Movement of molecules through cells

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University of Massachusetts Amherst
Peter Normark

01/28/13 Movement of molecules through cells – permeability, active & passive diffusion Membrane adaptions -->used to exploit laws of chemistry and physics 'Selective' permeability: (==> 'limited' permeability) Some small molecules diffuse easily through lipid bilayer: small non-polar O 2 CO ,2NO, ethylene, steroids, many others others have more/less trouble: H O, other small polar molecules 2 others can't at all: ions & big molecules Na , K , Ca ++, Cl , OH - Middle of bilayer is hydrophobic -> small hydrophobic molecules get through fine hydrophilic (charged, polar) and/or big molecules don't Permeability can be increased by ==> kinkiness of phospholipid tails ==> cholesterols in membrane Manipulate selective permeability with adaptions: protein channels: Little 'tubes' through membranes Each type of molecule requires it own unique protein channel Channel protein example:Aquaporins H 2 channels that speed up H O 2iffusion diffusion normally difficult because of hydrophobic area betw. membrane bilayers, aquaporins bypass this With ions, polar & large molecules: "membrane is permeable to..." means "membrane has a channel protein for.." Ions crossing membranes: one complication Ions carry charge: (+) or (-) charge = a force + + ++ H+; Na ; K ; Ca (+) repels (+) Cl-; PO4-3; NH3- (-) repels (-) Charge provides force that repels random diffusion is an exception to this Molecules always ricochet ==> force only creates tendency to move one direction diffusion = random movement + forced movement = spread + directional flow co-transport channels 2 molecule types transported together can only move as a pair => escort each other either direction, most common example in mammals Na+/K, in plants, H+ and sucrose 01/30/13 Physiology's Rules: Random diffusion --> individual molecules moves randomly --> net "flow" depends on relative concentratio
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