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Transcript
CELL MEMBRANES Topic 2.2b SPECIFICATION- TOPIC 2 2 Explain how models such as the fluid mosaic model of cell membranes are interpretations of data used to develop scientific explanations of the structure and properties of cell membranes. INTRODUCTION TO CELL MEMBRANES What’s the cause of CF symptoms? Can all substances pass across a membrane? The cell membrane is composed of the phospholipid bilayer containing proteins, glycoproteins, glycolipids and cholesterol. Phospholipids: like triglycerides but one fatty acid is replaced by a phosphate group. PHOSPHOLIPID Read pages 64-65 and answer these questions Phosphate head is hydrophilic, why? Fatty acid tail is hydrophobic, why? What happens when triglycerides are added to water? What happens when phospholipids are added to water? MEMBRANE STRUCTURE 1. How do phospholipds arrange themselves in a membrane. Explain why the phospholipids are arranged in this way. MEMBRANE STRUCTURE 2. Label the diagram below identifying the - peripheral proteins, integral proteins, glycolipids, glycoproteins, cholesterol, phospholipid heads, phospholipid tails MEMBRANE STRUCTURE 3. What is the function of the phospholipid bilayer? Keeps aqueous solutions separate- forming compartments. Only small non-polar substances can pass through because they can dissolve in lipids (because they have no charge)- e.g. oxygen, carbon dioxide 4. What are the functions of the glycoproteins and glycolipids? Involved in cell recognition- cells recognize other cells with similarly shaped glycolipids or glycoproteins and then stick together to form tissues Receptors on outside of a membrane- e.g. LDL receptors, hormone receptors. LDL or hormone binds to the receptor if the shape fits MEMBRANE STRUCTURE 5. List the functions of proteins in a membrane. Transport across the membrane- channel proteins and carrier proteins Structural- help build up cell membrane Enzymes on the surface of a membrane- catalyse reactions Receptors on outside of a membrane- e.g. LDL receptors, hormone receptors. LDL or hormone binds to the receptor if the shapes fit. Proteins on the outside- involved in cell recognitioncells recognize other cells with similarly shaped proteins and stick together to form tissues 6. Is the protein in the picture below a globular or fibrous protein? Explain your answer. 7. Which part of this protein is made of mainly hydrophillic amino acids? Hydrophobic amino acids? Explain how you knew this. THEORETICAL MODELS: CELL MEMBRANES Aims of the lesson • When scientists produce theoretical models, they use their imagination and creativity to think about data in new ways. The theoretical models that they produce are therefore more than careful descriptions of the data. • Because the models go beyond the data, more than one theoretical model can be supported by the available evidence. • In some cases new evidence is gathered which shows one model to be better than another. STRUCTURAL MODELS OF MEMBRANES Task 1 You should have a copy of sheet B1 .3 ‘Lipid layer evidence’. 1.1 From looking at the data in the table, would you agree with the conclusions of Gorter and Grendel? 1.2 What aspects of the membrane structure is there no evidence for in this data? Task 2 You should have been given sheet B2.1‘Electronmicrograph evidence’ and a description of two dif ferent models. 2.1 Which model (Danielli and Davson’s, Robertson’s (or both) is supported by the evidence so far? Explain your answer. Look at the “Timeline” 2.2 Describe what you think led to each model being devised. STRUCTURAL MODELS OF MEMBRANES Task 3 B3.1 ‘Freeze fracture electronmicrograph evidence’, B3.2 ‘NMR and X-ray dif fraction evidence’ and B3.3 ‘Singer and Nicholson’s model’. The time line will help you see the order these pieces of evidence and models came in. 3.1 How is each of the models, including Singer and Nicholson’s, supported or undermined by all the evidence now available? 3.2 Which do you think is the most useful model? Justify your answer. Look at the ‘Timeline’ to see how the models were developed. USEFUL ANIMATIONS https://youtu.be/Qqsf_UJcfBc https://youtu.be/jEY9Bie92aM- Mr. Pollock https://youtu.be/Pfu1DE9PK2w - rap SINGER AND NICHOLSON’S FLUID MOSAIC MODEL 8. The model above is based on Singer and Nicholson’s fluid mosaic model. Explain why it is called the fluid mosaic model. FEATURES OF THE MODEL AND EVIDENCE USE ALL THE EVIDENCE INCLUDING THAT IN THE TEXBOOK Feature of the model Evidence Lipids present Non-polar substances pass through easily, charged don’t Protein channels (hydrophobic/philic regions) Bilayer Charged substance (ions and polar covalent substances) can pass through • Phospholipid behaviour in water • Total surface area of lipids one molecule thick is 2 x surface area of cell membrane • Electronmicrographs (EM) FEATURES OF THE MODEL AND EVIDENCE Feature of the model Evidence Proteins scattered in mosaic Some proteins integral/ some peripheral Freeze fracture EM Fluid • X ray diffraction • NMR scanning • Fusing 2 cells- proteins from each cell intermixed • Q 2.14 Carbohydrate found on outer surface of the membrane only Lectin (which naturally bind to polysaccharides) only adheres to outer surface • Freeze fracture EM • Some easily removed • Some only removed if the whole membrane is disrupted MEMBRANE STRUCTURE 10.Why is the membrane more fluid with unsaturated fatty acids than with saturated fatty acids? MORE «kinks» NOT as closely packed LESS interactions Move MORE freely. EVIDENCE FOR FLUID MOSAIC MODEL OF MEMBRANE SPECIFICATION- TOPIC 2 2 Explain how models such as the fluid mosaic model of cell membranes are interpretations of data used to develop scientific explanations of the structure and properties of cell membranes.