Lipid Chemistry

Introduction to Lipids

• Lipids are a heterogeneous group of organic compounds containing carbon, hydrogen, and oxygen.
• They have a higher level of hydrogen than carbohydrates, making them a more energy-rich source for the body.
• Lipids are insoluble in water but soluble in organic solvents like ether, chloroform, benzene, and acetone.
• The human body requires 30% of its energy from lipids.
• Lipids are widely distributed in plants and animals, with sources including nuts, seeds, milk, egg, meat, liver, fish, and nuts.

Biological Significance of Lipids

• Lipids serve as fuel in the body and provide energy.
• Essential fatty acids (unsaturated) reduce inflammation, help regulate blood pressure, and cannot be manufactured by the body.
• Lipids protect the body from excessive heat or cold.
• Breakdown products of fats can be used for building biologically active materials like cholesterol.
• Lipids are necessary for absorbing fat-soluble vitamins A, D, E, and K.
• Lipids act to produce hormones.

Diseases Associated with Lipids

• Atherosclerosis
• Obesity

Classification of Lipids

• Simple Lipids: esters of fatty acids with alcohol
  • Triacylglycerol (TAG): a simple lipid consisting of glycerol and three fatty acids, serving as the main storage form of fatty acids and chemical energy.
  • Waxes: simple lipids consisting of fatty acid esters of long chain mono-hydroxy alcohols.
• Complex Lipids: phospholipids, glycolipids, and derived lipids
  • Phospholipids: contain phosphate group
  • Glycolipids: contain fatty acid, sphingosine, and carbohydrate
  • Derived Lipids: steroids, fatty acids, and fat-soluble vitamins

Fatty Acids

• Fatty acids are the major unit of lipids and are carboxylic acids with a long aliphatic chain.
• Fatty acids are insoluble in water but soluble in fat solvents like ether, chloroform, benzene, and acetone.
• The human body can synthesize saturated fatty acids from carbohydrates.
• Classification of fatty acids:
  • Saturated fatty acids: usually from animal origin, solid at room temperature, and have no carbon-to-carbon double bonds.
  • Unsaturated fatty acids (essential fatty acids): usually from plant origin, liquid at room temperature, and have one or more carbon-to-carbon double bonds.
• Nutritional classification of fatty acids:
  • Essential fatty acids: not manufactured by the body and must be taken in diet, e.g., linoleic acid and α-linolenic acid.
  • Non-essential fatty acids: can be formed in the body in good amounts, mainly from carbohydrates.

Sources of Fatty Acids

• Saturated fatty acids: whole milk, cheese, egg yolks, chicken skin, red meat, ice cream, butter, chocolate, coconuts, coconut milk, and coconut oil.
• Monounsaturated fatty acids: olives, olive oil, sesame oil, canola oil, peanut oil, cashews, almonds, and avocados.
• Polyunsaturated fatty acids: sunflower oil, cotton seed oil, corn, soybean, fish, omega-3 fatty acids (found in Mackerel, salmon, tuna, and cod), and omega-6 fatty acids (found in palm oil, soybean, and sunflower oil).

Functions of Dietary Fats

• Concentrated sources of energy
• Carriers of fat-soluble vitamins A, D, E, and K
• Sources of essential fatty acids
• Satiety value
• Impart taste and flavor to foods

Chemical Reactions of Fatty Acids

• Ester formation
• Salt formation
• Reduction
• Rancidity: development of bad odor and taste due to exposure to high temperature, oxygen, moisture, light, and metals.

Phospholipids

• Similar in structure to triacylglycerols, but with a phosphate group instead of a fatty acid.
• Sources: sunflower, chicken, eggs, bovine milk.
• Examples: lecithin, cephalin, and sphingomyelin.
• Importance:
  • Required for construction of biological membranes.
  • Important constituent of plasma lipoproteins, acting as carriers of triglycerides and cholesterol.
  • Form lung surfactant, which prevents alveolar collapse.

Glycolipids

• Found mainly in the myelin sheath and cell membrane of RBCs.
• Act as cell membrane receptors for hormones and external stimuli, providing recognition properties.

Cholesterol

• A white, waxy compound and the most important animal sterol.
• Present either free or esterified with fatty acid to form cholesteryl ester.
• Widely distributed in all tissues, but with higher concentrations in brain, nerves, liver, skin, and adipose tissue.
• Important biological functions:
  • One of the important members of membrane lipids.
  • Building block for bile salts and steroid hormones.
  • Precursor for vitamin D.