Lipids – Comprehensive Bullet-Point Study Notes

Introduction & Core Definition

Lipid = Water‐insoluble, non-polar organic compound
- Insoluble (or only sparingly soluble) in water, but soluble in non-polar solvents (ether, chloroform, benzene, alcohol)
- Historically called “fats” but term now covers a structurally diverse family
- Biological significance
- Major long-term energy & carbon storage form in organisms
- Mechanical protection & thermal insulation for vital organs (fat pads)
- Structural material for cell membranes (≈ 80 % of membrane mass)
- Defining parameter is solubility, not common structure ‑– explains huge structural diversity (triacylglycerols, waxes, steroids, phospholipids, etc.)

Two Complementary Classification Schemes

1. By Biochemical Function

Energy-storage lipids → Triacylglycerols (TAGs)
Membrane lipids → Phospholipids, sphingoglycolipids, cholesterol
Emulsification lipids → Bile acids / bile salts
Messenger lipids → Steroid hormones & eicosanoids
Protective-coating lipids → Biological waxes

2. By Chemical Reactivity (Saponifiability)

Saponifiable (hydrolysable → ≥ 2 smaller molecules)
- Triacylglycerols, phospholipids, sphingoglycolipids, biological waxes
Nonsaponifiable (do not hydrolyze to smaller units)
- Cholesterol, steroid hormones, bile acids, eicosanoids

Fundamental Building Blocks: Fatty Acids vs. Monosaccharides

Carbohydrates use monosaccharides (e.g., $D\text{-glucose},\;D\text{-fructose}$ ) as the repeat/assembly unit.
Lipids most commonly use fatty acids (FA) – naturally occurring monocarboxylic acids seldom found in the free state; instead they are esterified in complex lipids.
- General (unbranched) formula $CH<em>3(CH</em>2)_nCOOH$

Types of Fatty Acids (Three-fold)

1. Saturated Fatty Acids (SFA)

Only C–C single bonds in hydrocarbon chain.
Examples: lauric (12:0), myristic (14:0), palmitic (16:0), stearic (18:0), arachidic (20:0).

2. Monounsaturated Fatty Acids (MUFA)

One C=C double bond.
Characteristic features of natural MUFAs
1. Unbranched chain
2. Even number of C atoms
3. Double bond almost always cis configuration
Classic example: oleic acid $C_{18:1\,\Delta^9\,(\omega!\text{-}9)}$ .
Over 500 unique FAs identified in nature differing in chain length, degree & position of unsaturation.

3. Polyunsaturated Fatty Acids (PUFA)

≥ 2 C=C double bonds.
Examples & notation
- Linoleic acid $C_{18:2\,\Delta^{9,12}}$ (\omega-6)
- \alpha-Linolenic acid $C_{18:3\,\Delta^{9,12,15}}$ (\omega-3)
- Arachidonic acid $C_{20:4\,\Delta^{5,8,11,14}}$ (\omega-6)
- EPA $C_{20:5\,\Delta^{5,8,11,14,17}}$ (\omega-3)
- DHA $C_{22:6\,\Delta^{4,7,10,13,16,19}}$ (\omega-3)

Physical Properties of Fatty Acids

Determined chiefly by chain length & unsaturation:
1. Water insoluble; solubility in organic solvents high.
2. Solubility ↑ as chain length ↓ (inverse relationship).
3. Long-chain saturated FAs = solids at room T°; long-chain unsaturated FAs = liquids.
4. Boiling point ↑ with chain length (for saturated series).

Triacylglycerols (TAGs) – Energy Storage Lipids

Three FA residues esterified to glycerol backbone → neutral fats.
Non-polar, highly reduced → yield maximal ATP per gram during oxidation.

Complex (Membrane) Lipids

A. Phospholipids – common architecture = “head + two tails”

1. Glycerophospholipids

Platform = glycerol (\text{CHO-CHOH-CH2OH}).
Contains:
- 2 FA esters (non-polar tails; hydrophobic)
- Phosphate ester at C-3 → further esterified by an alcohol (choline, ethanolamine, serine, inositol, glycerol).
Amphipathic → spontaneously form bilayers & liposomes; primary structural element of cell membranes.

2. Sphingophospholipids (e.g., sphingomyelins)

Platform = sphingosine (18-C unsaturated amino-di-alcohol).
Composition: 1 FA (via amide bond) + phosphate + alcohol (commonly choline).
Major component of myelin sheath (insulation of nerves).

B. Sphingoglycolipids (Glycolipids)

Platform = sphingosine + fatty acid (ceramide) plus carbohydrate head (glycosidic linkage).
- Cerebrosides – simplest; single monosaccharide (glucose/galactose); abundant in brain and myelin.
- Gangliosides – complex, branched oligosaccharides (≤ 7 sugar units); concentrated in gray matter; involved in cell-cell recognition.

C. Cholesterol – third membrane lipid

Steroid nucleus = four fused rings ((C_{17}) skeleton) specifically $\text{cyclopentanoperhydrophenanthrene}$ .
Cholesterol is a C_{27} steroid; no glycerol, no sphingosine, no FA residues.
Functions
- Inserts between phospholipid tails → modulates fluidity & rigidity (prevents tight packing at low T° & excessive motion at high T°).
- Precursor for bile acids, vitamin D, steroid hormones, eicosanoids.
Distribution & transport
- Present in membranes, nerve/brain tissue, most body fluids.
- Blood plasma ≈ 50 mg/100 mL free cholesterol.
- Carried as lipoproteins:
- LDL (low-density) – delivers cholesterol to tissues ("bad" when in excess).
- HDL (high-density) – scavenges & returns cholesterol to liver ("good").
Sources
1. Dietary intake (animal products)
2. Endogenous synthesis (liver, intestine). Diet suppresses synthesis, but suppression < intake → total body level ↑ with high cholesterol diet.

