Chapter_4_Cell_Membrane_Transport_new

Chapter_4_Cell_Membrane_Transport_new - Chapter 4 Cell...

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Chapter 4 Cell Membrane Transport Review Cell Membrane Membrane mostly phospholipids & proteins and % of each varies with structure. Protein: Lipid RBCs = 50:50 Myelin sheath = 20:80 Inner mitochondrial membranes = 76 :24 Phospholipids : polar head & 2 non polar tails Micelles : droplets Liposomes : aqueous center used to transport anticancer drugs Crossing Cell Membranes Ability to cross a cell membrane depends on: Size, charge and lipid solubility of a molecule Properties of the membrane Presence of membrane proteins Water moves freely across open water filled channels Ions do NOT move freely across Movement Across Membranes Membranes - barrier between ECF & ICF Selective Permeability non-polar and small polar cross O 2 , CO 2 , fatty acids, water large polar molecules, ions can’t cross glucose, proteins, ions Passive Transport Versus Active Transport Active transport Requires energy Requires pumps From low energy to high Passive transport No energy required Simple diffusion or Mediated transport From high energy to low Driving Forces Acting on Molecules Direction = high energy to low
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Chemical forces Electrical forces Chemical Driving Force Concentration gradient = DC From high to low concentration Down gradient Spontaneous From low to high Up gradient Requires energy Chemical Driving Force – fig 4.1 Electrical Driving Force What charge the molecule has Potential difference Separation Membrane Potential Ions possess charge Cations have positive charge Anions have negative charge Number of cations in body = number of anions ; Body neutral Number of cations in ECF > number of anions Number of anions ICF > number of cations Charge separation across membrane = membrane potential Resting Membrane Potential Membrane potential given as inside relative to outside At rest, excess anions on inside make membrane potential negative Average resting membrane potential of neurons = -70 mV Membrane Potential Determines Electrical Driving Force Separation of charge = potential energy Electrical forces Opposite charges attract Like charges repel Membrane potential is negative
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Electrical driving force on cations – into cell Electrical driving force on anions – out of cell Magnitude of electrical driving force Size of membrane potential valence of ion Example: Equilibrium for Sodium Ions Sodium in greater concentration outside cell Chemical force directed inward Both electrical and chemical forces are for sodium to move into cell Strong electrochemical force for sodium to enter cell Determining Forces Acting on Potassium at the Resting Membrane Potential Potassium in greater concentration inside cell Chemical force directed outward Summary of Forces on Potassium and Sodium at Rest Potassium close to equilibrium Weak electrochemical force to move out of cell
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