lec 1b - directional transport of ions Crossing the channel...

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Hydration shell and selectivity of ion K + channels Pore = selectivity „filter‟
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-> selectivity = 1000x! Hydration shell and selectivity of ion K + channels
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O O O O K + O O O O K + K + H H O K + H H O plasma membrane (lipids) Channel „walls‟ „pore” Oxygen atoms from peptide bonds exposed along the aperture of the gate Water molecules of the hydration layer
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O O K + O O K + H H O O O Na + O O H H O Na + For K+, both situations are about equally favorable -> high probability to enter the channel For Na+, binding to O of the pore is energetically much less favorable than binding to water molecules -> very low probability to enter the channel
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O O O O K + O O O O K + K + H H O K + H H O K+ crossing the channel is a PASSIVE transport (no energy used) Reaction dehydration-binding to the pore is reversible, and happens on both sides of the gate, thus the transport is not directional !! ≠ ACTIVE transport such as Na+/K+ ATPase or H-pump, where ATP hydrolysis is coupled to
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Unformatted text preview: directional transport of ions Crossing the channel for each individual ion is only a probability, it may as well bind the gate and be released on the same side “Net transport”, i.e. the total number of ions transported from one side to the other of the membrane through a channel can be thought as the sum of the 4 following possible events: time If K+ concentrations are equal on both sides of the membrane, the net transport will be = 0 If the concentration of ions on one side is higher, then the probability of crossing the channel will not be equal, there will be a net transport toward the compartment with the lower concentration. For additional information see Lodish (6 th ed) p. 461-462...
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lec 1b - directional transport of ions Crossing the channel...

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