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Passive Transport Processes
  • Does not require energy expenditure of the cell membrane
  • passive process
  • molecules spread thru membrane
  • molecules move from area of high concentration to low concentration (down concentration gradient)
  • as molecules diffuse - equilibrium occurs
Simple Diffusion
  • molecules cross through phospholipid bilayer
  • solutes permeate membrane (membrane permeable)
  • diffusion of h2o thru selectively permeable membrane
  • limits diffusion of at least some solute particles
  • Osmotic Pressure - water pressure that develops as a result of osmosis
  • gain of volume on 1 side; lost of volume on another
Potential Osmotic Pressure
  • maximum pressure that could develop in solution when it is separated from pure h2o by selectively permeable membrane.
  • allows prediction of direction of osmosis and resulting change of pressure
2 fluids have same osmotic pressure
  • "higher pressure"
  • cells shrivel up.
  • "lower pressure"
  • cells swell up
Facilitated Diffusion
Channel Mediated Passive Transport
  • channels are specific - allow only 1 type of solute to pass thru
  • gated channels may be open/close - may be triggered by variety of stimuli
  • channels allow membranes to be selectively permeable
  • aquaporins - water channels that permit rapid osmotics
Facilitated Diffusion (cont'd)
Carrier-mediated passive transport
  • carriers attract & bind to solute, change shape, & release solute out the other side of carrier
  • usually reversible, depending on direction of concentration gradient
Active Transport Processes
  • requires expenditure of metabolic energy by cell
  • Transports by pumps
- pumps are membrane transporters that move a substance against concentration gradient (opposite of diffusion) - examples:  calcium and sodium pumps
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