Lecture 3: Diffusion – How stuff gets around

1. Lecture 3: Diffusion – How stuff gets around Readings ch 6 pp 99-116; Ch 37 p 832; ch 44 p 982 Mathbench: Diffusion module Dr. Karen 9/7/11

2. BSCI 207 goal • Learn how all organisms get the job done. • Unity vs diversity

3. Unity among organisms • Governed by same physical and chemical principles

4. Unity among organisms • Governed by same physical and chemical principles • Forces: gravitational, electric, magnetic • Thermodynamics: Enthalpy, entropy • Diffusion Dr. Bill Today

5. Unity among organisms

6. Unity among organisms • Evolved from common ancestor • Have common genetic information DNA, RNA, proteins Friday 9/9

7. Diversity among organisms • May be slightly different solutions to evolutionary problem

8. Diversity among organisms • Or radically different solutions

9. Diversity among organisms • Have different evolutionary history • Evolve along different paths Rest of semester

10. Diffusion • How stuff gets around • Within cell • Across cell membrane • Between cells

11. Diffusion – entropy in action • In a vacuum • Molecules follow straight path till run into something • In air or in liquid • Frequent collisions with neighboring molecules • Slows directional motion

12. Brownian motion

13. Brownian motion http://www.youtube.com/watch?v=6VdMp46ZIL8

14. Brownian motion – computer simulation Random motion due to heat energy – Velocity is proportional to temperature

15. Initial State Final State Diffusion – random molecular motion which redistributes molecules from high to low concentration

16. Initial State Final State Diffusion rate is proportional to concentration gradient - this gradient is the force which causes motion

17. Diffusion • Gibb’s free energy

18. Diffusion • Spontaneous process – driven by increase in entropy • Organisms rely on diffusion • Move, retain and get rid of essential molecules 0 Large

19. Diffusive flux • Flux of molecules moving through an area per unit time Area, A

20. Fick’s first law of diffusion D is diffusion coefficient is concentration gradient

21. Why the negative sign?? • Movement is in opposite direction to that along which concentration increases Increases

22. Why the negative sign?? • Movement is in opposite direction to that along which concentration increases Increases Molecular motion

23. What are the units?

24. What are the units of D? So units of D are:

25. What are the units of D? So units of D are:

26. What does D depend on? http://www.youtube.com/watch?v=H7QsDs8ZRMI

27. What does D depend on? • Mass (size) of solute • Temperature

28. What if we want to know the rate of molecules moving? • Flux = • So rate =

29. What if we want to know the rate of molecules moving? • Flux = • So rate = • How do we increase rate, dS/dt?

30. Typical values of D

31. Fick’s second law • Tells us how long it takes something to diffuse a given distance, x • t = diffusion time (s) • D = diffusion coefficient (cm2/s) • x = diffusion distance (cm)

32. How long does it take sucrose to diffuse 1 μm? • Dsucrose = 5 x 10-6 cm2 /s • x = 1 μm = 10-4 cm

33. How long does it take sucrose to diffuse 1 μm? • Dsucrose = 5 x 10-6 cm2 /s • x = 1 μm = 10-4 cm

34. Diffusion time of sucrose

35. Diffusion time of sucrose

36. vs diffusion time of O2Dsucrose=5 x 10-6 DO2=1.6x10-5 cm2 /s

37. Slow speed of diffusion • Not a problem for small organisms • Huge problem if you want to get bigger

38. MathBench “Introduction to Diffusion” module Problem with units – if someone finds error I’ll give the class a point

39. Additional problem with increasing size Surface to volume ratio changes SA = 4 πr2 Vol = 4/3πr3

40. Same problem for rectangular shapes SA = 6l2 Vol = l3

41. Surface area (SA) to volume ratio Side 1 2 3 4 cm SA 6 24 54 96 cm2 Vol 1 8 27 64 cm3 SA / vol 6 3 2 1.5

42. Ratio of surface area to volume decreases with cube length

43. Why is this a problem??

44. Why is this a problem?? • If nutrients come in through surface and waste products go out through surface - As animal gets bigger, there is disproportionately more mass inside that need nutrients to diffuse in - There is more waste generated that needs to diffuse out

45. Cell membranes Composed of phospholipid membrane Permeability determines how well a compound diffuses across membrane Fig 6.5

46. Membrane permeability Concentration gradient results in diffusion We can measure how fast different types of molecules diffuse through the membrane Solute (ion or molecule) Phospholipid membrane Fig 6.7

47. Solutes diffuse till equal in concentration on both sides 1. Start with different solutes on opposite sides of a lipid bilayer. Both molecules diffuse freely across bilayer. 3. Equilibrium is established. Solutes continue to move back and forth across the membrane but at equal rates. 2. Solutes diffuse across the membrane— each undergoes a net movement along its own concentration gradient. Fig 6.14

48. Size and charge affect permeability and diffusion rate Permeability scale (cm/sec) High permeability O2, CO2 H2O Glycerol, urea + Glucose Cl– Fig 6.8 K+ Na+ Low permeability

49. Size and charge affect permeability and diffusion rate Permeability scale (cm/sec) High permeability O2, CO2 H2O Glycerol, urea + Glucose Cl– Fig 6.8 K+ Na+ Low permeability

50. If solute can not diffuse.. 1. Start with more solute on one side of the lipid bilayer than the other, using molecules that cannot cross the selectively permeable membrane. 2. Water undergoes a net movement from the region of low concentration of solute to the region of high concentration. ..water will move to equalize concentration