# 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) = Ao (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.