6.021/Notes/2006-09-22

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Osmosis dynamics

change in both $A (t)$ (surface area) and $V^i(t) \rightarrow C^i_\Sigma(t)$
simplest case of concave cell (red blood cell)
- $A (t) = A o$ (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

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.