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Membrane Transport

Membrane Transport. Structure of Biological Membranes Diffusion and Fick’s First Law Partition Coefficients and Movement Osmosis and Osmotic Movement Charge Gradients Across the Membrane Donnan Equilibrium Passive Transport Straight Diffusion Facilitated Diffusion -- Cystic Fibrosis

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Membrane Transport

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  1. Membrane Transport Structure of Biological Membranes Diffusion and Fick’s First Law Partition Coefficients and Movement Osmosis and Osmotic Movement Charge Gradients Across the Membrane Donnan Equilibrium Passive Transport Straight Diffusion Facilitated Diffusion -- Cystic Fibrosis Active Transport -- Na/K ATPase Coupled Transport

  2. Membrane Protein/Lipid Ratio Myelin .25 Plama Membrane Liver Cell 1.0-1.4 Intestinal cell 4.6 RBC Ghost 1.5-4.0 E.R. .7-1.2 Size Plasma Membrane 9-10 nm Mitochondria outer 6 nm Golgi 8 nm

  3. Membrane Lipids • Membranes vary in thickness but all appear to be lipid bilayers • Components-- phosphoglycerides, sphingolipids and cholesterol • Cholesterol stabilizes membrane • 40%-60% lipid by weight • Most common lipid phosphatidylcholine

  4. Osmosis • Osmosis is the special case of the diffusion of water across a membrane • Osmosis = Gas constant x absolute temperature x concentration gradient • p = CRT where C = concentration in moles l-1; R = gas constant of .082 l atm/Ko-1 mole-1;T= temperature

  5. Water Movement • Diffusive water movement is much less than osmotic flux • Water pores have been postulated by never documented • Diffusion may be limited by unstirred layers on either side of the membrane

  6. Crossing Membranes • Diffusion • Fick’s First Law Revisited • Rate = DA [C]/x or the amount of a substance crossing the membrane per unit time = the diffusion coefficient (cm2/sec) x area of the membrane (cm2) x the concentration gradient (moles/cm3)/ the thickness of the membrane (cm) • Diffusion shows linear kinetics

  7. Permeability • The permeability coefficient in units of cm-sec is a measure of how easily a substance penetrates and crosses the membrane • Solutes must break hydrogen bonds with water, move into the lipid phase and cross to an aqueous phase • Solute charge, size of molecule, temperature and membrane type all determine permeability

  8. Donnan Equilibrium • Both concentration gradients and charge gradients contribute to the distribution of ions on either side of a membrane • If one side of the membrane contains a charge impermeant solute like protein, the concentrations of the permeant ions on either side will not be equal

  9. Transport Types • Straight Diffusion • Facilitated Diffusion through a protein channel or pore • Active transport • Coupled Transport

  10. Facilitated Diffusion • Movement is through a protein based channel in the membrane • Diffusion is greater than it would be without the channel • The chloride channel in cystic fibrosis is a good example of a regulated channel • Facilitated diffusion displays saturation kinetics

  11. Coupled Transport • In coupled transport, an actively established ion gradient is used to drive the movement of another solute • Example: sugar and amino acid uptake in the small intestine is linked to uptake of Na which depends upon a favorable gradient created by Na/K ATPase

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