Biological Membranes (Lipid Bilayers)

80 % lipid, 20 % protein (approx.)
- Lipid components: (1) phospholipids, (2) glycolipids, (3) cholesterol.
Bilayer arrangement: hydrophilic heads outward to aqueous phases; hydrophobic tails inward.
Integral & peripheral proteins + carbohydrates (glycoproteins) provide transport & signaling.

Messenger Lipids I – Steroid Hormones (Cholesterol Derivatives)

Hormone = ductless gland secretion acting as messenger.
Steroid hormones share the cholesterol nucleus; not stored – synthesised on demand; pass through membranes (lipophilic).

1. Sex Hormones

Estrogens (e.g., 17β-estradiol)
- Synthesised in ovaries & adrenal cortex.
- Develop female secondary sexual characteristics, regulate menstrual cycle, stimulate mammary gland development in pregnancy.
Androgens (e.g., testosterone)
- Synthesised in testes & adrenal cortex.
- Develop male secondary sexual characteristics, promote muscle growth.
Progesterone
- Produced by ovaries & placenta.
- Prepares endometrium for implantation, suppresses ovulation during pregnancy.

2. Adrenocorticoid Hormones (adrenal cortex)

Mineralocorticoids (e.g., aldosterone) → regulate Na^+ / K^+ balance & water retention.
Glucocorticoids (e.g., cortisol) → regulate glucose metabolism, anti-inflammatory, immunosuppressive.

Emulsification Lipids – Bile Acids & Bile Salts

Emulsification = mixing two immiscible liquids into stable dispersion (e.g., dietary fat in aqueous lumen).
Emulsifier = amphipathic molecule that stabilizes emulsion.
Bile acids
- Cholesterol derivatives synthesized in liver, conjugated with glycine or taurine → bile salts.
- Secreted in bile (fluid containing bile acids, cholesterol, phospholipids, bilirubin, electrolytes) → stored in gallbladder, released to small intestine.
- Function: solubilize & emulsify dietary TAGs, increasing surface area for lipase digestion; also increase cholesterol solubility in bile.

Messenger Lipids II – Eicosanoids (C_{20} Oxylipins)

Derived mainly from arachidonic acid $C_{20:4\,\Delta^{5,8,11,14}}$ .
Act locally (paracrine/autocrine); deactivated within seconds → tight regulation.
Broad physiological roles: inflammation, pain, fever, blood pressure, clotting, reproduction, sleep-wake cycle.

Three Principal Families

Prostaglandins – contain cyclopentane ring + hydroxyl/ketone groups.
- Actions: raise body temperature, inhibit gastric acid, enhance mucus secretion, modulate smooth muscle (vasodilation/bronchoconstriction), regulate water/electrolyte balance, pain, inflammation.
- Aspirin & other NSAIDs block cyclooxygenase (COX) enzymes → ↓ prostaglandin synthesis → anti-inflammatory & antipyretic.
Thromboxanes – cyclic ether ring.
- Synthesised in platelets; promote platelet aggregation & clot formation.
Leukotrienes – linear molecules with three conjugated C=C + hydroxyls.
- Produced in leukocytes; mediate inflammation & hypersensitivity (allergies, asthma).

Practical, Medical & Ethical Notes

Dietary fats: ratio of (\omega!\text{-}6/\omega!\text{-}3) PUFAs influences eicosanoid profile → implications for cardiovascular health & inflammation.
Cholesterol management: excess LDL linked to atherosclerosis; lifestyle & statins (HMG-CoA reductase inhibitors) target endogenous synthesis.
Essential FAs: linoleic & \alpha-linolenic acids cannot be synthesized by humans → must be obtained from diet; precursors for arachidonic acid & \omega!\text{-}3 eicosanoids (EPA/DHA).
Myelin disorders (e.g., multiple sclerosis) involve sphingophospholipid damage.
NSAIDs (aspirin, ibuprofen) – therapeutic modulation of prostaglandins/thromboxanes but risk of gastric ulcers (due to ↓ protective mucus).
Gallstones: imbalance in bile components (cholesterol precipitation) underscores importance of bile salts.

Key Numbers, Formulae & Notation Cheat-Sheet

Fatty acid shorthand: $C_{\text{carbon number} : \text{double bonds}\,\Delta^{\text{positions}}}$ ; (\omega)-system counts from methyl end.
Plasma free cholesterol concentration ≈ $50\,\text{mg}/100\,\text{mL}$ .
Cholesterol: $C<em>{27}H</em>{46}O$ ; steroid ring fusion = $A\text{–}B\text{–}C\text{–}D$ rings (three 6-membered + one 5-membered).
Bile acid conjugation: $\text{cholic acid} + \text{glycine} \rightarrow \text{glycocholate}$ (bile salt).

Concept Map / Interconnections

Cholesterol → precursor for (i) steroid hormones, (ii) bile acids, (iii) vitamin D, (iv) membrane regulator.
Fatty acid unsaturation governs membrane fluidity, melting points of TAGs, & substrates for eicosanoid biosynthesis.
Phospholipids + Cholesterol → lipoproteins for lipid transport; imbalance leads to atherosclerosis.
Saponifiable vs. nonsaponifiable informs laboratory hydrolysis assays for lipid identification.

Summary

Lipids encompass a vast family unified by hydrophobicity. Their roles span energy storage (triacylglycerols), structure (membrane lipids), signaling (steroid hormones & eicosanoids), digestion (bile salts), and protection (waxes). Understanding their chemistry – from fatty-acid building blocks to complex steroids – is fundamental for grasping metabolism, physiology, and many pathological states.