Q1 LONG ANSWER 10M ★ High-yield

Classify lipids with suitable examples. Describe their functions and the properties used to characterise them.

Biochemistry · 1st Professional — Pre-Clinical

Definition

Lipids = a heterogeneous group of organic compounds, insoluble in water but soluble in organic solvents such as ether, chloroform and benzene, related either actually or potentially to the fatty acids.

They are grouped by solubility, not by structure — which is why the class is so chemically varied

Bloor’s Classification

Class	Definition	Examples
Simple lipids	Esters of fatty acids with alcohols	Fats and oils (with glycerol); waxes (with a higher alcohol)
Compound (complex) lipids	Esters of fatty acids with alcohol, plus an additional group	Phospholipids, glycolipids, lipoproteins, sulpholipids
Derived lipids	Products of hydrolysis of the above	Fatty acids, glycerol, steroids, cholesterol, ketone bodies, fat-soluble vitamins

Simple Lipids

Triacylglycerol (triglyceride) — three fatty acids esterified to glycerol; the storage form of fat and about 95% of dietary lipid
Fats are solid at room temperature (mostly saturated); oils are liquid (mostly unsaturated)
Waxes — esters of fatty acids with long-chain monohydric alcohols; not digestible by man; found in cerumen and sebum

Compound Lipids

Type	Contains	Examples	Significance
Phospholipids — glycerophospholipids	Glycerol + 2 fatty acids + phosphate + base	Lecithin (choline), cephalin (ethanolamine), phosphatidylserine, phosphatidylinositol, cardiolipin	Membranes; surfactant; IP3/DAG signalling; VDRL antigen
Phospholipids — sphingophospholipids	Sphingosine instead of glycerol	Sphingomyelin	Myelin; accumulates in Niemann–Pick disease
Glycolipids	Sphingosine + fatty acid + carbohydrate, no phosphate	Cerebrosides, gangliosides, sulphatides	Brain and nerve; blood group antigens; lysosomal storage diseases
Lipoproteins	Lipid + apoprotein	Chylomicron, VLDL, LDL, HDL	Transport of lipids in plasma

Derived Lipids

Fatty acids — saturated and unsaturated
Steroids — based on the cyclopentanoperhydrophenanthrene ring: cholesterol, bile acids, steroid hormones, vitamin D
Ketone bodies — acetoacetate, β-hydroxybutyrate, acetone
Fat-soluble vitamins — A, D, E, K
Eicosanoids — prostaglandins, thromboxanes, leukotrienes

Functions of Lipids

Function	Detail
Energy storage	9 kcal/g — more than twice carbohydrate or protein; stored anhydrous, so far more compact
Structural	Phospholipids and cholesterol form all biological membranes
Insulation	Subcutaneous fat conserves heat
Protection	Cushions kidney, eyeball and other organs
Hormones and signalling	Steroid hormones, eicosanoids, IP3 and DAG
Vitamin absorption	Carrier for the fat-soluble vitamins A, D, E and K
Essential fatty acids	Linoleic and α-linolenic acid
Surfactant	Dipalmitoyl phosphatidylcholine prevents alveolar collapse

Properties Used in the Laboratory

Property	Meaning	Use
Saponification	Alkaline hydrolysis of a fat → soap + glycerol	Soap manufacture; the saponification number indicates chain length
Iodine number	Grams of iodine absorbed by 100 g of fat	Measures the degree OF unsaturation
Acid number	Free fatty acid content	Indicates rancidity
Rancidity	Unpleasant odour and taste from oxidative or hydrolytic breakdown	Prevented by antioxidants and airtight storage
Hydrogenation	Adding hydrogen to double bonds → solid fat	Vanaspati; produces harmful trans fats

Amphipathic Lipids and Micelles

Amphipathic = possessing both a polar (hydrophilic) and a non-polar (hydrophobic) region — phospholipids, cholesterol, bile salts, free fatty acids
In water they orient with the polar heads outward, forming micelles, bilayers or liposomes
Micelle formation is essential for fat digestion — bile salts emulsify fat and carry the products to the mucosal surface
The lipid bilayer of every membrane is the direct consequence of this property
Liposomes are used as drug delivery vehicles, as in liposomal amphotericin B and doxorubicin
Neutral lipids such as triglyceride and cholesteryl ester are not amphipathic — they form the core of a lipoprotein, not its surface

Applied Aspects

Trans fats, produced by partial hydrogenation, raise LDL and lower HDL — the worst combination; now legally restricted in India and many other countries
Respiratory distress syndrome of the newborn follows deficiency of the phospholipid surfactant
Lipid storage diseases — Gaucher, Niemann–Pick, Tay–Sachs and Krabbe disease all result from failure to degrade a complex lipid
Cardiolipin is the antigen used in the VDRL test for syphilis, and antibodies to it explain the biological false positive in lupus
Essential fatty acid deficiency causes scaly dermatitis and poor wound healing, seen in prolonged fat-free parenteral nutrition
Steatorrhoea — failure to digest or absorb fat gives bulky, pale, offensive stools that float; caused by pancreatic insufficiency, bile salt deficiency or mucosal disease

All questions in Chemistry of Lipids, Proteins & Amino Acids

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