Lipids and Their Classification

Study Notes

Lipids are insoluble in water but soluble in fat solvents (non-polar solvents) like benzene, chloroform, acetone, and ether.
Lipids are physiologically important for humans and have four major functions.
Lipids serve as structural components of biological membranes.
Lipids provide energy reserves, mainly in the form of triacylglycerols.
Lipids and lipid derivatives function as vitamins and hormones.
Bile acids and bile salts aid in fat emulsification.

Lipids are classified into simple lipids (fatty acid + glycerol), compound lipids (fatty acid + alcohol + other group), and derived lipids (hydrolysis products of simple or compound lipids).
Simple lipids include neutral fats (TAGs) and waxes.
Compound lipids include phospholipids and glycolipids.
Derived lipids include steroids, carotenoids, and fat-soluble vitamins.

Fatty acids are long-chain hydrocarbon molecules with a carboxylic acid group (-COOH) at one end.
The hydrocarbon chain (R) makes fatty acids hydrophobic.
The -COOH group gives fatty acids hydrophilic properties.
Fatty acids have an amphipathic nature, meaning they have both hydrophobic and hydrophilic regions.

Fatty acids are classified as saturated or unsaturated based on the presence or absence of double bonds.
Saturated fatty acids have no double bonds.
Unsaturated fatty acids have one or more double bonds.

Oleic acid (C18:1)
Linoleic acid (C18:2)
Linolenic acid (C18:3)
Arachidonic acid (C20:4)
Polyunsaturated fatty acids (PUFAs) have two or more double bonds.
Linoleic and linolenic acids are essential fatty acids because the body cannot make them.

Eicosanoids are hormone-like molecules produced by most mammalian tissues.
They're derived from polyunsaturated fatty acids, specifically those with 20 carbon atoms (eicosanoids).
Examples include arachidonic acid.
Eicosanoids act locally within the same cells or adjacent cells.

Eicosanoids are classified into cyclic and acyclic compounds.
- Cyclic compounds include prostaglandins, prostacyclins, and thromboxanes.
- Acyclic compounds include leukotrienes and lipoxins.

Prostaglandins are cyclic eicosanoids produced by most tissues.
They play roles in inflammation, uterine contractions, and gastric secretions, and some produce vasodilation or vasoconstriction.

Prostacyclins are cyclic eicosanoids produced by the endothelium of blood vessels.
Prostacyclins cause vasodilation and inhibit platelet aggregation.

Leukotrienes are acyclic eicosanoids produced by leukocytes, platelets, and mast cells.
They are released during allergic reactions; and increase vascular permeability, and cause bronchospasm.