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A Closer Look at Cell Membranes

This chapter explores the organization of cell membranes, including the lipid bilayer structure, fluid mosaic model, and variations in membrane composition. It also discusses the different types of membrane proteins and their functions, as well as diffusion, membrane permeability, and passive and active transport mechanisms. The impacts and issues surrounding cystic fibrosis are also examined.

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A Closer Look at Cell Membranes

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  1. A Closer Look at Cell Membranes Chapter 5 阮雪芬 10/2/2012

  2. Impacts, Issues:One Bad Transporter and Cystic Fibrosis • Transporter proteins regulate the movement of substances in and out of cells; failure of one of these proteins causes cystic fibrosis (囊腫性纖維化)

  3. Video: One bad transporter and cystic fibrosis

  4. 5.1 Organization of Cell Membranes • The basic structure of all cell membranes is the lipid bilayer with many embedded proteins • A membrane is a continuous, selectively permeable barrier (選擇性通透)

  5. Revisiting the Lipid Bilayer • Phospholipid molecules in the plasma membrane have two parts • Hydrophilic heads interact with water molecules • Hydrophobic tails interact with each other, forming a barrier to hydrophilic molecules

  6. Cell Membrane Organization

  7. The Fluid Mosaic Model • Fluid mosaic model • Describes the organization of cell membranes • Phospholipids drift and move like a fluid • The bilayer is a mosaic mixture of phospholipids, steroids, proteins, and other molecules

  8. Fluid Mosaic Model

  9. Variations on the Model • Differences in membrane composition(組成差異) • Different kinds and numbers of carbohydrates attached to membrane proteins • Different kinds of phospholipids(通常至少有一個脂肪酸尾部不飽和而彎曲) • Differences in fluidity(流體性差異) • Some proteins are attached to the cytoskeleton,因此固定不遊走; others drift around • Archaeans have more rigid membranes than bacteria or eukaryotes(因為磷脂尾部間形成共價鍵,使古代菌細胞膜異常堅固)

  10. Two Studies of Membrane Structure

  11. 5.2 Membrane Proteins • Cell membrane function begins with the many proteins associated with the lipid bilayer

  12. Membrane Proteins • Each type of protein in a membrane has a special function • Adhesion proteins • Recognition proteins • Receptor proteins • Enzymes • Transport proteins (active and passive)

  13. Common Types of Membrane Proteins

  14. Adhesion Protein Enzyme Receptor Protein Recognition Protein Passive Transporter Active Transporter Stepped Art Fig. 5-5, pp. 80-81

  15. 5.1-5.2 Key Concepts: Membrane Structure and Function • Cell membranes have a lipid bilayer that is a boundary between the outside environment and the cell interior • Diverse proteins embedded in the bilayer or positioned at one of its surfaces carry out most membrane functions

  16. 5.3 Diffusion, Membranes, and Metabolism • Ions and molecules tend to move from one region to another, in response to gradients

  17. Membrane Permeability • Selective permeability • The ability of a cell membrane to control which substances and how much of them enter or leave the cell • Allows the cell to maintain a difference between its internal environment and extracellular fluid • Supplies the cell with nutrients, removes wastes, and maintains volume and pH

  18. The Selectively Permeable Nature of Cell Membranes

  19. Concentration Gradients • Concentration • The number of molecules (or ions) of substance per unit volume of fluid • Concentration gradient • The difference in concentration between two adjacent regions • Molecules move from a region of higher concentration to one of lower concentration

  20. Diffusion • Diffusion • The net movement of molecules down a concentration gradient • Moves substances into, through, and out of cells • A substance diffuses in a direction set by its own concentration gradient, not by the gradients of other solutes around it (只跟自種粒子的濃度梯度差有關,與他種粒子無關)

  21. Examples of Diffusion

  22. The Rate of Diffusion • Rate of diffusion depends on five factors • Size(小分子移動耗能小,故擴散快) • Temperature • Steepness of the concentration gradient • Charge • Pressure

