Isoprenoids and Steroids: Biological Functions and Types

Isoprenoids: Structure and Classification

Isoprenoids are molecules derived from the polymerization of a monomer called 2-methyl-1,3-butadiene (isoprene). These linear or cyclical chains are classified by the number of molecules they contain:

• Monoterpenes: 2 isoprene molecules. These compose plant essences such as menthol in mint.
• Diterpenes: 4 isoprene molecules. Examples include vitamins A, E, and K.
• Triterpenes: 6 isoprene molecules. For example, squalene, from which cholesterol is synthesized in the liver.
• Tetraterpenes: 8 isoprene molecules. These are found in photosynthetic pigments called carotenoids.
• Polyterpenes: More than 8 molecules. Notably, rubber is a polymer composed of thousands of isoprene molecules.

Steroids: Lipid Derivatives

Steroids are lipid derivatives. There are two primary classes:

Sterols

Sterols possess a hydroxyl group attached to carbon 3 and an aliphatic chain at carbon 17. This is the largest group of steroids, including:

• Cholesterol: A vital component of animal cell membranes.
• Bile acids: Produced in the liver from cholesterol; they derive from bile salts.
• Vitamin D group: A set of sterols that regulate calcium metabolism and intestinal absorption.
• Estradiol: A hormone responsible for regulating the appearance of female secondary sexual characteristics.

Steroid Hormones

These hormones have an oxygen atom attached to carbon 3 via a double bond. They are divided into two groups:

• Suprarenal hormones: Includes aldosterone, which increases the reabsorption of sodium and chloride ions in the kidney, and cortisol, which promotes glucose and glycogen synthesis as well as the catabolism of lipids and proteins.
• Sex hormones: Includes progesterone, which prepares female sexual organs for pregnancy, and testosterone, responsible for male sexual characteristics.

Prostaglandins

Prostaglandins are substances derived from prostanoic acid, which consists of a cyclopentene ring and two aliphatic chains.

Biological Roles of Lipids

Lipids fulfill several essential functions in the body:

• Energy Reserve: Lipids serve as the body's principal energy reserves.
• Structural: Lipids form the lipid bilayer of cell membranes.
• Protection: Glycerides line organs to protect them from physical shocks.
• Biocatalysis: Vitamins, hormones, and prostaglandins act as lipid biocatalysts.
• Transport: Lipids are transported from the intestine to storage or usage sites via lipid emulsions formed by bile acids and proteolipids.