Lipids: Structure, Properties, and Biological Roles

Les lipides / Lipids

Introduction

Lipids are a diverse group of hydrophobic or amphipathic molecules essential for biological membranes, energy storage, and signaling. They include fatty acids, triglycerides, phospholipids, and sterols, among others. This section focuses on the structure, properties, and classification of fatty acids and simple lipids, as well as their biological significance.

Properties of Fatty Acids

Physical Properties

• Solubility: Fatty acids (FAs) are soluble in non-polar organic solvents such as benzene and chloroform. Short-chain fatty acids (e.g., butyric acid, C4) are soluble in water, but solubility decreases with increasing chain length. Fatty acids with more than 10 carbons are insoluble in water.

• Organization in Water: Insoluble fatty acids organize at the water-air interface as molecular films (mono-, bi-, or multilayers) or form micelles (emulsions).

Melting Point (T*)

• Melting point is the transition from solid to liquid state.

• Depends on:

  • Number of carbon atoms (C): Higher C increases melting point.

  • Number of double bonds: More double bonds decrease melting point.

  • Presence of methylation.

• Fatty acids are liquid at 20°C if n < 10 C, and solid if n > 10 C.

Examples:

• Arachidic acid (C20:0): +76.5°C (high C, no double bonds)

• Arachidonic acid (C20:4, 5,8,11,14): -49.5°C (high C, many double bonds)

Table: Melting Points of Fatty Acids

Category	Common Name	Formula	Melting Point (°C)
Saturated FAs	Lauric	C12:0	43.5
Saturated FAs	Palmitic	C16:0	63
Saturated FAs	Stearic	C18:0	70
Saturated FAs	Arachidic	C20:0	76.5
Monounsaturated FAs	Palmitoleic	C16:1Δ9	-0.5
Monounsaturated FAs	Oleic	C18:1Δ9	13.5
Polyunsaturated FAs	Linoleic	C18:2Δ9,12	-5
Polyunsaturated FAs	Linolenic	C18:3Δ9,12,15	-11
Polyunsaturated FAs	Arachidonic	C20:4Δ5,8,11,14	-49.5

Chemical Properties of Fatty Acids

1. Oxidation of Double Bonds

• Oxidation by atmospheric oxygen leads to rancidity of fats.

• Intracellular enzymatic oxidation (e.g., by cyclo-oxygenase) of arachidonic acid produces prostaglandins, which are potent, short-lived mediators.

• Powerful chemical oxidants (e.g., permanganate ion, MnO4-, in alkaline medium) cleave unsaturated fatty acids into mono- and dicarboxylic acids.

2. Addition Reactions with Halogens

• Double bonds in fatty acids react with halogens (e.g., I2), allowing quantification of unsaturation (iodine value).

• Equation: $\text{R-CH=CH-R} + I_2 \rightarrow \text{R-CH(I)-CH(I)-R}$

• Iodine Value Formula: $II = d \times MI2 \times 100 / M_{lip}$

• Where $d$ = number of double bonds, $MI2$ = molar mass of I2, $M_{lip}$ = mass of lipid.

3. Hydrogenation

• Hydrogenation converts unsaturated fatty acids in edible oils into saturated fats (e.g., margarine), making them solid at room temperature and less prone to oxidation.

• Equation: $\text{R-CH=CH-R} + H_2 \rightarrow \text{R-CH_2-CH_2-R}$

4. Esterification

• Fatty acids react with alcohols to form esters and water.

• Equation: $\text{R-COOH} + \text{R'-OH} \rightarrow \text{R-COO-R'} + H_2O$

5. Saponification

• Alkaline hydrolysis of lipids produces sodium or potassium salts of fatty acids (soaps).

• Equation: $\text{R-COOH} + KOH \rightarrow \text{RCOO}^- K^+ + H_2O$

• Soaps are amphiphilic molecules with hydrophobic and hydrophilic regions, giving them detergent properties.

Acid and Saponification Values

• Acid Value: Mass of KOH (mg) needed to neutralize free fatty acids in 1 g of fat. Used to assess oil quality and determine molecular mass of pure fatty acids.

• Formula: $I_A = M_{KOH} \cdot 10^3 / M_{ag}$

• Saponification Index: Mass of KOH (mg) required to neutralize free fatty acids and saponify esters in 1 g of lipid.

Synthesis of Fatty Acids

Mono-unsaturated Fatty Acids

• Oleic acid (C18:1, ω9) is synthesized in both plants and animals by Δ9 desaturase.

• Oleic acid is less sensitive to oxidation and is abundant in olive oil, which is associated with cardiovascular health benefits.

Poly-unsaturated Fatty Acids (PUFAs)

• Plants and animals differ in their desaturation capacity, leading to the concept of essential fatty acids (EFAs).

• Δ15 and Δ12 desaturases are present in plants, not in animals; thus, linoleic (C18:2, ω6) and α-linolenic (C18:3, ω3) acids are essential in the human diet.

Essential Fatty Acids

• Linoleic Acid (C18:2, ω6): Required for growth, development, membrane phospholipids, kidney function, and reproduction.

• Alpha-Linolenic Acid (C18:3, ω3): Essential for membrane biogenesis (nervous system, retina), critical during development, and facilitates growth.

• Daily requirements: Linoleic acid ~10 g/day; α-linolenic acid 2–4 g/day.

Biological Importance of Omega-3 Fatty Acids

• Omega-3 fatty acids are crucial for brain and vision development.

• Populations with high fish consumption (rich in omega-3s) have lower rates of cardiovascular disease, despite high lipid intake.

Classification of Lipids

Overview

Lipids are classified based on their structure and saponifiability:

• Saponifiable lipids (true lipids): Contain fatty acids and can be hydrolyzed to yield fatty acids and alcohols.

• Non-saponifiable lipids: Do not contain fatty acids (e.g., steroids, terpenes).

Simple Lipids

• Glycerides (Acylglycerols): Esters of glycerol and fatty acids. Mono-, di-, and triglycerides depending on the number of fatty acids esterified.

• Sterides: Esters of cholesterol and fatty acids.

• Cérides: Esters of long-chain fatty acids and long-chain alcohols.

Table: Simple Lipids

Type	Alcohol Component	Fatty Acid Component	Example
Glycerides	Glycerol	Fatty acids	Triglycerides (fats, oils)
Sterides	Cholesterol	Fatty acids	Cholesteryl esters
Cérides	Long-chain alcohols	Fatty acids	Waxes

Biological Roles of Triglycerides

• Major energy reserve in animals and plants.

• Yield twice as much energy as carbohydrates upon oxidation.

• Stored in a compact, anhydrous form in adipose tissue.

• Provide thermal insulation in certain animals.

Additional info: The notes above integrate both the French and English content from the slides, and expand on definitions, examples, and formulas for clarity and completeness.