Chpt_4membtransnot

Chpt_4membtransnot - Chapter 4 Cell Membrane Transport...

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Chapter 4 Cell Membrane Transport Chapter Outline 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 Micelles: Liposomes: aqueous center used to Phospholipids 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 Selective Permeability non-polar and small polar cross large polar molecules, ions can’t cross I. Factors Affecting the Direction of Transport Passive Transport Versus Active Transport Driving Forces Acting on Molecules Passive Versus Active Transport Active transport Requires energy Requires pumps From low energy to high
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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 Chemical forces Electrical forces Chemical Driving Force Concentration gradient = C From high to low concentration Down gradient Spontaneous From low to high Up gradient Requires energy Chemical Driving Force Electrical Driving Force 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 = Separation of Charge Across Membrane 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
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Membrane Potential Determines Electrical Driving Force Separation of charge = potential energy Electrical forces Opposite charges attract Like charges repel Membrane potential is negative 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 Direction of Electrical Driving Forces Chemical force = concentration gradient direction: high low Electrical force = charge separation direction: opposites attract likes repel Example: Equilibrium for Sodium Ions Sodium in greater concentration outside cell Chemical force directed inward To be at equilibrium (no net force) Electrical force must be directed outward
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Chpt_4membtransnot - Chapter 4 Cell Membrane Transport...

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