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BIO 105

BIO 105. Cell Membranes. Cell Membranes. Cell Membranes. The eukaryotic cell possesses many membranes The outer membrane is called the plasma membrane . Many organelles are bounded by membranes…..ER, mitochondria, Golgi, nucleus All membranes have similar properties. Plasma Membrane.

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BIO 105

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  1. BIO 105 Cell Membranes

  2. Cell Membranes

  3. Cell Membranes • The eukaryotic cell possesses many membranes • The outer membrane is called the plasma membrane. • Many organelles are bounded by membranes…..ER, mitochondria, Golgi, nucleus • All membranes have similar properties.

  4. Plasma Membrane • The interface between the cell and the external environment • Physical and chemical barrier • Selective • Responsive to signals • Only 2 molecules thick • No plasma membrane….no cell • Comprised of 2 layers of lipid molecules and specific proteins….the cell’s personality

  5. Membrane Structure • 10,000 membranes = thickness of sheet of paper • Lipids form the foundation of the membrane • Lipids are phospholipids

  6. Phospholipids • Consist of: • 3 carbon backbone derived from glycerol • 2 fatty acids • Phosphorylated alcohol group (charged)

  7. Structure of Phospholipids

  8. Some Definitions • Hydrophilic (water-loving) molecules that are charged (polar) and will attract H2O . • Hydrophobic (water-hating) molecules that are uncharged (non-polar)and will shun water.

  9. Structure of Phospholipids

  10. So, what happens when you place a phospholipid into H2O? • The hydrophilic (charged) region is just fine and attracts H2O. • But what happens to the hydrophobic region (the fatty acids)….no place to run • A single phospholipid molecule in H2O never happens in nature. • But multiple phospholipid molecules do occur in H2O ….what happens then? • They form a bilayer.

  11. Phospholipid Bilayer OUTSIDE INSIDE

  12. Phospholipid Bilayer • Forms spontaneously • Polar (hydrophilic) portion faces outside H2O and inside H2O • Non-polar (hydrophobic) fatty acids face each other and form the middle of the “sandwich”. • Phospholipids are free to float laterally within each of the bilayers. • About the same viscosity (flow) as olive oil.

  13. Phospholipid Bilayer OUTSIDE INSIDE

  14. The Fluid Mosaic Model

  15. Features of the Model • Cell membranes comprised of: • A phospholipid bilayer • Proteins • Peripheral – on exterior or interior surface • Transmembrane – spanning the entire width

  16. The Fluid Mosaic Model

  17. Transmembrane Proteins • Two types: • Free to float laterally within the lipid bilayers • Fixed in a specific place of the membrane by the cytoskeleton

  18. The Fluid Mosaic Model

  19. Free FloatingTransmembrane Proteins

  20. Fixed Transmembrane Proteins

  21. Membrane Protein Functions • Transporter – inside  outside

  22. Membrane Protein Functions 2. Enzyme – substrate  product

  23. Membrane Protein Functions 3. Cell Surface Receptors – hormones, etc.

  24. Membrane Protein Functions 4. Cell Surface identity marker -- glycoprotein

  25. Membrane Protein Functions 5. Cell Adhesion – shared molecules

  26. Membrane Protein Functions 6. Attachment to cytoskeleton

  27. Structure of Transmembrane Proteins

  28. Structure of Transmembrane Proteins • Portions of protein exposed to outside H2O and inside H2O (both are polar and hydrophilic) • Portions of protein that spans the width of the non-polar, hydrophobic, fatty acid width of membrane several times

  29. Structure of Transmembrane Proteins Based on neutral and charged AA sequence

  30. Diffusion • Molecules in water are in constant random motion. • This motion causes these molecules to move from regions of high  low concentration. • This process is called diffusion. • When molecules are in equal concentration in all regions, the substance is said to be in equilibrium.

  31. Diffusion Equilibrium • Solvent • Solute • Aqueous solution

  32. Diffusion Across a Membrane • Both water and solutes will diffuse down their concentration gradients. • But what happens when we consider the biological world and consider the plasma membrane (PM). • Only H2O and hydrophobic (non-polar) molecules can freely pass across the lipophilic (fatty acids) portion of the PM. • Most solutes (ions, AA, sugars) are not lipid soluble and can’t diffuse across H2O the PM. • H2O can diffuse down its gradient across the PM.

  33. Diffusion Across a Membrane

  34. Osmosis • Osmosis is the movement of H2O across a membrane that permits H2O flow but not that of one or more solutes. • Osmotic concentration – the concentration of all solutes on one side of a selective membrane. • If the concentration of a solution is higher on one side of a membrane, the solution is said to be hyperosmotic. • If the concentration of a solution is lower on one side of a membrane, the solution is said to be hypoosmotic.

  35. Osmosis (cont.) • If the osmotic concentration of solutions on both sides is equal, the solutions are said to be isosmotic. • Water will flow across a membrane in the direction of the hyperosmotic solution in order to dilute the solution.

  36. Osmosis Demonstration hydrostatic (osmotic) pressure How do polar molecules get across the PM?

  37. Bulk Passage In and Out of Cells • Endocytosis PM engulfment of surrounding fluids or large particles such as bacteria. Three types: are:

  38. Endocytosis • Pinocytosis – ingestion of surrounding extracellular fluids – non-selective • Phagocytosis – ingestion of particulate matter and bacteria

  39. 3. Receptor-mediated 3. Selective engulfment

  40. Bulk Passage In and Out of Cells • Exocytosis • Reverse of endocytosis • Extrusion of bulk materials via membrane-bound vesicles • Adds membrane to PM

  41. Question • Bulk transport aside, what about hydrophilic (polar) molecules? • How do they get across the PM? • Transport proteins • They are trans-membrane proteins within the PM

  42. 1. Ion Channels • These proteins have a hollow channel that spans the entire width of the PM. • Channel is filled with H2O. • Ion channels are specific for specific ions– Ca++, Cl-, Na+, K+ • Direction of ion flow (in or out of cell) is determined by concentration gradient. • These channels are ! In nerve and muscle cells

  43. 1. Ion Channels (cont.)

  44. 2. Facilitative Diffusion • Proteins that physicallybind specific solutes • Transport solutes from one side of membrane • Release them on other side of membrane • Direction depends on [solute] gradient • Passive diffusion vs. facilitative diffusion

  45. 2. Facilitative Diffusion (cont.) • Can be saturated…limited by the number of transport proteins rate of transport of solute (mol/sec) [solute]

  46. 2. Facilitative Diffusion (cont.)

  47. Passive Diffusion • Ion channel and facilitative transport are examples of passive diffusion. • They do not require energy. • Solutes are driven by concentration gradient (high  low) • But what about transporting solutes against the concentration gradient?

  48. Active Transport • Ability of cell membrane to move solutes against their concentration gradient • Can occur in either direction (in or out) • Requires significant expenditure of cells’ energy (ATP)

  49. 3. Na+/K+ Pump • Most animal cells have low internal [Na+] and high internal [K+]. • They do this by actively pumping Na+ out of the cell and actively pumping K+ into the cell. • This process requires ~ 1/3 of a cell’s total energy (ATP) output.

  50. Na+/K+ Pump

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