Chapter 2 — Lipids
Key Concepts Quick Recap
| Type | Structure | Function |
|---|
| Triglyceride (fat/oil) | Glycerol + 3 fatty acids (ester bonds) | Energy storage; insulation |
| Phospholipid | Glycerol + 2 fatty acids + phosphate group + head | Cell membrane bilayer |
| Cholesterol | 4-ring steroid nucleus | Membrane fluidity; precursor of hormones |
| Wax | Long-chain fatty acid + long-chain alcohol | Protective coating |
| Saponification value | mg KOH to saponify 1 g fat | Measure of molecular weight of fatty acids |
| Iodine value | g iodine absorbed by 100 g fat | Measure of unsaturation |
I. Choose the Best Answer (MCQs)
1. The basic unit of a triglyceride is: (a) Glucose + Fatty acids (b) Glycerol + 3 Fatty acids (c) Glycerol + Phosphate (d) Cholesterol + Fatty acid
Answer(b) Glycerol + 3 Fatty acids Triglyceride (triacylglycerol): one glycerol esterified with 3 fatty acids via ester bonds. Hydrolysis (lipase) releases glycerol + 3 fatty acids.
2. Which of the following is an essential fatty acid? (a) Palmitic acid (b) Stearic acid (c) Linoleic acid (d) Oleic acid
Answer(c) Linoleic acid Essential fatty acids cannot be synthesised by the human body → must be obtained from diet. Essential: linoleic acid (ω-6, C18:2) and α-linolenic acid (ω-3, C18:3). Also arachidonic acid (conditionally essential).
3. Cell membranes are primarily composed of: (a) Triglycerides (b) Cholesterol only (c) Phospholipids arranged in a bilayer (d) Waxes
Answer(c) Phospholipids arranged in a bilayer Phospholipids: amphipathic — hydrophilic head (phosphate + polar group) + hydrophobic tail (2 fatty acids). In water, spontaneously form bilayer: heads face water, tails face interior.
4. Saponification is the hydrolysis of fats by: (a) Acid (b) Lipase enzyme (c) Alkali (NaOH or KOH) (d) Water alone
Answer(c) Alkali (NaOH or KOH) Saponification: fat + alkali → soap (sodium/potassium salt of fatty acid) + glycerol. Used in soap making. Saponification value: higher value = lower molecular weight fatty acids (e.g. coconut oil has high saponification value).
5. Iodine value measures: (a) Acidity of fat (b) Degree of unsaturation of fatty acids (c) Molecular weight of fat (d) Melting point of fat
Answer(b) Degree of unsaturation of fatty acids Iodine adds across C=C double bonds. Higher iodine value = more double bonds = more unsaturated. Linseed oil (high, ~180) > olive oil (~85) > butter (low, ~35).
6. Rancidity of fats is caused by: (a) Protein denaturation (b) Oxidation of unsaturated fatty acids (c) Saponification (d) Iodination
Answer(b) Oxidation of unsaturated fatty acids Rancidity: decomposition of fats by oxidation (auto-oxidation) or microbial action → aldehydes, ketones, acids with unpleasant smell/taste. Prevention: antioxidants (Vitamin E, BHA, BHT), refrigeration, vacuum packing.
7. Cholesterol is an example of: (a) Phospholipid (b) Triglyceride (c) Steroid (d) Wax
Answer(c) Steroid Cholesterol: 4-ring steroid nucleus. Functions: membrane fluidity, precursor of bile acids, steroid hormones (cortisol, testosterone, oestrogen, aldosterone), Vitamin D.
II. Short Answer Questions
Q1. Compare saturated and unsaturated fatty acids.
Answer
| Feature | Saturated Fatty Acids | Unsaturated Fatty Acids |
|---|
| Structure | No C=C double bonds | One or more C=C double bonds |
| Physical state | Solid at room temperature | Liquid (oils) at room temperature |
| Melting point | Higher | Lower |
| Examples | Palmitic (C16:0), Stearic (C18:0) | Oleic (C18:1), Linoleic (C18:2) |
| Health effects | Associated with cardiovascular disease | Protective (especially ω-3, ω-6) |
| Iodine value | Low (no double bonds to add to) | High |
Q2. What are phospholipids and why are they important?
Answer
Phospholipid: glycerophospholipid = glycerol + 2 fatty acids + phosphate + polar head group.
Amphipathic nature: hydrophilic head (charged phosphate + polar group) + hydrophobic tail (two fatty acids).
Importance: 1. Cell membrane: Form lipid bilayer — backbone of all cellular membranes. 2. Selective permeability: Hydrophobic core blocks ions and polar molecules; allows non-polar molecules. 3. Signalling: Phosphatidylinositol → IP₃ (second messenger in cell signalling). 4. Lung surfactant: Dipalmitoylphosphatidylcholine (DPPC) reduces surface tension in alveoli.
Exam Checklist
- Triglycerides: glycerol + 3 fatty acids; energy storage (9 kcal/g).
- Saturated (solid, no C=C) vs unsaturated (liquid, C=C bonds).
- Essential fatty acids: linoleic (ω-6) and α-linolenic (ω-3).
- Phospholipids: amphipathic; lipid bilayer of cell membranes.
- Cholesterol: steroid; membrane fluidity; precursor of steroid hormones.
- Saponification: fat + alkali → soap + glycerol.
- Iodine value: measures unsaturation; rancidity: oxidation of unsaturated fats.
