Biological Membranes
- Protection – e.g. protecting plant leaves from drying up
- Acting as hormones
- Act as the structural component of the body and provide the hydrophobic barrier that permits partitioning of the aqueous contents of cell and sub cellular structures.
- Lipids are major sources of energy in animals and high lipid-containing seeds.
- Activators of enzymes eg. glucose-6-phosphatase, stearyl CoA desaturase and ω-mono oxygenase, and β-hydroxy butyric dehydrogenase (a mitochondrial enzyme), require phosphatidyl choline micelles for activation.
- Smith, C. M., Marks, A. D., Lieberman, M. A., Marks, D. B., & Marks, D. B. (2005). Marks’ basic medical biochemistry: A clinical approach. Philadelphia: Lippincott Williams & Wilkins.