Lecture 28: Chemistry of Lipids

Study Notes

Lipids are a diverse group of compounds including fats, oils, steroids, waxes and related compounds.
They are mainly characterized by their physical properties rather than chemical ones.
Lipids are largely insoluble in water but soluble in nonpolar solvents like ether and chloroform.

Discuss the classification of lipids, fatty acids, phospholipids, triacylglycerols, cholesterol, and prostaglandins, and their biochemical actions.

Lipids have various functions, including being an energy source, creating a protective barrier, and acting as electrical insulation.
Lipids are important components of cell membranes.

Storage of energy: Triglycerides stored in adipose tissue.
Structural components of biomembranes: Phospholipids and cholesterol.
Metabolic regulators: Steroid hormones and prostaglandins.
Surfactants, detergents and emulsifying agents: Amphipathic lipids.
- Molecules containing both hydrophobic and hydrophilic groups.
- Orient at oil-water interfaces with polar groups facing water and nonpolar groups facing oil.
Electrical insulators: Lipids such as myelin insulate nerve cell axons.

Lipids are classified into 4 categories:
- Chemical composition (simple, complex, derived, miscellaneous)
- Fatty acids
- Requirements (essential/non-essential)
- Sources (visible/invisible)
Lipids are further categorized into simple, compound and derived lipids.

Fats and Oils: Esters of fatty acids and glycerol (also known as triglycerides). These constitute a major portion of dietary lipid and body fats (98-99%).
Waxes: Esters formed from long-chain fatty acids and long-chain alcohols. They protect plants from dehydration and invasion by microorganisms.

Phospholipids: Contain phosphoric acid and a nitrogenous base in addition to fatty acids and glycerol. Examples include lecithin and cephalin. Abundant in brain & nerve tissues.
Glycolipids: Complex lipids containing carbohydrates in combination with fatty acids and glycerol. Examples include cerebrosides.
Sulfolipids: Possess a sulfur-containing functional group and are found in photosynthetic organisms.
Lipoproteins: Carriers of lipids in the blood, involved in cell membrane formation. Examples include LDL and HDL.

Sterols: Solid alcohols that form esters with fatty acids. Cholesterol is a crucial waxy, fat-like substance found in all cells; synthesized by the liver and acts as a precursor for steroid hormones, Vitamin D and bile.
Fatty Acids: Key building blocks and energy sources for cells. Classified as saturated/unsaturated. Examples include oleic, linoleic, and linolenic acids.

These molecules are derived from 20-carbon polyunsaturated fatty acids, most commonly arachidonic acid.
They include prostaglandins, thromboxanes, leukotrienes, and lipoxins.
They act as local hormones and influence neighboring cells.

First discovered in semen, they cause uterine contractions and lower blood pressure.
Synthesized in most tissues (except red blood cells).
Are highly potent and have very short lifespans.
Affect smooth muscle contraction, blood pressure, and inflammation.
Nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin inhibit prostaglandin synthesis by blocking an enzyme called cyclooxygenase.

Abnormal lipid metabolism is linked to diseases like obesity, atherosclerosis, diabetes mellitus, hyperlipoproteinemia, and fatty liver.

Membrane structure and function: Essential for cell membranes and associated processes.
Vision: Crucial for retinal function, particularly DHA.
Nervous system: High concentrations of fatty acids are important for brain function.
Deficiency: Associated with skin problems, growth retardation in children, infections, and impaired wound healing.

Students should be able to classify lipids, outline lipid structure and biochemical functions, and explain the clinical importance of essential fatty acids.