6.021/Notes/2006-09-22

Osmosis dynamics

• change in both [math]\displaystyle{ A(t) }[/math] (surface area) and [math]\displaystyle{ V^i(t) \rightarrow C^i_\Sigma(t) }[/math]
• simplest case of concave cell (red blood cell)
  • [math]\displaystyle{ A(t)=A_o }[/math] (constant surface area)
  • not exponential solution. solve differential equation numerically
  • find that shrinking is faster than swelling
  • explained by fact that more water is needed to swell cell to twice volume as water lost to shrink to half volume
• for spherical cell, dynamics look identical to constant surface area case
• General conclusion: simple model of swelling agrees with equilirium and kinetic response of simple cells.
• But there are cases where it doesn't fit

Water channels

• family of aquaporins
• first discovered by Agre (AQP1, 28kDa)
• normal protein dyes don't stain this protein
• 1D, 2D, 3D structures solved
• positive charge in middle of this channel
• on one side water's negative side points towards it. once passed the middle of channel, the water flips direction.