Explain the structure and functions of glycerophospholipids.

You're on a roll — here are the answers to the selected questions from the exam paper: Section A 1.c. Explain the structure and functions of glycerophospholipids. Glycerophospholipids are a class of lipids that are major components of cell membranes. Structure: A glycerophospholipid consists of a glycerol backbone to which two fatty acid chains are attached via ester linkages at the first and second carbon atoms. A phosphate group is attached to the third carbon of the glycerol backbone. This phosphate group is often further esterified to a small, polar alcohol molecule (e.g., choline, ethanolamine, serine, inositol). This structure gives glycerophospholipids an amphipathic* nature, meaning they have both a hydrophilic (water-loving) head (the phosphate and alcohol group) and hydrophobic (water-fearing) tails (the fatty acid chains). Functions: Cell Membrane Formation: Their amphipathic nature allows them to spontaneously form lipid bilayers in aqueous environments, which are the fundamental structure of all biological membranes. This bilayer acts as a barrier, regulating the passage of substances into and out of the cell. Cell Signaling: Some glycerophospholipids, such as phosphatidylinositol, are precursors to important intracellular signaling molecules (second messengers) that regulate various cellular processes like growth, metabolism, and immune responses. 1.e. What is the significance of omega-3 and omega-6 fatty acids in human health? Omega-3 and omega-6 fatty acids are polyunsaturated fatty acids (PUFAs) that are essential for human health, meaning the body cannot synthesize them and they must be obtained through diet. Omega-3 Fatty Acids: These have their first double bond at the third carbon atom from the methyl end of the fatty acid chain. Key omega-3 fatty acids include alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). They are significant for: Anti-inflammatory Effects: They produce eicosanoids that have anti-inflammatory properties, which can help reduce the risk of chronic diseases. Brain and Eye Health: DHA is a major structural component of the brain and retina, crucial for cognitive function and vision development. Cardiovascular Health: They help lower triglyceride levels, reduce blood pressure, and prevent blood clot formation, thus supporting heart health. Omega-6 Fatty Acids: These have their first double bond at the sixth carbon atom from the methyl end. The primary omega-6 fatty acid is linoleic acid (LA). They are significant for: Growth and Development: Essential for normal growth and development. Skin Health: Contribute to maintaining skin barrier function. Pro-inflammatory Effects: While necessary, excessive intake of omega-6 fatty acids relative to omega-3s can lead to the production of pro-inflammatory eicosanoids, potentially contributing to chronic inflammation and disease. The balance between omega-3 and omega-6 fatty acids in the diet is crucial for optimal health. 2.a. Define active site and explain its significance in enzyme catalysis. Active Site: The active site is a specific, three-dimensional region on an enzyme molecule where substrate molecules bind and undergo a chemical reaction. It is typically a small cleft or crevice formed by the folding of the enzyme's polypeptide chain, bringing together specific amino acid residues from different parts of the primary sequence. Significance in Enzyme Catalysis: Specificity: The unique shape and chemical properties of the active site allow it to bind only specific substrate molecules (or a small range of structurally similar molecules), ensuring the enzyme catalyzes only particular reactions. This is often described by the "lock and key" or "induced fit" models. Catalysis: The active site provides an optimal microenvironment for the chemical reaction to occur. It positions the substrate molecules correctly, facilitates bond breaking and formation, and often contains catalytic amino acid residues that directly participate in the reaction mechanism (e.g., by acting as acid/base catalysts or forming transient covalent bonds). This significantly lowers the activation energy of the reaction, thereby increasing the reaction rate by millions of times compared to uncatalyzed reactions. Section B 4.a. Define the terms purines and pyrimidines. What are their significance to base pairing in DNA and RNA? Purines: Purines are nitrogenous bases characterized by a double-ring structure (a six-membered ring fused to a five-membered ring). In nucleic acids, the purines are adenine (A) and guanine (G)*. Pyrimidines: Pyrimidines are nitrogenous bases characterized by a single six-membered ring structure. In DNA, the pyrimidines are cytosine (C) and thymine (T). In RNA, uracil (U)* replaces thymine. Significance to Base Pairing: Purines and pyrimidines are crucial for the formation of the double-helical structure of DNA and the various structures of RNA through specific hydrogen bonding* interactions, known as Watson-Crick base pairing: In DNA, adenine (A) always pairs with thymine (T) via two hydrogen bonds (A=T). Guanine (G) always pairs with cytosine (C) via three hydrogen bonds (G≡C). In RNA, adenine (A) pairs with uracil (U) via two hydrogen bonds (A=U). This specific pairing ensures the accurate replication and