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Lecture 3 – Diffusion – How stuff gets aroundUnity among organisms- Governed by same physical and chemical principleso Forces: gravitational, electric, magnetico Thermodynamics: Enthalpy, entropy, diffusion- Evolved from common ancestor- Have common genetic informationo DNA, RNA, proteinsDiversity among organisms- May be slightly different solutions to evolutionary problem- Or radically different solutions- Have different evolutionary history- Evolve along different pathsDiffusion- How stuff gets aroundo Within cello Across cell membraneo Between cellsDiffusion – Entropy in action!- In a vacuumo Molecules follow straight path till run into something- In air or in liquid  Brownian Motiono Frequent collisions with neighboring moleculeso Slows directional motionBrownian Motion- Random motion due to heat energyo Velocity is proportional to temperatureDiffusion – random molecular motion which redistributes molecules from high to lowconcentration- Diffusion rate is proportional to concentration gradient – this gradient is the forcewhich causes motion- Gibb’s free energy Diffusion Flux- Flux of molecules moving through an area per unit timeFick’s First Law of Diffusion-flux ( J )=knetforce( ΔF )length( Δx) J =−DΔCΔx- D is diffusion coefficient-ΔCΔx is concentration gradientWhat does D depend on?- Mass (size) of soluteo Larger mass, slower diffusion- Temperatureo Increase in temperature means an increase in the molecule’s speed(kinetic energy)  diffusion rate is now fasterWhat if we want to know the rate of molecules moving?- Flux = moleculesarea∗time- So rate = molecules/time = flux*area = dS/dtdSdt=J∗A=−D∗A∗ΔCΔx- How do we increase rate, ∆S/∆t?o Increase area moving througho Increase concentration gradiento Increase diffusion coefficientFick’s Second Law of Diffusion- Tells us how long it takes something to diffuse a given distance, x t=x22 Do T = diffusion time (s)o D = diffusion coefficient (cm2/s)o x = diffusion distance (cm)- How long does it take sucrose to diffuse 1 μm? o Dsucrose = 5 x 10-6 cm2 /so x = 1 μm = 10-4 cmot=x22 D=(10−4cm )22∗5 x 10−6cm2/ s=10−8cm210−5cm2/s=10−3s=1 msSlow speed of diffusion- Not a problem for small organisms- Huge problem if you want to get bigger!Additional problem with increasing size- Surface to volume ratio changeso SA = 4 πr2 Vol = 4/3πr3 o So if the radius doubles, SA increases by factor of 4 but the volume by factorof 8Same problem for rectangular shapes- SA = 6l2- Vol = l3Why is this a problem?- If nutrients come in through surface and waste products go out through surfaceo As an animal gets bigger, there is disproportionality - more mass inside that need nutrients to diffuse ino There is more waste generated that needs to diffuse out- Overall, if there is a big surface area it will take longer for cells to diffuse substances in and out of the cellCellMembranes- Composed of phospholipidmembrane-Permeability determines how well a compound diffuses across membraneMembrane Permeability- Concentration gradient results in diffusion- We can measure how fast different types of molecules diffuse through the


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UMD BSCI 207 - Lecture 3 – Diffusion

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