Lipids: Classification, Properties, and Biological Functions
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Topic 12 — Key Facts for Kenyatta University (Kenya) Core concept: Lipids are a diverse class of hydrophobic biological molecules that are soluble in non-polar organic solvents but insoluble in water; they include fatty acids, triglycerides, phospholipids, steroids, and fat-soluble vitamins High-yield point: Saturated fatty acids have no C=C double bonds (straight chains, pack tightly → high melting point); unsaturated fatty acids have C=C double bonds (kinks, pack loosely → low melting point); trans fats are unsaturated fatty acids with trans double bonds — health risk不同于 natural cis unsaturated fats ⚡ Exam tip: Triacylglycerols (fats and oils) are esters of glycerol (propane-1,2,3-triol) with three fatty acids; saponification of a triglyceride with NaOH yields glycerol and three fatty acid sodium salts (soap); iodine number measures the degree of unsaturation of a fat
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Lipids: Hydrophobic Biological Molecules
Lipids are defined by their physical property (solubility) rather than a specific functional group. They are compounds that are soluble in non-polar organic solvents (chloroform, ether, acetone) but insoluble in water. This hydrophobic nature arises because lipids consist primarily of long hydrocarbon chains (non-polar) with few polar functional groups.
Lipids play several essential biological roles:
- Energy storage (triglycerides)
- Structural components of cell membranes (phospholipids)
- Hormones and signalling molecules (steroids, eicosanoids)
- Vitamins (A, D, E, K — the fat-soluble vitamins)
- Thermal insulation (subcutaneous adipose tissue)
- Protective padding (fat around organs)
Classification of Lipids
1. Simple Lipids:
- Fats and Oils (triacylglycerols/triglycerides): Esters of glycerol with fatty acids
- Waxes: Esters of long-chain fatty alcohols with fatty acids
2. Complex Lipids:
- Phospholipids: Glycerol backbone + 2 fatty acids + phosphate + nitrogenous base
- Glycolipids: Lipid + carbohydrate (no phosphate)
- Sphingolipids: Based on sphingosine backbone (not glycerol)
3. Derived Lipids:
- Fatty Acids
- Steroids (cholesterol, hormones)
- Fat-soluble vitamins (A, D, E, K)
- Eicosanoids (prostaglandins, thromboxanes, leukotrienes)
- Terpenes (essential oils, rubber, beta-carotene)
Fatty Acids
Fatty acids are carboxylic acids with long hydrocarbon chains. The general formula is R–COOH where R is a long-chain alkyl group (typically C₄ to C₃₀).
Saturated Fatty Acids (no C=C double bonds):
| Name | Formula | Melting Point (°C) | Source |
|---|---|---|---|
| Butyric acid | C₃H₇COOH | −7.9 | Butter |
| Lauric acid | C₁₁H₂₃COOH | 44.2 | Coconut oil |
| Myristic acid | C₁₃H₂₇COOH | 54.4 | Nutmeg butter |
| Palmitic acid | C₁₅H₃₁COOH | 62.9 | Palm oil |
| Stearic acid | C₁₇H₃₅COOH | 69.6 | Animal fat |
| Arachidic acid | C₁₉H₃₉COOH | 75.4 | Peanut oil |
Unsaturated Fatty Acids (contain C=C double bonds):
| Name | Formula | Melting Point (°C) | Notes |
|---|---|---|---|
| Oleic acid | C₁₈H₃₄O₂ (one cis C=C) | 13.4 | Olive oil (most abundant) |
| Linoleic acid | C₁₈H₃₂O₂ (two cis C=C) | −5 | ω-6 essential fatty acid |
| Linolenic acid | C₁₈H₃₀O₂ (three cis C=C) | −11 | ω-3 essential fatty acid |
| Arachidonic acid | C₂₀H₃₂O₂ (four C=C) | −49 | Precursor of eicosanoids |
Essential Fatty Acids: Linoleic acid (ω-6) and α-linolenic acid (ω-3) cannot be synthesised by the human body and must be obtained from the diet.
