# 6.021/Notes/2006-09-22

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## 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.