Lecture #5 – Plant Transport

1. Lecture #5 – Plant Transport Image of waterfall

2. Key Concepts: • The importance of water • Water potential: Ψ = P - s • How water moves – gradients, mechanisms and pathways • Transpiration – water movement from soil to plant to atmosphere • The pressure flow model of phloem transport

3. WHY WATER??? • Required for metabolism and cytoplasm • Nutrients are taken up and transported in water-based solution • Metabolic products are transported in water-based solution • Water movement through the plant affects gas exchange and leaf T Diagram – movement of water through a tree

4. Water Potential (Ψ): • Controls the movement of water • A measure of potential energy • Water always moves from an area of HIGH water potential to an area of LOW water potential • Controlled by physical pressure, solute concentration, adhesion of water to cell structures and to soil particles, temperature, and gravity Ψ = P - s

5. Diagram – water moves from high water potential to low water potential, sometimes toward a negative value; same next 3 slides

7. minus 4 is MORE NEGATIVE than minus 1

8. High Low

9. Diagram – water potential is universal, including with waterfalls

10. Water Potential (Ψ): • Controls the movement of water • A measure of potential energy • Water always moves from an area of HIGH water potential to an area of LOW water potential • Controlled by physical pressure, solute concentration,adhesion of water to cell structures and to soil particles, temperature, and gravity Ψ = P - s

11. P – Pressure Potential • By convention, set to zero in an open container of water (atmospheric pressure only) • In the plant cell, P can be positive, negative or zero • A cell with positive pressure is turgid • A cell with negative pressure is plasmolyzed • A cell with zero pressure is flaccid

12. Turgid P > 0 Plasmolyzed P < 0 Flaccid P = 0

13. What are the little green things??? Micrograph – photosynthetic cells: turgid on left, plasmolyzed on right; same on next 3 slides

14. Turgid Plasmolyzed

15. Critical Thinking • How can you tell this tissue was artificially plasmolyzed?

16. Critical Thinking • How can you tell this tissue was artificially plasmolyzed? • Observe the cell on the far right – it is still turgid 

17. Crispy means plasmolyzed beyond the permanent wilting point  Image – turgid plant on left, plasmolyzed on right

18. s s s s – Solute Potential • s = zero for pure water • Pure H2O = nothing else, not a solution • Adding solutes ALWAYS decreases the potential energy of water • Some water molecules now carry a load – there is less free water

19. Remember, Ψ = P – s Diagram – effect on water potential of adding salts to solutions separated by semi-permeable membrane

20. Ψ = P – s Pressure can be +, -, or 0 Solutes always have a negative effect Simplest way to calculate Ψ is by this equation

21. Flaccid cell in pure water – what happens??? …..what do you know??? ….what do you need to know???

22. Ψ = ? Flaccid cell in pure water – what happens???

23. Ψ = ? P = ?.......s = ? Flaccid cell in pure water – what happens???

24. Ψ = ? P = 0.......s = about 0.7 MPa Flaccid cell in pure water – what happens???

25. Ψ = -0.7 MPa P = 0.......s = about 0.7 MPa Flaccid cell in pure water – what happens???

26. Ψ = ? Flaccid cell in pure water – what happens??? …..what do you know??? ….what do you need to know???

27. Ψ = ? P = ?.......s = ? Flaccid cell in pure water – what happens???

28. Ψ = ? P = 0.......s = 0 Flaccid cell in pure water – what happens???

29. Ψ = 0 MPa P = 0.......s = 0 Flaccid cell in pure water – what happens???

30. Ψ = -0.7 MPa ? Ψ = 0 MPa Will water move into the cell or out of the cell??? Flaccid cell in pure water – what happens???

31. Ψ = -0.7 MPa Ψ = 0 MPa Water moves from high Ψ to low Ψ Flaccid cell in pure water – what happens???

32. Ψ = -0.7 MPa Ψ = 0 MPa Then what happens???

33. Ψ = -0.7 MPa Ψ = 0 MPa P in cell goes up….. Then what happens???

34. Ψ = 0 MPa Ψ = 0 MPa Dynamic equilibrium! Then what happens???

35. Hands On • Prepare a section of plump celery and stain with T-blue • Examine and describe • Introduce a drop of salt water • Any change??? • Examine the stalk of celery that was in salt water vs. one that was in fresh water • Explain your observations in your lab notes.

36. Water Movement • Osmosis – the diffusion of water one molecule at a time across a semi-permeable membrane • Controlled by both P and s • Bulk Flow – the movement of water in bulk – as a liquid • Controlled primarily by P

37. Osmosis Diagram – osmosis across a semi-permeable membrane; next slide also Critical Thinking: Where does water move by osmosis in plants???

38. Osmosis Critical Thinking: Where does water move by osmosis in plants??? Cell membrane is semi-permeable

39. Water Movement • Osmosis – the diffusion of water one molecule at a time across a semi-permeable membrane • Controlled by both P and s • Bulk Flow – the movement of water in bulk – as a liquid • Controlled primarily by P

40. Water Movement • Osmosis – the diffusion of water one molecule at a time across a semi-permeable membrane • Controlled by both P and s • Bulk Flow – the movement of water in bulk – as a liquid • Controlled primarily by P – no membrane, no solute gradient!

41. Critical Thinking • Where does water move by bulk flow in plants???

42. Critical Thinking • Where does water move by bulk flow in plants??? • Primarily in the xylem, also in phloem and in the cell walls

43. Routes of water transportsoil  root  stem  leaf  atmosphere Cell Wall Cell Membrane Cytoplasm Diagram – apoplast, symplast and transmembrane pathways; same on next slide

44. Routes of water transportsoil  root  stem  leaf  atmosphere Cell Wall Cell Membrane Cytoplasm

45. Diagram – Casparian strip; same on next 2 slides

46. The Casparian Strip is a band of suberin in the transverse and radial (but not the tangential) walls of the endodermis cells Water CANNOT PASS THROUGH the Casparian Strip Water must GO AROUND the Casparian Strip – through the tangential face of the endodermis

47. The Casparian Strip is a band of suberin in the transverse and radial (but not the tangential) walls of the endodermis cells Water CANNOT PASS THROUGH the Casparian Strip Water must GO AROUND the Casparian Strip – through the tangential face of the endodermis

48. Critical Thinking • Apoplast water is forced into the symplast at the Casparian Strip • What does this mean for the water??? • What is the function of the Casparian Strip???

49. Critical Thinking • Apoplast water is forced into the symplast at the Casparian Strip • What does this mean for the water??? • It has to cross a cell membrane (easy for water!) • What is the function of the Casparian Strip???

50. Critical Thinking • Apoplast water is forced into the symplast at the Casparian Strip • What does this mean for the water??? • It has to cross a cell membrane (easy for water!) • What is the function of the Casparian Strip??? • Solute uptake is regulated at the membrane!!!