# Steroids and Prostaglandins This section details two important classes of lipids: steroids, which act as hormones and structural components, and prostaglandins, which function as local signaling molecules involved in processes like inflammation. ## Steroids Steroids are a class of lipids characterized by a specific four-ring carbon structure. Unlike acylglycerols, they do not contain fatty acids but are classified as lipids because they are hydrophobic and insoluble in water. **General Properties:** - High molecular weight. - Can be crystalline. - Synthesized from isoprene units. **Structure:** - The core structure is a **steroid nucleus** composed of 17 carbon atoms arranged in four fused rings. - Three rings have six carbon atoms each (cyclohexane rings). - One ring has five carbon atoms (cyclopentane ring). - Different steroids are distinguished by the length and structure of the side chains attached to these rings. **Examples and Functions:** - **Cholesterol:** A crucial structural component of animal cell membranes (providing fluidity) and the precursor molecule for all other steroids. It is generally absent from prokaryotic membranes. - **Bile Acids/Salts:** Emulsify fats in the digestive system, aiding in their breakdown and absorption. - **Sex Hormones:** - **Testosterone:** Male sex hormone. - **Progesterone and Estrogen:** Female sex hormones. - **Vitamin D:** Regulates calcium metabolism in the body. ## Prostaglandins Prostaglandins are lipid compounds that act as local hormones, meaning they are produced in and act on the tissues where they are needed. **Source:** Derived from the 20-carbon unsaturated fatty acid called **arachidonic acid**. **Location:** Found in virtually every mammalian tissue. **Functions:** Their roles are diverse and often specific to the tissue: - **Blood Pressure Regulation:** Some prostaglandins reduce blood pressure, while others raise it. - **Immune Response:** They help induce fever and inflammation and intensify the sensation of pain. - **Blood Clotting:** They regulate the aggregation of platelets, which is an initial step in forming blood clots. ## Science, Technology and Society Connections **Inflammation and Aspirin:** - Prostaglandins are key mediators of inflammation, a protective response characterized by redness, heat, pain, and swelling. - They cause local blood vessels to dilate, increasing blood flow, and become more permeable, allowing white blood cells to move into the affected tissue. - The drug aspirin reduces fever and pain by inhibiting the synthesis of prostaglandins (specifically by inhibiting the enzyme cyclooxygenase). ## Biochemical Tests for Lipids (STEAM Activity 2.3) Since steroids are lipids, their presence can be detected using general lipid tests. | Experiment | Observations | Inference | | :--- | :--- | :--- | | **A. Spot Test** | A clear or translucent greasy spot appears on filter paper. | Lipid is present. | | **B. Solubility Test** | Fat (e.g., butter) and water remain as separate layers. | Fat is insoluble in water. | | **C. Emulsion Test** | Adding an alcohol/lipid solution to cold water forms a cloudy white suspension (emulsion). | Lipid is confirmed. | | **E. Sudan-III Test** | Adding Sudan-III solution to a lipid-water mixture forms a dark red colored ring or layer at the surface. | Lipid is present. | ## Possible Questions/Answers **Q: What is the fundamental structure of all steroids?** **A:** A steroid nucleus consisting of 17 carbon atoms arranged in four attached rings (three 6-carbon rings and one 5-carbon ring). **Q: Name two functions of cholesterol.** **A:** 1. It is a structural component of animal cell membranes. 2. It is the precursor for other important steroids like sex hormones and Vitamin D. **Q: What are prostaglandins, and what are they derived from?** **A:** Prostaglandins are local hormones that act on nearby tissues and are derived from the fatty acid arachidonic acid. **Q: How does aspirin work to relieve pain and fever?** **A:** Aspirin inhibits the enzyme responsible for synthesizing prostaglandins. Since prostaglandins mediate pain, fever, and inflammation, blocking their production reduces these symptoms.