Cis vs Trans Fatty Acids:
- Natural unsaturated fatty acids have cis double bonds (both H atoms on the same side of the double bond)
- This causes a “kink” in the chain, preventing tight packing → low melting point → liquid at room temperature
- Trans fats (partially hydrogenated oils) have trans double bonds (H atoms on opposite sides) → straighten the chain → higher melting point → solid at room temperature
- Trans fats are associated with increased cardiovascular disease risk
⚡ Health Note: Industrially produced trans fats (in margarine, fried foods, baked goods) raise LDL (“bad” cholesterol) and lower HDL (“good” cholesterol), significantly increasing coronary heart disease risk. Many countries have banned or restricted partially hydrogenated oils.
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Triglycerides (Fats and Oils)
Structure
A triglyceride is an ester of glycerol (propane-1,2,3-triol) with three fatty acids:
CH₂–O–COR₁
CH–O–COR₂ (three ester linkages)
CH₂–O–COR₃Types of Triglycerides:
- Simple triglyceride: Same fatty acid in all three positions
- Mixed triglyceride: Two or three different fatty acids
- Most natural fats and oils are mixed triglycerides
Fats vs Oils:
| Property | Fats | Oils |
|---|---|---|
| Physical state at 20°C | Solid (more saturated) | Liquid (more unsaturated) |
| Source | Usually animal | Usually plant |
| Examples | Lard, butter, tallow | Olive oil, sunflower oil |
| Melting point | Higher (more saturated) | Lower (more unsaturated) |
Chemical Reactions of Triglycerides
1. Saponification (Alkaline Hydrolysis): Triglyceride + NaOH/KOH → glycerol + fatty acid salts (soap):
Triglyceride + 3NaOH → Glycerol + 3 R–COONa (soap)Hard soap: Sodium salts of fatty acids (solid) Soft soap: Potassium salts of fatty acids (liquid)
Soap Mechanism:
- Soap molecules have a polar head (–COO⁻Na⁺) and non-polar tail (hydrocarbon chain)
- The non-polar tail dissolves in grease/oil
- The polar head interacts with water
- This forms micelles (spherical aggregates) that solubilise the grease
- The grease is literally surrounded by soap molecules in a water-dispersible micelle
⚡ Exam Tip: Soap is an emulsifying agent. It doesn’t actually “dissolve” grease — it forms an emulsion (micellar solution). In hard water, soap reacts with Ca²⁺ and Mg²⁺ ions to form calcium/magnesium stearate (scum). Synthetic detergents (like SDS — sodium dodecyl sulphate) are designed not to form this precipitate.
2. Hydrogenation: Unsaturated triglycerides + H₂ (Ni catalyst) → saturated triglycerides:
Olein (triolein, liquid) + 3H₂ → Stearin (solid)This converts liquid vegetable oils (like peanut oil) to semi-solid fats (like margarine). Partial hydrogenation is used to make partially hydrogenated vegetable oils (PHOs) — the source of industrial trans fats.
3. Rancidity: Fats can become rancid by:
- Hydrolytic rancidity: Lipases (enzymes) hydrolyse triglycerides → free fatty acids + glycerol → unpleasant odour
- Oxidative rancidity: Atmospheric oxygen oxidises C=C double bonds → peroxides → aldehydes and ketones → unpleasant flavours
- Antioxidants (BHT, BHA, vitamin E) are added to prevent oxidative rancidity
4. Halogenation (Iodine Value): Iodine (I₂) adds across C=C double bonds in a triglyceride:
I₂ + C=C → C–C–I (addition product)Iodine Value (IV): Grams of I₂ (or equivalent) absorbed by 100g of fat.
