LIPIDS: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future pharmacists and healthcare professionals!
The word lipid is derived from the Greek word lipos meaning fat; universally present in all plants and animal cells. They are naturally occurring waxy, greasy, or oily organic compounds and are known as oils and fats. Lipids are hydrophobic in nature, i.e., insoluble in water but soluble in non-polar solvents (chloroform, benzene, ether, etc.).
Dpharmguru’s exam insights:
In my years of teaching biochemistry, I have observed that students often get confused between fats and oils. Remember: Fats are solid at room temperature (animal sources), while oils are liquid at room temperature (plant sources). This distinction is frequently tested in exams!
CLASSIFICATION OF LIPIDS
1. Simple Lipids
- Fats and Oils: Esters of fatty acids containing glycerol. Examples: tristearin, triolein, tripalmitin.
- Waxes: Esters of fatty acids containing high molecular weight monohydric alcohols. Examples: paraffin wax, beeswax.
2. Compound Lipids
- Phospholipids: Consist of fatty acids, an alcohol, and a phosphoric acid residue. Examples: lecithins, cephalins, plasmalogens.
- Glycolipids (Glycosphingolipids): Consist of a fatty acid, sphingosine, and carbohydrate. Examples: gangliosides, cerebrosides.
3. Derived Lipids
These are hydrolysed derivatives of simple or compound lipids. Examples: fatty acids, glycerol, steroids, terpenes, carotenoids.
QUALITATIVE TESTS OF LIPIDS
- Grease Spot Test: A greasy spot penetrating the paper is formed as lipids do not wet paper.
- Test for Free Fatty Acids: Pink colour disappears due to neutralisation of alkali by free fatty acids.
- Emulsification: Oil suspends in bile salt solution in the form of minute droplets.
- Saponification Test: Hydrolysis of esters by alkali yields soap.
- Tests for Unsaturation: Bromine water decolourisation indicates unsaturation.
- Tests for Glycerol: Acrolein test (pungent odour) and Dichromate test (blue colour).
Dpharmguru’s exam insights:
Remember: The Acrolein test is specific for glycerol (pungent odour). The Grease Spot test is a simple test for lipids. These are frequently asked in practical exams!
FUNCTIONS OF LIPIDS
- Storage: Reserve energy in the form of triglycerides in adipocytes.
- Energy Source: Provide 9 kcal of energy per gram (2x more than proteins/carbohydrates).
- Cell Membranes: Important components of cell membranes.
- Vitamins: Source for fat-soluble vitamins (A, D, E, K).
- Electrical Insulation: Act as electrical insulators to nerve fibres (myelin sheath).
- Thermal Insulation: Subcutaneous fat provides insulation and protection from cold.
- Signalling: Act as signalling molecules.
- Essential Fatty Acids: Supply essential fatty acids not synthesised in the body.
TRIGLYCERIDES (FATS AND OILS)
Triglycerides are made up of three fatty acids and glycerol. They are our primary source of energy.
Structure of Triglycerides
Triglycerides are esters containing 3 molecules of one or more different fatty acids linked to the alcohol glycerol. If a triglyceride is solid at 25°C, it is a fat; if liquid, it is an oil.
Properties of Triglycerides
- Physical: Colourless, odourless, tasteless; lighter than water; poor conductors of heat and electricity.
- Chemical: Hydrolysis (saponification), hydrogenation, oxidation (rancidity).
Fat Constants
- Rancidity: Unpleasant odour and taste developed on ageing due to hydrolysis or oxidation.
- Acid Number: mg of KOH required to neutralise free fatty acids in 1g of fat.
- Reichert-Meissl Number: ml of 0.1N KOH required to neutralise soluble volatile fatty acids from 5g of fat.
- Iodine Number: Grams of iodine absorbed by 100g of fat (measures unsaturation).
- Saponification Number: mg of KOH required to saponify 1g of fat.
Dpharmguru’s exam insights:
Remember: Iodine number indicates the degree of unsaturation (higher iodine number = more unsaturation). Saponification number indicates the average molecular weight of fatty acids. These are frequently tested in exams!
