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7 - Membrane Structue and Function
- 1. Membrane: Structure and Function UNIVERISTY OF TEHRAN Ahmad V.Kashani, PhD
- 2. Outlines • Cellular membrane, a mosaic fluid of lipids and proteins • Membrane fluidity • Membrane Proteins • Membrane carbohydrates and cell-cell recognition • Membrane structure and selective permeability • The permeability of the lipid layer • Passive transport • Diffusion • Facilitated Transport by transport proteins • Active transport • Bulk transport • Endocytosis • Exocytosis
- 3. Cell Membrane • The edge of life • Primary functions: • Separating a living cell from surrounding • Controlling the passage of substances • Selective permeability • Structure: • Phospholipids • Hydrophilic heads • Hydrophobic fatty acid chain • Proteins • Integral proteins • Channel proteins • Peripheral proteins • Certain carbohydrates
- 4. Cell Membrane • Lipid bilayer arrangement is a fluid mosaic model: • The membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids • Proteins are not randomly distributed • Lipid rafts
- 5. Cell Membrane - Fluidity • Phospholipid and protein movement • Most of them shift sideways • Lipids also flip-flop across the membrane • Proteins move using lipid rafts
- 6. Cell Membrane - Fluidity • Saturated and unsaturated fatty acid • Cholesterol in different temperatures • In high temperature the fluidity is increased resulting into decreases cohesion • In low temperature it inhibits solidification (Positive point) • In moderate temperature maintains membrane structure by reducing phospholipid movement
- 7. Cell Membrane – Membrane Proteins • Two major population of membrane proteins: • Integral proteins • Peripheral proteins
- 8. Cell Membrane – Membrane Proteins
- 9. Cell Membrane – Membrane Proteins • Proteins and their significant role in: • Drug delivery • Diagnosis
- 10. Cell Membrane – Membrane Carbohydrates • Cell-cell recognition, a cell’s ability to distinguish one type of neighboring cell from another • Membrane Carbohydrates are usually short, branched chains of fewer than 15 sugar units • Some are covalently bonded to lipids, forming molecules called glycolipids. • most are covalently bonded to proteins, which are thereby glycoproteins • Attach by the process of glycosylation • All membrane components are synthesized in ER and modified in Golgi apparatus
- 11. Cell Membrane - Review
- 12. Cell Membrane – Selective Permeability • Molecules: • Nonpolar: Dissolve in the lipid bilayer and cross it easily e.g. • Hydrocarbons • CO2 and O2 • etc. • Polar: Pass slowly due to their charge e.g. • Glucose and other sugars • All other charges molecules and atoms • Transport proteins regulate transport!
- 13. Cell Membrane – Selective Permeability • Transport Proteins • Carrier proteins: hold onto their passengers and change shape in a way that shuttles them across the membrane • Proteins channels: function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane • Ion Channels function as gate channels • Aquaporins are channels that facilitate the passage of water molecules in certain cells. • allows entry of up to 3 billion water molecules per second. Without this protein, the rate of water passage through the membrane is so low. • Transport Proteins are selective for the substance they translocate
- 14. Cell Membrane – Passive Transport • Passive transport • The diffusion of the substance across a biological membrane is called passive transport. • No energy is used • Based on molecules’ thermal energy which is derived from their constant motion • Diffusion is the movement of particles of any substance so that they spread out into the available space. • A simple rule of diffusion: In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated (Concentration gradient) https://images.app.goo.gl/MZikzdhUgeo4x5V59
- 15. Cell Membrane – Diffusion
- 16. Cell Membrane – Osmosis and water balance • Osmosis: The diffusion of free water (low solute concentration) across a selectively permeable membrane, whether artificial or cellular • Water Balance of cells without cell wall • Tonicity: the ability of a surrounding solution to cause a cell to gain or lose water • Isotonic: Water diffuses at the same rate • Hypertonic: The cell wall loses water • Hypotonic: Water enters faster than it leaves
- 17. Cell Membrane – Osmosis and water balance • Water Balance of Cells with Cell Walls • Turgor pressure: the force within the cell that pushes the plasma membrane against the cell wall. It is also called hydrostatic pressure, and defined as the pressure measured by a fluid, measured at a certain point within itself when at equilibrium. • Turgid: very firm • Flaccid: plant wilts and become limp • Plasmolysis: plant can severely wilt and it can lead to death
- 18. Cell Membrane – Active transport • Active Transport • It uses energy to move solutes against their gradients. To pump a solute across a membrane against its gradient requires energy • The transport proteins are all carrier proteins
- 19. Cell Membrane – Active transport
- 20. Cell Membrane – Active vs passive transport
- 21. Cell Membrane – Active transport Based on electrochemical gradient
- 22. Cell Membrane – Bulk transport • Bulk transport • large molecules generally cross the membrane in bulk, packaged in vesicles: • Proteins • Polysaccharides • Larger particles • Require energy • Two major classes: • Endocytosis • Phagocytosis • Pinocytosis • Receptor-mediated endocytosis • Exocytosis
- 24. Summary • Cellular membrane, a mosaic fluid of lipids and proteins • Membrane fluidity • Membrane Proteins • Membrane carbohydrates and cell-cell recognition • Membrane structure and selective permeability • The permeability of the lipid layer • Passive transport • Diffusion • Facilitated Transport by transport proteins • Active transport • Bulk transport • Endocytosis • Exocytosis