- Higher IV = more unsaturated the fat
- Linseed oil: IV ~180 (very unsaturated)
- Coconut oil: IV ~10 (very saturated)
- Olive oil: IV ~80 (moderately unsaturated)
⚡ Exam Tip: Iodine value is an analytical parameter used to characterise fats and determine their degree of unsaturation. It is particularly important in the paint and varnish industry where drying oils (high IV) are preferred.
Phospholipids
Phospholipids are the structural lipids of cell membranes. They have:
- Glycerol backbone (2 positions esterified to fatty acids, 1 position to phosphate)
- Phosphate group (negatively charged)
- Nitrogenous base attached to phosphate (choline, serine, ethanolamine, or inositol)
Phosphatidylcholine (lecithin):
Glycerol + 2 fatty acids + phosphate + choline- Most abundant phospholipid in cell membranes
- The choline head group is positively charged (at physiological pH ~7.4)
- Phosphatidylcholine is zwitterionic (both positive and negative charges on the same molecule)
Membrane Structure — The Fluid Mosaic Model:
- Cell membranes are composed of a phospholipid bilayer
- The bilayer is fluid (lipids can diffuse laterally within the layer)
- Proteins are embedded in or attached to the bilayer (mosaic pattern)
- Cholesterol is interspersed in animal cell membranes (rigidifying agent)
Steroids
Steroids are a class of lipids with a characteristic four-ring structure (three 6-membered rings + one 5-membered ring). The core structure is the cyclopentanoperhydrophenanthrene ring (17 carbon atoms in 4 rings).
Cholesterol:
- The most important steroid in animals
- Precursor to all other steroid hormones (testosterone, oestrogen, cortisol, aldosterone)
- Component of animal cell membranes (rigidifies the membrane)
- Human dietary intake: Egg yolks, meat, dairy
- Elevated blood cholesterol (particularly LDL) is associated with atherosclerosis
Important Steroid Hormones:
| Hormone | Source | Function |
|---|---|---|
| Testosterone | Testes | Male sex characteristics |
| Oestradiol | Ovaries | Female sex characteristics |
| Progesterone | Corpus luteum | Menstrual cycle, pregnancy |
| Cortisol | Adrenal cortex | Stress response, metabolism |
| Aldosterone | Adrenal cortex | Electrolyte balance |
| Calcitriol | Skin (from vitamin D) | Calcium absorption |
Fat-Soluble Vitamins
| Vitamin | Chemical Name | Function | Source |
|---|---|---|---|
| A | Retinol | Vision, immune function, skin | Carrots (β-carotene), liver |
| D | Cholecalciferol (D₃) | Calcium absorption, bone health | Sunlight, fish oils |
| E | Tocopherol | Antioxidant | Vegetable oils, nuts |
| K | Phylloquinone (K₁) | Blood clotting factors | Leafy greens, K₂ from bacteria |
⚡ Deficiency Diseases:
- Vitamin A deficiency: Night blindness, xerophthalmia, keratomalacia
- Vitamin D deficiency: Rickets (children), osteomalacia (adults)
- Vitamin E deficiency: Haemolytic anaemia (RBC breakdown)
- Vitamin K deficiency: Bleeding disorders (delayed coagulation)
Prostaglandins and Eicosanoids
Eicosanoids are signalling molecules derived from arachidonic acid (a 20-carbon polyunsaturated fatty acid). They include:
- Prostaglandins (PG): Local hormones involved in inflammation, pain, fever
- Thromboxanes (TX): Promote blood clot formation (aspirin inhibits TXA₂ synthesis)
- Leukotrienes (LT): Bronchoconstriction (asthma drugs target LT receptors)
Aspirin (acetylsalicylic acid) works by irreversibly inhibiting cyclooxygenase (COX), preventing prostaglandin and thromboxane synthesis from arachidonic acid. This reduces inflammation, pain, and fever. However, it also inhibits thromboxane A₂ production in platelets, reducing clot formation — hence aspirin’s use in cardiovascular disease prevention.
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