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  • 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 [math]\displaystyle{ \phi_n(t)=P_n(c^i_n(t)-c^o_n(t), P_n=\frac{D_nk_n}{k} }[/math]
  • 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 [math]\displaystyle{ P_n }[/math] and [math]\displaystyle{ k_n }[/math]
    • clear correlation between [math]\displaystyle{ P_n }[/math] and [math]\displaystyle{ k_n }[/math] 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