Lipids: Chemical Composition and Classification
A complete and comprehensive note on one of the most important topics of Biochemistry.
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Table of Contents
Introduction to Lipids
Definition of Lipids:
Organic compounds that are made up of fatty acids, and alcohols and may contain other compounds called Lipids.
General Characteristics of Lipids
- Lipids are non-soluble in water or any other polar solvent. So they are Hydrophobic.
- They are soluble in non-polar solvents only e.g. in ether, chloroform, benzene, acetone, etc.
- Being lighter they have a low density than water.
- They leave greasy marks on paper.
- Examples of lipids: Triglycerides, Phospholipids, Cholesterol, and Cholesterol derivatives, such as Steroid hormones, Bile salts, Fats-soluble vitamins, Prostaglandins (PG), etc.
Fatty Acids - The Major component of Lipids
- They are made up of a minimum of two carbon atoms.
- The chain length of lipid-forming fatty acids ranges from 4 to 24 carbon atoms.
- Fatty Acids, which occur in neutral fats, usually contain an even number of Carbon atoms.
Classification of Fatty acids
Fatty acids can broadly be divided into saturated fatty acids and unsaturated Fatty Acids.
1. Saturated Fatty Acids
The fatty acids that do not contain double bonds, between their carbon atoms, are known as saturated fatty acids. Examples: Butyric acid, Caproic acid, Palmitic acid, Stearic acid, etc.
2. Unsaturated Fatty Acids
The Fatty acids that contain at least one double bond between their Carbon atoms are called Unsaturated Fatty acids. Classified into:
- Mono-Unsaturated: Contain one double bond. e.g. Oleic Acid.
- Poly-Unsaturated: Contain more than one double bond. (Linoleic, Linolenic, Arachidonic acids).
Properties of Fatty Acids
Physical Properties
- For saturated Fatty acids, the melting point increases with the increase in the number of carbon atoms.
- For unsaturated fatty acids the melting point increases with the decrease in the number of double bonds.
- Solubility increases with the increase of the number of double bonds and vice versa.
Chemical Properties
Formation of salts: Upon boiling with alkali metals, Fatty acids form salts. The salts of sodium, potassium, calcium, and magnesium are soaps. This process is called saponification.
Classification of Lipids
1. Simple Lipids
Esters of Fatty acids with various alcohols. Further classified into neutral fats and waxes.
A. Neutral fats or Tri-acyl-glycerols (TAG): Esters of fatty acids with glycerol. "Acyl" refers to the fatty acid moiety. Can be mono, di, or triglycerols. Mixed TAGs have different fatty acid compositions.
Functions: Role in digestion, metabolic intermediates, and major storage/transport form. Physical properties: Liquid (Oils) or non-crystalline solids. Pure fats are colorless, odorless, and tasteless. Yellow butter is due to Carotene.
Chemical Properties of Neutral fats (TAGs)
- Hydrolysis: Yields glycerol and fatty acids (via lipases).
- Saponification: Formation of soaps from boiling with alkalis (NaOH, KOH).
- Hydrogenation: Adding hydrogen to double bonds (Oils to ghee).
- Halogenation: Reaction with Halogens at double bonds.
- Rancidity: Developing unpleasant odor and taste.
2. Compound Lipids
The lipids that contain other substances in addition to fatty acids and alcohols. Includes Phospholipids, Glycolipids, Gangliosides, Sulfo-lipids, Lipoproteins, and Lipopolysaccharides.
A. Phospholipids
Biologically important in membranes, lipoproteins, bile, and surfactants. Roles include Electron Transport Chain and Oxidative Phosphorylation.
i. Glycerol-phospholipids (GPL): Fatty acids + Glycerol + H3PO4 + Nitrogenous base. Types: Phosphatidic Acid (simplest/parent), Lecithins (with choline), Cephalins (with ethanol-amine/serine/inositol), and Plasmalogens (Platelet activation factors).
ii. Sphingo-phospholipids (SPL): Contain Sphingosine (18-C alcohol). Chief SPL is Sphingo-myelin (Ceramide + Phosphoryl-choline). Present in myelin sheath.
B. Glycolipids (Cerebrosides)
Ceramide sugars (Ceramide + Hexose). Mostly in brain white matter and myelin sheath. Cerebronic acid (24-C) is common in all types.
C. Gangliosides
Ceramide + Galactose or Glucose + N-Acetyl Neuraminic acid or N-Acetyl Hexosamine. Found in brain, spleen, RBCs. Role in tissue immunity and nerve impulse transmission.
D. Sulfo-lipids & VI. Lipopolysaccharides
Sulfo-lipids: Ceramide + Galactose + Sulfate. Lipopolysaccharides: Lipids + Polysaccharides (Bacterial cell walls).