  23. How Substances Cross Membranes • Gases and nonpolar molecules diffuse freely across a lipid bilayer (課本說水也是但…無視) • Ions and large polar molecules require other mechanisms to cross the cell membrane • Passive transport • Active transport • Endocytosis and exocytosis

  24. Membrane-Crossing Mechanisms Fig. 5-8 (a-c), p. 83

  25. Fig. 5-8 (d-e), p. 83

  26. 5.4 Passive and Active Transport • Many types of molecules and ions diffuse across a lipid bilayer only with the help of a specific transport protein

  27. Passive Transport • Passive transport (facilitated diffusion) • Requires no energy input • A passive transport protein allows a specific solute (such as glucose) to follow its concentration gradient across a membrane • A gated passive transporter changes shape when a specific molecule binds to it. An open-channel transporter doen not. (葡萄糖的是gated,因此運輸過程形狀會改變)

  28. Passive Transport

  29. Passive Transport

  30. Passive Transport

  31. Active Transport • Active transport • Requires energy input (usually ATP) • Moves a solute against its concentration gradient, to the concentrated side of the membrane • Calcium pumps • Active transporters move calcium ions across muscle cell membranes into the sarcoplasmic reticulum

  32. Active Transport: Calcium Pump

  33. Active Transport: Calcium Pump

  34. Cotransport

  35. Cotransport: Sodium-Potassium Pump

  36. 5.3-5.4 Key Concepts: Diffusion and Membrane Transport • Gradients drive the directional movements of substances across membranes • Transport proteins work with or against gradients to maintain water and solute concentrations

  37. 5.5 Membrane Trafficking • By processes of endocytosis and exocytosis, vesicles help cells take in and expel particles that are too big for transport proteins, as well as substances in bulk • Membrane trafficking (膜轉運) • Formation and movement of vesicles formed from membranes, involving motor proteins and ATP 這必定耗能,因此必定牽涉馬達蛋白和ATP

  38. Exocytosis and Endocytosis • Exocytosis (胞吐) • The fusion of a vesicle with the cell membrane, releasing its contents to the surroundings • Endocytosis (胞吞) • The formation of a vesicle from cell membrane, enclosing materials near the cell surface and bringing them into the cell

  39. Endocytosis and Exocytosis

  40. Exocytic Vesicle

  41. Three Pathways of Endocytosis • Bulk-phase endocytosis (內陷成泡) • Extracellular fluid is captured in a vesicle and brought into the cell; the reverse of exocytosis • 非選擇性,因不使用receptor • Receptor-mediated endocytosis (受體媒介) • Specific molecules bind to surface receptors, which are then enclosed in an endocytic vesicle • 具選擇性 • Phagocytosis (偽足) • Pseudopods engulf target particle and merge as a vesicle, which fuses with a lysosome in the cell • 其實也有用到receptor,具選擇性

  42. Receptor-Mediated Endocytosis

  43. Phagocytosis

  44. Membrane Cycling • Exocytosis and endocytosis continually replace and withdraw patches of the plasma membrane 因此細胞膜會不斷進行修補、更新 • New membrane proteins and lipids are made in the ER, modified in Golgi bodies, and form vesicles that fuse with plasma membrane

  45. 5.5 Key Concepts: Membrane Trafficking • Large packets of substances and engulfed cells move across the plasma membrane by processes of endocytosis and exocytosis • Membrane lipids and proteins move to and from the plasma membrane during these processes

  46. 5.6 Which Way Will Water Move? • Water diffuses across cell membranes by osmosis (滲透壓) • Osmosis is driven by tonicity (液張力), and is countered by turgor (膨壓)

  47. Osmosis • Osmosis • The movement of water down its concentration gradient – through a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration • Tonicity (液張力) • The relative concentrations of solutes in two fluids separated by a selectively permeable membrane

  48. Osmosis

  49. Tonicity • For two fluids separated by a semipermeable membrane, the one with lower solute concentration is hypotonic (低張), and the one with higher solute concentration is hypertonic (高張) • Water diffuses from hypotonic to hypertonic • Isotonic (等張) fluids have the same solute concentration

  50. Experiment: Tonicity

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