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Chapter 7

Chapter 7 . Membrane Structure and Function. Plasma Membrane. Boundary Thickness. Figure 7.1. Function of Plasma Membrane. 1. Selective Permeability. WATER. Hydrophilic head. Hydrophobic tail. WATER. Figure 7.2. Plasma Membrane Characteristics. Lipid bilayer Components:

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Chapter 7

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  1. Chapter 7 Membrane Structure and Function

  2. Plasma Membrane • Boundary • Thickness

  3. Figure 7.1 Function of Plasma Membrane 1. Selective Permeability

  4. WATER Hydrophilic head Hydrophobic tail WATER Figure 7.2 Plasma Membrane Characteristics • Lipid bilayer Components: • Phospholipids AMPHIPATHIC

  5. Plasma Characteristics cont • Proteins a. Integral protein b. Peripheral protein

  6. Hydrophobic region of protein Phospholipid bilayer Figure 7.3 Hydrophobic region of protein

  7. Cholesterol Membrane fluidity Buffer

  8. Carbohydrates Cell to cell recognition a. Glycolipids b. Glycoprotein

  9. Fluid Mosaic Model • Fluidity Membrane held together by Movement

  10. Lateral movement (~107 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids Figure 7.5 A Phospholipids on the move

  11. Membranes can solidify Fluidity affected by: 1. Unsaturated hydrocarbons 2. Saturated hydrocarbons 3. Cholesterol

  12. Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails (b) Membrane fluidity Figure 7.5 B

  13. Cholesterol Figure 7.5 (c) Cholesterol within the animal cell membrane Cholesterol has different effects on fluidity at different temperatures

  14. Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Microfilaments of cytoskeleton Peripheral protein Cholesterol Integral protein CYTOPLASMIC SIDE OF MEMBRANE Figure 7.7 Fluid Mosaic Model • Mosaic

  15. N-terminus C-terminus CYTOPLASMIC SIDE a Helix Figure 7.8 Mosaic • Integral proteins 1. Penetrate membrane 2. Transmembrane b. Peripheral proteins

  16. ATP Enzymes Signal Receptor Figure 7.9 Function of membrane proteins • Transport • Enzymatic Activity • Signal Transduction

  17. Glyco- protein Figure 7.9 Function of Membrane Proteins • Cell to Cell recognition • Intercellular joining • Attachment to the cytoskeleton and extracellular matrix

  18. Selectively Permeability • A cell must exchange materials with surroundings • Controlled by • Examples

  19. Permeability of Lipid Bilayer • Hydrophobic molecules • Polar molecules

  20. How do molecules cross the membrane? Transport protein Types 1. Channel proteins 2. Carrier proteins

  21. (a) Molecules of dye Membrane (cross section) Equilibrium Net diffusion Net diffusion Figure 7.11 A Transport Mechanism • Passive Transport a. Diffusion

  22. (b) Equilibrium Net diffusion Net diffusion Net diffusion Equilibrium Net diffusion Figure 7.11 B • Solutes diffuse down their concentration gradient, the difference in concentration of a substance from one area to another

  23. Figure 7.12 Passive Transport • Osmosis affected by the concentration gradient of dissolved substances

  24. Osmosis • 1. Tonicity a. b. c.

  25. Tonicity Types • Isotonic Solution • Hypertonic Solution • Hypotonic Solution

  26. Hypotonic solution Hypertonic solution Isotonic solution (a) Animal cell. An animal cell fares best in an isotonic environ- ment unless it has special adaptations to offset the osmotic uptake or loss of water. H2O H2O H2O Normal Shriveled Lysed Figure 7.13 Osmoregulation • Animal cells require special adaptations

  27. Help! • Plant Cell Walls can aid with water balance • Turgid • Flaccid

  28. (b) Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environ- ment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. H2O H2O H2O H2O Turgid (normal) Flaccid Plasmolyzed Figure 7.13

  29. Transport Mechanisms 1. Facilitated Diffusion Types of proteins • Channel proteins • Carrier proteins

  30. EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein (purple) has a channel through which water molecules or a specific solute can pass. Figure 7.15 Channel Proteins

  31. Solute Carrier protein (b) A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute. Figure 7.15 Carrier proteins

  32. Transport Mechanism • Active Transport a. b.

  33. 6 2 5 1 3 4 [Na+] high [K+] low Na+ Na+ Na+ Na+ Na+ ATP [Na+] low [K+] high P Na+ ADP CYTOPLASM Na+ binding stimulates phosphorylation by ATP. Cytoplasmic Na+ binds to the sodium-potassium pump. Na+ Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside. Na+ Na+ K+ P K+ K+ is released and Na+ sites are receptive again; the cycle repeats. K+ K+ K+ K+ Loss of the phosphate restores the protein’s original conformation. Extracellular K+ binds to the protein, triggering release of the Phosphate group. Example of Active Transport • The sodium-potassium pump

  34. Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP. ATP Diffusion. Hydrophobic molecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer. Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins, either channel or carrier proteins. Compare Passive & Active Transport

  35. – + H+ ATP H+ + – H+ Proton pump H+ – + H+ – + H+ Diffusion of H+ Sucrose-H+ cotransporter H+ – + – Sucrose + Figure 7.19 Transport Mechanisms • CoTransport: Coupled Pump

  36. Transport Mechanisms • Bulk Transport A. Exocytosis B. Endocytosis

  37. Exocytosis • Transport Vesicles arise from Golgi Body • Migrate • Fuse

  38. Endocytosis • Cell takes in macromolecules by forming new vesicles from the plasma membrane Three types: • Phagocytosis • Pinocytosis • Receptor-mediated

  39. EXTRACELLULAR FLUID 1 µm CYTOPLASM Pseudopodium Pseudopodium of amoeba “Food” or other particle Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM). PINOCYTOSIS 0.5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). . Vesicle

  40. RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Coated pit Ligand A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs). Coat protein Plasma membrane 0.25 µm

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