A Closer Look at Cell Membranes

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