Lecture 8 Diffusion 1 Diffusion a Random molecular motion Brownian motion which redistributes molecules from high b Gibbs free energy drives diffusion will always be spontaneous because not a reaction to low concentration so delta h is 0 c Diffusive flux number of molecules moving through an area per unit time 2 Fick s first law of diffusion x D is diffusion coeff fraction is concentration gradient a Flux diffusion coefficient concentration gradient b J D C c Mass ions energy diffusion mass mostly relevant for this class d Negative sign because movement is opposite to direction of increasing concentration e Can just pay attention to the magnitude of the flux f C x g Units of D are cm2 s h D depends on x is the thickness of the cell wall C outside Cinside x i Temperature ii Size of molecule iii Properties of medium Spherical molecule D kT fluid i j D increases on temperature k D decreases with bigger molecular radius 3 Rate molecules time flux area 6 r T temp r molecular radius viscosity of surrounding a Flux is rate out of only one area but rate is flux of the whole cell i Ex flux students out one door rate students out all of doors b Can increase rate by Increasing area i 1 Ex microvilla in small intestine Increasing concentration gradient by ii 1 Increase concentration difference a Ex counter current exchange 2 Decrease thickness of diffusion zone Increasing temperature iii iv Decreasing viscosity v Decreasing radius of diffusing solute a Ex capillary blood vessels have super thin walls 1 Typically cant change these factors 4 Fick s second law t x2 2D i Above is 1 dimensional form for 2D and 3D denominator is 4D and 6D t diffusion time x diffusion distance D diffusion coefficient a b Ex How long does it take sucrose to diffuse 1 micrometer respectively i X 1 micrometer 10 4 cm ii Dsucrose 5x10 6 cm2 s t x2 2 D 10 4cm 2 2 5 x10 6cm2 s 10 8 cm2 10 5 cm2 s 10 3 s 1ms iii c Diffusion time can get pretty long for bigger molecules and bigger areas 5 Slow speed of diffusion a Not a problem for small organisms big problem for smaller ones b Big cells have consumption demand higher than diffusion ability 6 Cell membranes control transport a Permeability of phospholipid layer determines how well compound diffuses across membrane b For cell membranes size and charge effect permeability and diffusive flux c Hydrophobic small uncharged polar molecules have no problem oxygen co2 water d e Large uncharged polar molecules go across with difficulty glycerol glucose Ions can t get through membrane Cl K Na i Transport proteins channels carrier proteins pumps 7 Summary a Diffusion is EXTREMELY slow b Diffusion time goes as distance2 c Diffusion depends on temp solute size concentration gradient d Rate isn t flux i Rate depends on area ii Flux is per unit are
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