The article by P.R. Cullis and B. de Kruijff explores the functional roles of lipids in biological membranes, focusing on lipid polymorphism and its implications for membrane structure and function. The authors highlight that while the bilayer structure of membranes is well-established, the diversity of lipids within these membranes suggests additional functional roles for lipids beyond simply maintaining the bilayer. They discuss the ability of lipids to adopt non-bilayer configurations, such as hexagonal (HII) phases, and how these structures can influence membrane processes like fusion and transport. Key points include: 1. **Lipid Polymorphism**: Lipids can exist in various phases, including bilayer, hexagonal (HII), micellar, and other non-bilayer structures. Techniques like 31P NMR and freeze-fracture microscopy have been used to study these phases. 2. **Model Systems**: Studies on model membrane systems, such as erythrocyte ghosts and endoplasmic reticulum membranes, have shown that non-bilayer structures can form under certain conditions, such as the presence of divalent cations like Ca²⁺. 3. **Functional Roles**: - **Membrane Fusion**: Non-bilayer structures, particularly inverted micelles, may facilitate membrane fusion events, including exo- and endocytosis. - **Transbilayer Transport**: The formation of intracellular inverted lipid structures can enable rapid transbilayer movements of lipids, such as "flip-flop," which is crucial for transport processes. 4. **Cholesterol and Sphingomyelin**: Cholesterol can stabilize bilayer structures, while sphingomyelin may preserve bilayer integrity in the presence of high cholesterol levels. 5. **Biological Membranes**: Evidence from various biological membranes, including erythrocyte, endoplasmic reticulum, and sarcoplasmic reticulum membranes, supports the presence of non-bilayer structures and their functional roles. The authors conclude that the polymorphic capabilities of lipids provide new insights into the molecular mechanisms of membrane processes and suggest that a reevaluation of lipid functions is necessary to fully understand membrane biology.The article by P.R. Cullis and B. de Kruijff explores the functional roles of lipids in biological membranes, focusing on lipid polymorphism and its implications for membrane structure and function. The authors highlight that while the bilayer structure of membranes is well-established, the diversity of lipids within these membranes suggests additional functional roles for lipids beyond simply maintaining the bilayer. They discuss the ability of lipids to adopt non-bilayer configurations, such as hexagonal (HII) phases, and how these structures can influence membrane processes like fusion and transport. Key points include: 1. **Lipid Polymorphism**: Lipids can exist in various phases, including bilayer, hexagonal (HII), micellar, and other non-bilayer structures. Techniques like 31P NMR and freeze-fracture microscopy have been used to study these phases. 2. **Model Systems**: Studies on model membrane systems, such as erythrocyte ghosts and endoplasmic reticulum membranes, have shown that non-bilayer structures can form under certain conditions, such as the presence of divalent cations like Ca²⁺. 3. **Functional Roles**: - **Membrane Fusion**: Non-bilayer structures, particularly inverted micelles, may facilitate membrane fusion events, including exo- and endocytosis. - **Transbilayer Transport**: The formation of intracellular inverted lipid structures can enable rapid transbilayer movements of lipids, such as "flip-flop," which is crucial for transport processes. 4. **Cholesterol and Sphingomyelin**: Cholesterol can stabilize bilayer structures, while sphingomyelin may preserve bilayer integrity in the presence of high cholesterol levels. 5. **Biological Membranes**: Evidence from various biological membranes, including erythrocyte, endoplasmic reticulum, and sarcoplasmic reticulum membranes, supports the presence of non-bilayer structures and their functional roles. The authors conclude that the polymorphic capabilities of lipids provide new insights into the molecular mechanisms of membrane processes and suggest that a reevaluation of lipid functions is necessary to fully understand membrane biology.