6.021/Notes/2006-09-15

Microfluidics

 * small channels about width of hair
 * supports experiments not possible macroscopically
 * small size inhibits mixing by convection
 * mixing primarily due to diffusion
 * no turbulence, laminar flow

Diffusion into cells

 * Dissolve and diffuse model
 * Fick's law for membranes $$\phi_n(t)=P_n(c^i_n(t)-c^o_n(t), P_n=\frac{D_nk_n}{k}$$
 * Charles Overton: 1st studies of diffusion through cell membranes
 * tested 100s of solutes, dozens of cell types
 * Surprising results: everyone thought membranes were impermeant to essentially everything but water
 * Overton's rules
 * Cell membranes are semi-permeable
 * plant and animal cells are similar
 * permeabilities correlate with solubility of solute in organic solvents
 * hypothesized that membrane is lipid (cholesterol and phospholipis) (was correct)
 * some cells concentrate solute (active transport)
 * Meyer-Overton theory of narcosis: potentcy of anesthetics correlated with lipid solubility
 * muscles don't contract in sodium free media
 * Collander: first quantitative studies
 * find permeabilities and solubilities
 * enormous range of $$P_n$$ and $$k_n$$
 * clear correlation between $$P_n$$ and $$k_n$$ but large scatter (many orders of magnitude)
 * can be attributed to differences in diffusivity (due to molecular weight)

Transport of many substances consistent with dissolve and diffuse mechanism