Biology Chapter: Steroids and Sterols

Steroids and Sterols

• Steroids are lipids with a four-ring structure (cyclohexane rings and cyclopentane).
• They are important for cell membrane integrity and hormonal signaling.
• Sterols are lipids with a steroid nucleus, a hydroxyl group at carbon 3, and a side chain at carbon 17.
• They are essential membrane components in eukaryotes, maintaining membrane rigidity, fluidity, and permeability.

Cholesterol

• Found in animal tissues and some fungi, produced by nucleated animal cells.
• A structural component of cell membranes, crucial for membrane stability and fluidity.
• A precursor for steroid hormones, vitamin D, and bile acids.
• Chemical formula: C27H46O
• Has a hydroxyl group at C3, a double bond between C5 and C6, and an 8-carbon side chain at C17 making it weakly amphiphilic.

Ergosterol

• Found in fungal and protozoan cell membranes.
• Similar functions to cholesterol in animal cells.
• Plays a role in membrane structure, formation of lipid rafts, and growth regulation.
• A precursor to vitamin D2.
• Targeted by antifungal drugs.
• Structure includes ergostane with double bonds at 5,6-, 7,8-, and 22,23- positions, and a 3β-hydroxy group making it a 3β-sterol and phytosterol.

Vitamin D

• Vitamin D2: has a double bond between carbons 22 and 23 and a methyl group at carbon 24.
• Regulates calcium and phosphorus levels, supporting bone health and immune function.
• Derived from ergosterol in yeast and fungi when exposed to UV light.
• Vitamin D3: has a single bond between carbons 22 and 23 and lacks the methyl group at carbon 24.
• More potent than Vitamin D2.
• Naturally produced in the skin from sunlight and found in animal products like fatty fish and egg yolks.

Clinical Relevance

• Membrane Structure: Amphipathic lipids (like phospholipids) form cell membranes; defects can cause diseases like cystic fibrosis.
• Drug Delivery: Liposomes (using amphipathic lipids) are used for targeted drug delivery, especially in cancer treatments.
• Cholesterol and Cardiovascular Disease: Cholesterol imbalances (HDL/LDL) contribute to atherosclerosis, increasing heart attack and stroke risk.
• Bile Acids: Critical for fat digestion; defects cause gallstones and bile acid malabsorption.
• Neurodegenerative Diseases: Sphingolipids maintain myelin integrity; abnormalities lead to multiple sclerosis and Fabry disease.
• Inflammation: Amphipathic lipids are involved in immune signaling; imbalances contribute to conditions like rheumatoid arthritis and asthma.

Liposomes

• Formed when amphipathic lipids in an aqueous medium are subjected to sonification.
• Have intermittent aqueous phases within the lipid bilayer.
• Used as drug carriers to target tissues, often combined with tissue specific antigens.

Emulsion

• Formed when nonpolar lipids (like triacylglycerols) are mixed with water.
• Particles are larger in size.
• Stabilized by emulsifying agents (often amphipathic lipids), such as bile salts and phospholipids.

Soaps vs. Detergents

• Soaps: Sodium or potassium salts of fatty acids, produced by saponification of fats.
• Hard sodium soaps result in bar soaps.
• Cleansing agents that emulsify oils and remove dirt.
• Detergents: Synthetic cleansing agents (e.g., sodium lauryl sulfate).
• Superior cleansing action compared to soaps.
• Used in clothes washing and toothpaste.

Explore the critical roles of steroids and sterols in cellular functions. This quiz covers their structures, significance in membranes, and the vital functions of cholesterol and ergosterol. Test your knowledge on lipid biology and hormonal signaling mechanisms.