Lipid Chemistry Basics

Lipid Chemistry

• Lipids are organic substances relatively insoluble in water but soluble in organic solvents like chloroform, ether, and benzene.
• Lipids are classified into three main categories: simple lipids, complex lipids, and derived lipids.

Simple Lipids

• Simple lipids are esters of fatty acids with various alcohols.
• They consist of long-chain fatty acids that may be either free or combined with an alcohol by ester linkage.
• Hydrolysis of simple lipids yields glycerol and fatty acids, sterols, and fatty acids, etc.
• Alcohols in simple lipids can be subclassified as:
  • Neutral fats or oils (alcohol is glycerol)
  • Waxes (alcohol is other than glycerol)

Neutral Fats or Oils

• Esters of fatty acids with glycerol.
• Triglycerides are esters of trihydric alcohol or glycerol with 3 fatty acids.
• Uncharged.

Waxes

• Esters of fatty acids with higher molecular weight monohydric alcohols.
• Examples: lanolin, beeswax, cerumen (earwax).
• Waxes are esters of higher fatty acids with long-chain monohydric alcohols.
• Contain about 12-32 carbon atoms.

Complex Lipids

• Esters of fatty acids with alcohol containing additional (prosthetic) groups, such as phosphoric acid, sugars, nitrogenous bases, or proteins.
• Subclassified according to the type of prosthetic group:
  • Phospholipids
  • Glycolipids
  • Lipoproteins

Phospholipids

• Esters of fatty acids with alcohol and phosphoric acid.
• Frequently have nitrogen-containing bases.
• Phospholipids are major membrane lipids that consist of lipid bilayers.
• Classified based on the type of alcohol present:
  • Glycerophospholipids (alcohol is glycerol)
  • Sphingophospholipids (alcohol is sphingosine)

Functions of Phospholipids

• In association with proteins, phospholipids form the structural components of membranes and regulate membrane permeability.
• Participate in the absorption of fat from the intestine.
• Essential for the synthesis of different lipoproteins and thus participate in the transport of lipids.
• Accumulation of fat in the liver (fatty liver) can be prevented by phospholipids, hence they are regarded as lipotropic factors.
• Arachidonic acid, an unsaturated fatty acid liberated from phospholipids, serves as a precursor for the synthesis of eicosanoids.

Derived Lipids

• Derivatives obtained on the hydrolysis of group 1 and group 2 lipids, which possess the characteristics of lipids.
• Examples: fatty acids, steroids, cholesterol, and vitamins A and D.

Functions of Lipids

• Structural component of cell membranes.
• Storage form of energy.
• Precursor of many steroid hormones and vitamin D.
• Acts as thermal insulators in lipids.
• Helps in the absorption of fat-soluble vitamins.
• Acts as electric insulators in neurons.

Test for Purity of Fats and Oils

• Acid number: measures the number of milligrams of potassium hydroxide required to completely neutralize the free fatty acids present in 1 g of the oil or fat.
• Saponification number: measures the average molecular size of the fatty acids present.
• Iodine number: measures the relative unsaturation of fats and is directly proportional to the content of unsaturated fatty acids.
• Reichert-Meissl (RM) number: measures the number of ml 0.1 N KOH required to completely neutralize the soluble volatile fatty acids distilled from 5 g of fat.