Lipids are vital components of the human body involved in energy storage, hormone production, membrane structure and nutrient absorption. Understanding triglycerides, fatty acids, cholesterol, lipoproteins and their functions helps pharmacy students connect biochemistry with health and disease management.
ACTUAL NOTES:
Lipids: Definition, Classification, Triglycerides, Fatty Acids, Cholesterol, Lipoproteins and Functions
Lipids are essential biomolecules that act as the body’s major energy reserve, protect organs, form cell membranes and help absorb fat-soluble vitamins. These notes explain lipids in simple, student-friendly language for D.Pharmacy learners.
Definition of Lipids
Lipids are organic compounds made mainly of carbon, hydrogen and oxygen. They are insoluble in water but soluble in organic solvents like chloroform and ether. In simple terms, lipids are fats and fat-like substances present in food and the body.
Classification of Lipids
1. Simple Lipids
- Fats and oils: Triglycerides made of glycerol + fatty acids (e.g., butter, vegetable oil)
- Waxes: Esters of fatty acids with long-chain alcohols (e.g., beeswax)
2. Compound (Complex) Lipids
- Phospholipids: Contain phosphate group (e.g., lecithin in cell membranes)
- Glycolipids: Lipids + carbohydrates (important in the brain and nerves)
- Lipoproteins: Lipid + protein complexes that transport fats in blood
3. Derived Lipids
- Fatty acids
- Steroids like cholesterol
- Fat-soluble vitamins: A, D, E, K
Structure and Properties of Triglycerides
Triglycerides are the most common dietary lipids. They are formed by:
Glycerol + 3 Fatty Acids → Triglyceride
Structure: One glycerol molecule forms ester bonds with three fatty acid molecules.
Properties of Triglycerides
- Oils are liquid at room temperature (rich in unsaturated fatty acids).
- Fats are solid at room temperature (rich in saturated fatty acids).
- Triglycerides are insoluble in water but soluble in organic solvents.
- They provide 9 kcal of energy per gram (more than carbohydrates and proteins).
- Undergo hydrolysis to form glycerol and fatty acids.
Classification of Fatty Acids
1. Based on Chemical Structure
a) Saturated Fatty Acids
No double bonds.
Examples: Stearic acid, Palmitic acid.
b) Unsaturated Fatty Acids
Contain one or more double bonds.
Examples: Oleic acid, Linoleic acid, Linolenic acid.
2. Based on Nutritional Requirements
a) Essential Fatty Acids (EFAs)
Must be taken through diet because the body cannot synthesize them.
Examples: Linoleic acid, Linolenic acid, Arachidonic acid.
Importance: Needed for growth, skin health, brain development and prostaglandin synthesis.
b) Non-Essential Fatty Acids
Body can synthesize them.
Examples: Oleic acid, Palmitic acid.
Cholesterol: Structure and Functions
Cholesterol is a steroid lipid found in cell membranes and body fluids. It has a four-ring structure (steroid nucleus) with a hydrocarbon tail and hydroxyl group.
Functions of Cholesterol
- Essential part of cell membranes
- Precursor for steroid hormones (cortisol, estrogen, testosterone)
- Required for bile acid formation
- Needed for synthesis of Vitamin D
- Maintains membrane fluidity and stability
Lipoproteins
Lipoproteins are complexes made of lipids + proteins. They transport fats (which are insoluble in blood) throughout the body.
Types, Composition and Functions
1. Chylomicrons
Composition: Highest fat content, very low protein.
Function: Transport dietary triglycerides from intestine to tissues.
2. VLDL (Very Low-Density Lipoproteins)
Composition: Rich in triglycerides.
Function: Carry triglycerides from liver to tissues.
3. LDL (Low-Density Lipoproteins)
Composition: High cholesterol content.
Function: Delivers cholesterol to cells; known as “bad cholesterol” because excess leads to heart disease.
4. HDL (High-Density Lipoproteins)
Composition: High protein content.
Function: Removes excess cholesterol from tissues; known as “good cholesterol.”
Qualitative Tests for Lipids
- Solubility Test: Lipids dissolve in organic solvents but not in water.
- Grease Spot Test: Lipids leave a translucent spot on paper.
- Saponification Test: Reaction with alkali forms soap and glycerol.
- Sudan III Test: Lipids stain red with Sudan III dye.
- Acrolein Test: On heating with potassium bisulfate, fats give a pungent smell (acrolein).
Biological Functions of Lipids
- Energy storage: Provide 9 kcal/g energy.
- Structural role: Form cell membranes (phospholipids).
- Insulation: Maintain body temperature.
- Protection: Cushion organs and nerves.
- Absorption: Help absorb fat-soluble vitamins A, D, E and K.
- Hormone synthesis: Cholesterol forms steroid hormones.
- Cell signaling: Lipids participate in cell communication.
