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Membrane Structure and Function

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meira
16 diapositives · 413 Views · Jul 31, 2014

This chapter explores the structure and function of the cell plasma membrane, highlighting its amphipathic nature and selective permeability. It discusses how hydrophobic substances like hydrocarbons, CO2, and O2 can easily pass through, while hydrophilic substances such as amino acids and ions require transport proteins. The phospholipid bilayer's role in membrane fluidity, the impact of cholesterol, and the various protein functions are explained. Additionally, it covers passive and active transport mechanisms, including osmosis, facilitated diffusion, bulk transport, and receptor-mediated endocytosis.

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Membrane Structure and Function
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  1. Chapter 7 Membrane Structure and Function

  2. Cell/Plasma Membrane • Selectively permeable and amphipathic • some substances can pass easily (hydrophobic/nonpolar) • hydrocarbons, CO2, O2 • other substances cannot pass easily (hydrophilic/polar) • amino acids, nucleic acids, carbohydrates, proteins, ions • Small molecules like glucose, H2O and some ions flow through the membrane constantly AND slowly due to the hydrophobic layer of fatty acids slowing them down

  3. A Phospholipid Bilayer • The plasma membrane is made of phospholipids with a phosphate head and 2 fatty acids tails with varying saturation (some have kinks while others don’t) • FYI - The ability of some organisms to regulate the fluidity of their cell membranes by altering lipid composition is called homeoviscous adaptation. • Also has proteins and carbohydrates held together by weak interactions = FLUIDITY • FLUID MOSAIC MODEL

  4. Fluidity • Cholesterol reduces fluidity by reducing movement but at low temps it hinders solidification by preventing the “packing” of phospholipids

  5. Fluid Mosaic Model

  6. Besides the phospholipids… • Proteins • Integral Proteins are embedded in the membrane and span across it (transmembrane) • Peripheral Proteins are loosely bound to the membrane surface

  7. 6 functions of proteins in the membrane

  8. Besides the phospholipids… • Carbohydrates • Used in cell to cell recognition (important for immune function and developing organisms)

  9. How does everything get through the membrane? • Hydrocarbons, CO2, O2 can easily pass • Ions and polar molecules usually pass through transport proteins that span the membrane • H2O pass through the membrane slowly AND through aquaporins (hydrophilic channels) quickly = up to 3,000,000,000 H2O molecules/second

  10. Passive Transport • Diffusion of a substance across a membrane with NO energy required (generally moving DOWN the concentration gradient from HIGH to LOW concentration) • Osmosis – the diffusion of water • Facilitated Diffusion – ions and polar molecules can pass through transport proteins that provide: • A hydrophilic channel for molecules to pass through OR • The bind loosely to molecules and carry them through the membrane

  11. Active Transport • Movement of solutes across a membrane AGAINST the concentration gradient (LOW to HIGH) = REQUIRES ENERGY • Ex – the sodium/potassium pump – pumps Na+ out of the cell and K+ into the cell against the gradient for nerve function

  12. Passive v. Active Transport

  13. Cotransport • After pumping the H+ against the gradient, the H+ can do work when they move with the gradient across the membrane • May bring sucrose with it though it is moving against the SUCROSE gradient • Like pumping water uphill and having it do WORK as is comes back down

  14. Bulk Transport • Large molecules are moved across the membrane by endocytosis and exocytosis • This requires vesicles entering or leaving the cell • Ex- phagocytosis (eating) and pinocytosis (drinking)

  15. Receptor-Mediated Endocytosis • Specific substances bind to receptors on the cell membrane causing a vesicle to form around the substance and move into the cell