Lipids Classification, Fatty Acids and Biological Roles

Lipids: Classification and Key Characteristics

LIPIDS: Lipids are classified into three major groups: fatty acids (saturated and unsaturated), saponifiable lipids (simple and complex, including acylglycerides and waxes) and unsaponifiable lipids (terpenes, steroids and prostaglandins).

Fatty Acids: Structure and Properties

Fatty acid: a hydrocarbon chain typically from 12 to 22 carbon atoms with a terminal COOH (carboxyl) group. They are divided into:

• Saturated fatty acids — chains with no double bonds; they adopt an extended, zigzag conformation and have stronger van der Waals interactions between hydrocarbon chains, producing higher melting points.
• Unsaturated fatty acids — chains that contain one or more double bonds; double bonds create kinks (approximate bond angles ≈120°), reduce packing and van der Waals contacts, lower melting points, and make these acids susceptible to autoxidation.

Fatty acids participate in chemical reactions such as esterification (fatty acid + alcohol → ester + water) and saponification (fatty acid + hydroxide → soap + water). They are amphipathic molecules (a polar carboxyl group and a nonpolar, lipophilic hydrocarbon chain) and are generally insoluble in water.

Saturated chains have stronger intermolecular interactions (van der Waals forces) and therefore higher melting points; unsaturated chains, with double bonds and fewer van der Waals contacts, have lower melting points and undergo autoxidation (reaction with O2 that can break double bonds, yielding aldehydes responsible for rancid odors). Some fatty acids are essential and must be obtained from the diet because the organism cannot synthesize them.

Saponifiable Lipids: Simple and Complex

Saponifiable lipids include:

• Simple lipids
  • Glycerides (acylglycerides): formed from fatty acids and glycerol; can be monoacylglycerols, diacylglycerols and triacylglycerols. They undergo saponification to yield soap. Saturated fatty acyl chains tend to form solids (fats), while unsaturated chains form liquids (oils).
  • Waxes (cerides): esters of fatty acids and long-chain fatty alcohols joined by long-chain ester bonds; they function in protection and can serve as energy reserves in some organisms.
• Heterolipids (complex lipids)
  • Glycerolipids and glycerophospholipids: examples include glyco‑/glycoglycerolipids (e.g., glyceroglycolipids: two fatty acyl chains + glycerol + a monosaccharide) and glycerophospholipids (two fatty acyl chains + glycerol + phosphoric acid + an alcohol or aminoalcohol). Glycerophospholipids are major components of biological membranes.
  • Sphingolipids: contain a backbone derived from sphingosine (an amino alcohol), a fatty acid (usually amide-linked) and a polar head group; they are important membrane components and signaling molecules.

Unsaponifiable Lipids: Terpenes, Steroids, Prostaglandins

Unsaponifiable lipids are lipids that do not yield soaps on saponification. They include:

• Terpenes — biosynthetic precursors for compounds such as vitamin A and vitamin K, and constituents of natural rubber.
• Steroids and sterols — derived from cholesterol; cholesterol is a precursor for bile salts, vitamin D, sex hormones, cortisol (cortisone family) and aldosterone.
• Prostaglandins (prostaglandin/prostanoic acid derivatives) — derived from fatty acids (e.g., arachidonic acid) and exert local hormonal (autocrine/paracrine) actions.

Summary of Key Reactions

• Esterification: fatty acid + alcohol → ester + water.
• Saponification: fatty acid (ester) + hydroxide → soap + water.
• Autoxidation of unsaturated fatty acids: reaction with O2 that can break double bonds and yield aldehydes responsible for rancid odors.

Notes on Terminology

Some terms in older or mixed-language texts may appear as acylglycerides, cerides, glyceroglycolipids (glyceroglycolipids), or Spanish-derived forms. In modern biochemical nomenclature:

• "Ac" refers to acid (e.g., fatty acid).
• "Ceridos" (Spanish) corresponds to waxes (cerides).
• "Gliceroglucolipidos" corresponds to glyceroglycolipids (glycerol + fatty acids + sugar).

Functional Importance

Lipids perform diverse biological roles: energy storage (triacylglycerols), membrane structure and dynamics (glycerophospholipids, sphingolipids, cholesterol), chemical signaling (prostaglandins, steroid hormones), protection and waterproofing (waxes), and as vitamin precursors (terpenes leading to vitamins A and K).