6.021/Notes/2006-09-22

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

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.