FATTY ACIDS
Chemically, fatty acids are monocarboxylic acids with aliphatic carbon chain. They are obtained in esterified form from animal or vegetable fat and oil or wax.
Chemical Classification of Fatty Acids
- Saturated Fatty Acids: Single bonds, maximum hydrogen. General formula: CH₃-(CH₂)n-COOH.
- Unsaturated Fatty Acids: One or more double bonds. Includes:
- Trans Fatty Acids: Generated when liquid vegetable oils are hydrogenated.
- Cis Fatty Acids: Divided into Monoenoic (one double bond) and Polyunsaturated Fatty Acids (PUFA) (more than one double bond).
Nutritional Classification of Fatty Acids
- Essential Fatty Acids: Cannot be produced in human body; must be consumed through food. Examples: α-linolenic acid, linoleic acid, arachidonic acid.
- Non-essential Fatty Acids: Generated in large quantities in human bodies. Not necessary to include in diet.
CHOLESTEROL
Cholesterol primarily belongs to a group of compounds known as sterols. It is found in foods of animal origin and is absent in food of plant origin.
Functions of Cholesterol
- Constituent of cell membranes; prevents crystallisation of hydrocarbons.
- Maintains permeability of cell membrane.
- Synthesises androgens and oestrogens (sex hormones).
- Participates in synthesis of adrenal hormones (cortisol, corticosterone, aldosterone).
- Aids in bile production.
- Converts sunlight into vitamin D.
- Insulates nerve fibres.
Dpharmguru’s exam insights:
Remember: Cholesterol is the precursor for bile acids, vitamin D, and steroid hormones. Also, LDL is “bad cholesterol” (transports cholesterol to tissues), while HDL is “good cholesterol” (transports cholesterol from tissues to liver). This is frequently tested!
LIPOPROTEINS
Lipoproteins are lipid and protein-containing molecular complexes (conjugated proteins). In blood plasma, they act as lipid transporters.
Types of Lipoproteins
- Chylomicrons: Exogenous (dietary) triacylglycerol transporters; highest lipid content (99%), lowest protein content (1%).
- VLDL (Very Low Density Lipoproteins): Produced in liver and intestine; transport endogenously synthesised triacylglycerols.
- LDL (Low Density Lipoproteins): Transport cholesterol from liver to the rest of the body.
- HDL (High Density Lipoproteins): Transport cholesterol from peripheral tissues to the liver.
- Free Fatty Acids — Albumin: Free fatty acids attached to albumin in circulation.
Composition of Lipoproteins
| Characteristics | Chylomicrons | VLDL | LDL | HDL |
|---|---|---|---|---|
| Protein (%) | 2 | 10 | 20 | 40 |
| Lipid (total) (%) | 98 | 90 | 80 | 60 |
| Triacylglycerol (%) | 88 | 55 | 12 | 12 |
| Cholesterol (%) | 4 | 24 | 59 | 40 |
| Phospholipids (%) | 8 | 20 | 28 | 47 |
Dpharmguru’s exam insights:
Lipids are essential biomolecules that serve as energy stores, membrane components, and signalling molecules. In exams, pay special attention to: (1) The classification of lipids, (2) The differences between saturated and unsaturated fatty acids, (3) The functions of cholesterol, and (4) The types and roles of lipoproteins (LDL and HDL). Remember: “Lipids are the energy reservoirs of the body!”
REFERENCES AND FURTHER READING
- Lehninger, A. L., Nelson, D. L., & Cox, M. M. (2017). Lehninger Principles of Biochemistry (7th ed.). W. H. Freeman.
- Berg, J. M., Tymoczko, J. L., & Stryer, L. (2019). Biochemistry (9th ed.). W. H. Freeman.
- Satyanarayana, U., & Chakrapani, U. (2021). Biochemistry (5th ed.). Elsevier.
- Vasudevan, D. M., Sreekumari, S., & Vaidyanathan, K. (2022). Textbook of Biochemistry for Medical Students (9th ed.). Jaypee Brothers.
- National Institutes of Health (NIH). (2022). Lipid Metabolism Resources. Retrieved from https://www.nih.gov.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.
written by:
Dr. N. Sujith Kumar
