Lipids: Structure, Properties, and Biological Roles

Posted on Apr 4, 2025 in Human Biology

Lipids: An Overview

Lipids are a diverse group of biomolecules characterized by their insolubility in water and solubility in nonpolar solvents. They are primarily composed of carbon, hydrogen, and oxygen, with a lower proportion of oxygen compared to carbohydrates. Some lipids also contain phosphorus, nitrogen, or sulfur.

Classification of Lipids

Lipids can be broadly classified into two categories:

  • Saponifiable Lipids: These lipids can be hydrolyzed by alkali (e.g., NaOH) to yield soap. Examples include:
    • Acylglycerols (e.g., triglycerides)
    • Waxes (cerides)
    • Phosphoglycerides
    • Phosphosphingolipids
    • Glycosphingolipids
  • Nonsaponifiable Lipids: These lipids cannot be hydrolyzed into soap. Examples include:
    • Isoprenoids (terpenes)
    • Steroids
    • Prostaglandins

Fatty Acids

Fatty acids are long-chain molecules consisting of a hydrocarbon chain with a carboxyl group (-COOH) at one end.

Saturated Fatty Acids

Saturated fatty acids contain only single bonds between carbon atoms, resulting in linear chains (e.g., stearic acid, palmitic acid).

Unsaturated Fatty Acids

Unsaturated fatty acids contain one or more double bonds between carbon atoms, creating kinks in the chain (e.g., oleic acid (monounsaturated), linoleic acid (polyunsaturated)).

Properties of Fatty Acids

  • Esterification: Fatty acids react with alcohols to form esters and water (Fatty acid + Alcohol = Ester + H2O).
  • Saponification: Fatty acids react with a base (e.g., NaOH) to form soap and water (Fatty acid + NaOH = Soap + H2O). Soaps form micelles in water, creating colloidal dispersions.
  • Solubility: Short-chain aliphatic acids (4-8 carbons) are more soluble in water. As the chain length increases, solubility decreases, and they become more soluble in nonpolar solvents.
  • Soaps: Soaps are amphipathic molecules with a hydrophilic (polar) carboxyl group and a hydrophobic (nonpolar) hydrocarbon chain. The hydrophilic portion interacts with water molecules through electrostatic attraction, while the hydrophobic portion interacts with lipid molecules through van der Waals forces.

Melting Point

Saturated Fatty Acids

As the hydrocarbon chain length increases, the melting point increases due to the increased energy required to break the intermolecular forces (e.g., myristic acid [CH3-(CH2)12-COOH], palmitic acid [(CH2)14], stearic acid [(CH2)16], lignoceric acid [(CH2)22]).

Unsaturated Fatty Acids

Unsaturated fatty acids have lower melting points due to the kinks introduced by the double bonds, which reduce the efficiency of molecular packing (e.g., oleic acid [CH3-(CH2)7-CH=CH-(CH2)7-COOH], linoleic acid [CH3-(CH2)4-CH=CH-CH2-CH=CH-(CH2)7-COOH]).

Complex Lipids

Phosphoglycerides

Phosphoglycerides are esters formed from two fatty acids, one glycerol molecule (HO-CH-CH2-OH), one phosphoric acid molecule, and one alcohol (e.g., ethanolamine [OH-CH2-CH2-NH2]). Examples include cephalin (brain), lecithin (liver).

Phosphosphingolipids

Phosphosphingolipids are esters formed from one fatty acid, one phosphate group, and one sphingosine (an amino alcohol).

Glycosphingolipids

Glycosphingolipids are esters formed from one fatty acid, one carbohydrate, and one sphingosine. They are abundant in neurons of the brain. Examples include cerebrosides (containing a single monosaccharide) and gangliosides (containing a complex oligosaccharide with sialic acid).

Nonsaponifiable Lipids

Isoprenoids (Terpenes)

Isoprenoids are derived from the polymerization of isoprene (2-methyl-1,3-butadiene) units. These chains can be linear or cyclic. Examples include:

  • Monoterpenes (2 isoprene units, e.g., menthol from mint, limonene from lemon)
  • Diterpenes (4 isoprene units, e.g., phytol, a component of chlorophyll)
  • Triterpenes (6 isoprene units, e.g., squalene)
  • Tetraterpenes (8 isoprene units, e.g., carotenoids, pigments)
  • Polyterpenes (more than 8 isoprene units, e.g., rubber)

Steroids

  • Sterols (e.g., Cholesterol): Cholesterol is a component of animal cell membranes, providing stability. Bile acids are formed from cholesterol in the liver.
  • Steroid Hormones:
    • Estradiol: A hormone responsible for female secondary sexual characteristics.
    • Adrenal Steroid Hormones (e.g., aldosterone, which increases sodium and chloride absorption in the kidney, and cortisol).
    • Sex Hormones (e.g., progesterone, important for pregnancy, and testosterone, responsible for male sexual characteristics).

Functions of Lipids

  • Energy Storage: 1 gram of fat provides approximately 9 kcal of energy.
  • Structural Component: Lipids form the lipid bilayer of cell membranes.
  • Protective Function: Acylglycerols protect bodies from impacts. Waxes protect the skin.
  • Biocatalytic Function: Lipid vitamins and prostaglandins act as hormones.
  • Transport Function: Lipids transport other lipids from the intestine to other parts of the body through emulsification by bile acids and proteolipids.