Lipids – Acylglycerols, Esterification & Fatty-Acid Chemistry

General Characteristics of Lipids

• Lipids = large, loosely-defined group of non-polar molecules

  • Insoluble in water (due to overall non-polar nature).

  • Soluble in organic solvents such as ether, chloroform, alcohol, benzene, etc.

• Major lipid classes (mentioned):

  • Acylglycerols (fats and oils)

  • Waxes

  • Phospholipids

  • Terpenes

  • Steroids

  • Prostaglandins

• Functional importance (implied / widely known):

  • Energy storage, membrane structure, hormones & signaling, waterproofing, insulation.

Acylglycerols (Fats & Oils)

• Composed of two fundamental sub-units:

  1. Glycerol (a trihydroxy alcohol)

  2. Fatty acids (long-chain carboxylic acids)

• Physical state at room temperature forms the everyday distinction:

  • Liquid ⇒ “oils”
    • Dominant in plants (e.g.
    – peanut oil,
    – corn oil,
    – castor oil).

  • Solid ⇒ “fats”
    • Dominant in animals (e.g. lard, butter, tallow).

• Provide the bulk of long-term energy reserves in most organisms.

Chemical Nature: Ester Linkage

• Ester = product of a condensation (dehydration) reaction between an alcohol (–OH) and an acid (–COOH), releasing water.

  • General reaction: $\text{R–OH} + \text{R'–COOH} \;\xrightarrow{\;\text{condensation}\;}\; \text{R–O–CO–R'} + \text{H}_2\text{O}$

  • Textbook illustration (methanol + acetic acid): $\text{CH}<em>3\text{OH} + \text{HOOCCH}</em>3 \longrightarrow \text{CH}<em>3\text{OCOCH}</em>3 + \text{H}_2\text{O}$

• For lipids, the hydroxyl groups of glycerol react with the carboxyl groups of fatty acids to form ester bonds—yielding mono-, di-, or tri-acylglycerols.

Triacylglycerols (Triglycerides / Neutral Lipids)

• Most abundant form of acylglycerols in biological systems.

• Structure: one glycerol + three fatty acids (same or different).

  • Diagrammatically: $\text{Glycerol\ (3C)} \;\xrightarrow{\;3\,\text{ester bonds}\;} \;\text{FA}<em>1 + \text{FA}</em>2 + \text{FA}_3$

• Term “neutral” because the molecule carries no net charge at physiological pH (all acidic groups tied up as esters).

Glycerol (Backbone)

• 3-carbon (
  $\text{C}<em>3\text{H}</em>8\text{O}_3$
  ) alcohol.

• Each C bears an –OH group: $\text{CH}<em>2\text{OH}–\text{CHOH}–\text{CH}</em>2\text{OH}$

• Provides three attachment sites for fatty acids → determines the potential diversity of triacylglycerols.

Fatty Acids (FA)

• Long hydrocarbon chain (even number of C, typically $4 \le n \le 30$) + terminal carboxyl group (–COOH).

• Responsible for physical properties (melting point, density, fluidity) of acylglycerols.

• Variation:

  • Length of hydrocarbon tail.

  • Degree of saturation (number of C=C double bonds).

  • Geometry / branching (straight in animals, occasionally branched or ringed in plants).

Saturated vs. Unsaturated Fatty Acids

• Saturated FA

  • No C=C double bonds.

  • Every internal C is fully “saturated” with H atoms.

  • Straight chains ⇒ efficient packing ⇒ high melting points (solid at RT).

• Unsaturated FA

  • Contain 1–6 C=C double bonds.

  • Each C=C replaces two H ⇒ fewer than maximum H atoms.

  • Bent / kinked chains (especially with cis double bonds) ⇒ less packing ⇒ low melting points (liquid at RT).

  • Terminology:
    • Mono-unsaturated ⇒ one double bond.
    • Poly-unsaturated (PUFA) ⇒ two or more double bonds.

Trends in Physical Properties

• Melting Point (M.P.)

  • Increases with chain length (more van-der-Waals interactions).

  • Decreases with degree of unsaturation (kinks interfere with packing).

• Solubility in non-polar organic solvents increases with chain length; remains negligible in water for most chain lengths.

Dehydration (Condensation) Synthesis of Triacylglycerol

• Each FA attaches to glycerol via esterification (loss of one $\text{H}<em>2\text{O}$ per bond): $\text{Glycerol\ (3 –OH)} + 3 \;\text{Fatty Acids\ (3 –COOH)} \longrightarrow \text{Triacylglycerol} + 3 \;\text{H}</em>2\text{O}$

• Reversible reaction: hydrolysis regenerates FA + glycerol (basis of fat digestion).

Biological & Practical Implications

• Plant oils (rich in unsaturated FA) are liquid → facilitate seed dispersal & energy-dense storage.

• Animal fats (rich in saturated FA) are solid → thermal insulation & protective cushioning.

• Nutrition & health linkage: high saturated-fat diets correlated with cardiovascular risk; unsaturated (esp. PUFA) considered heart-healthy.

• Industrial relevance:

  • Hydrogenation converts unsaturated → saturated (e.g., margarine manufacturing), potentially forming trans-fats.

  • Saponification (alkaline hydrolysis) produces soaps (fatty acid salts).

Key Terminology Recap

• Lipophilic – “fat-loving,” soluble in non-polar media.

• Ester linkage – $\text{R–O–C(=O)–R'}$ bond in lipids.

• Neutral lipid – no net charge molecule (e.g., triacylglycerol).

• Saturated / Unsaturated – absence / presence of C=C double bonds.