E. Lipo-proteins
Neutral lipid core surrounded by apo-lipo-proteins, phospho-lipids, and cholesterol. Transport lipids in blood.
| Type | Apoproteins | Major Characteristics/Functions |
|---|---|---|
| Chylomicrons | apoB-48, C-II, E | Lowest density, largest size. Synthesized in intestinal cells. Major carrier of TAGs. |
| VLDL | A-1, B-100, C-II, E | Synthesized in liver. Carriers of TAG. Hydrolyzed to IDL in peripheral tissues. |
| IDL | -- | Denser than chylomicrons. High TAG content. Degraded to LDL. |
| LDL | B-100 | Highest cholesterol concentration. Strong association with Cardiovascular diseases. |
| HDL | Apo-A, AI, CII, E | Densest; highest protein. "Cardio-protective." Carries cholesterol to liver. |
Summary: Comparison of Phospholipids
| Phospholipid Type | Backbone Alcohol | Distinguishing Components | Biological Role/Location |
|---|---|---|---|
| Phosphatidic Acid | Glycerol | H3PO4 + Fatty Acids | Simplest form; parent compound for other GPLs. |
| Lecithins | Glycerol | Choline | Abundant in serum/bile; lung surfactant; emulsifier. |
| Cephalins | Glycerol | Ethanol-amine or Serine | High concentration in brain tissue; helps blood clotting. |
| Plasmalogens | Glycerol | Ether-linked alkene | Platelet activation; causes neutrophils to generate radicals. |
| Sphingomyelin | Sphingosine | Ceramide + Phosphoryl-choline | Primary component of the myelin sheath in nerves. |
Metabolism of Chylomicrons
Chylomicrons are synthesized in the intestinal cells from dietary lipids. Because they have the highest concentration of Tri-acyl-Glycerol (TAG) and very few proteins, they are the largest and least dense of all lipoproteins.
- Synthesis: Formed in the intestine with major lipoproteins apoB-48, C-II, and E.
- Hydrolysis: As they circulate, the enzyme Lipoprotein Lipase breaks down the TAGs into Fatty Acids and Glycerol.
- Fate of Fatty Acids: These are either oxidized for immediate energy or converted back to TAGs for storage.
- Fate of Glycerol: Used to synthesize new TAGs or converted to Dihydroxy acetone Phosphate (DHAP), which enters glycolysis or gluconeogenesis in the liver.
- Remnant Clearance: The "Chylomicron Remnants" are taken up by the liver and degraded by Lysosomal enzymes. Their components (amino acids, cholesterol, etc.) are released into the cytosol for reuse.
Metabolism of VLDL and IDL
Very-low Density Lipoproteins (VLDL) are the liver's primary way of exporting internally synthesized fats to the rest of the body.
- Liver Synthesis: The liver combines TAG, cholesterol, and phospholipids with apoproteins A-1, B-100, C-II, and E.
- Peripheral Processing: In adipose and muscle tissues, Lipoprotein Lipase hydrolyzes the VLDL's TAG content.
- Transformation to IDL: Once the VLDL loses a significant portion of its TAGs, it becomes an Intermediate-density Lipoprotein (IDL).
- Final Degradation: In the blood, IDL is further stripped of lipids until it becomes Low-density Lipoprotein (LDL).
Clinical Note: While VLDL is a major carrier of TAG, its byproduct LDL has the highest concentration of cholesterol and is strongly associated with cardiovascular disease risk in inactive individuals.
The Role of HDL (The Reverse Pathway)
High-Density Lipoprotein (HDL) acts as the cleanup crew. Released from the liver with proteins Apo-A, AI, CII, and E, it is the densest lipoprotein because it has the most protein and the least TAG.
It carries cholesterol from peripheral tissues back to the liver. Here, the cholesterol is either used for VLDL synthesis or converted into bile salts, making HDL "cardio-protective" against hyperlipidemia and coronary heart disease.
Laboratory Identification of Lipids
In biochemistry, several qualitative tests are used to detect the presence of lipids and differentiate between various types of fatty acids and cholesterol.
1. Solubility Test
This is the most basic test based on the hydrophobic nature of lipids. Lipids are insoluble in water (polar) but dissolve readily in organic solvents like ether, chloroform, or benzene.
2. Translucent Spot Test (Grease Spot)
As mentioned in your notes, lipids leave greasy marks on paper. When a drop of lipid is placed on paper, it creates a translucent spot that does not disappear even after drying, as lipids do not evaporate like water.
Chemical Color Tests
| Test Name | Reagent Used | Positive Result | Principle |
|---|---|---|---|
| Sudan III Test | Sudan III Dye | Shiny Red Droplets | A fat-soluble dye that stains lipid droplets red. |
| Salkowski Test | Conc. H₂SO₄ | Red/Cherry Color | Detects cholesterol. Sulfuric acid dehydrates cholesterol to form a colored product. |
| Acrolein Test | Potassium Bisulphate | Pungent Irritating Odor | Detects Glycerol. When heated, glycerol is dehydrated to Acrolein. |
| Iodine Test | Iodine Solution | Disappearance of Pink/Brown | Tests for Unsaturation. Iodine adds to the double bonds of fatty acids. |
Note on Saponification Value: This is a quantitative measure that represents the number of milligrams of KOH required to saponify 1 gram of fat. It helps in determining the molecular weight of the fatty acids present in the lipid.
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