1. Resource Acquisition and Transport in Vascular Plants
Chapter 36
Resource Acquisition and Transport in Vascular Plants

2. Focus of Chapter
How do plants acquire and move materials from one location to another?

3. Levels of Plant Transport
1. Cellular
2. Short Distance
3. Long Distance

4. Cellular Transport
The transport of solutes and water across cell membranes.
Types of transport:
1. Passive Transport
2. Active Transport
3. Water Transport

5. 1. Passive Transport Diffusion and Osmosis.
Requires no cellular energy.
Materials diffuse down concentration gradients.

6. Problems Usually very slow. How can diffusion be assisted?
Transport Proteins and facilitated Diffusion
Ex. K+ channel

7. 2. Active Transport Requires cell energy.
Moves solutes against a concentration gradient.
Ex: Proton Pumps- - use ATP to move H+ and create a gradient or cell potential.

8. Membrane Potentials Allow cations to moved into the cell.
Ex: Ca+2, Mg+2
Allow anions to move by co-transport.
Ex: NO3

9. Summary

10. 3. Water Transport
Osmosis - water moves from high concentration to low concentration.

11. Water Potential
The potential energy of water to move from one location to another.
Abbreviated as y

12. Problem Cell wall creates a pressure in the cells.
Water potential must account for this pressure.
Pressure counteracts the tendency for water to move into plant cells.

13. y = yr + yp Water Potential Has two components: Pressure potential: yr
Solute potential: yp
y = yr + yp

15. Comment
Negative pressure or tension can be an important way to get water to flow from one point to another.
Tension “pulls” water from place to place.

16. Bulk Flow
The movement of water between two locations due to positive fluid pressure .
Bulk flow is one of the mechanism for long distance transport in plants (more on this later).

17. Comment
Both bulk flow and tension are much faster than osmosis and diffusion.

18. Plant Vacuoles Create Turgor Pressure against the cell wall.
Affect water potential by controlling water concentrations inside cells.

19. Tonoplast Name for the vacuole membrane. Has proton pumps.
Comment – genetic modification of these pumps gives plants salt tolerance.

21. Result Water moves into the vacuole. Vacuole swells.
Turgor pressure increases.

22. Turgor Pressure Important for non-woody plant support. Wilting:
Loss of turgor pressure.
Loss of water from cells.

23. Flaccid
Turgid

24. Aquaporins Water specific facilitated diffusion transport channels.
Help water move more rapidly through lipid bilayers.

25. Aquaporins with GFP

26. Short Distance Transport
1. Transmembrane route
2. Symplast route
3. Apoplast route

27. 1. Transmembrane
Materials cross from cell to cell by crossing each cell's membranes and cell walls.

28. 2. Symplast
The continuum of cytoplasm by plasmodesmata bridges between cells.

29. 3. Apoplast
Extracellular pathway around and between cell walls.

30. Long Distance Transport
Problem: diffusion is too slow for long distances.
Answer: tension and bulk flow methods.

31. Root Hairs Main site of absorption of water and minerals.
Comment - older roots have cork and are not very permeable to water.

32. Root Cortex
Very spongy.
Apoplast route very common.

33. Problem
Can't control uptake of materials if the apoplast route is used.

34. Solution
Endodermis with its Casparian Strip.

35. Casparian Strip Waxy layer of suberin in the endodermis.
Creates a barrier between the cortex and the stele.
Forces materials from apoplast into endodermis symplast.

36. Casparian Strip
Endodermis

37. Result Plant can now control movement of materials into the stele.
Filters out harmful materials in the water, controls amount of ions etc.

38. Mycorrhizae Symbiotic association of fungi with roots of plants.
Help with water and mineral absorption (replaces root hairs in some plants).
May also prevent toxins from entering the plant.

39. Mycorrhizae

40. Xylem Sap
Solution of water and minerals loaded into the xylem by the endodermis.
Endodermis - also prevents back flow of water and minerals out of the stele.

41. Xylem Sap Transport Methods
1. Root Pressure
2. Transpiration (Ts)

42. Root Pressure Root cells load minerals into xylem.
Water potential (yp) is lowered.
Water flows into xylem.

43. Result Volume of water in xylem increases
Xylem sap is pushed up the xylem tissues creating root pressure.

45. Comments Root Pressure: limited way to move xylem sap.
Most apparent at night.
Excess water may leave plant through Guttation.

46. Transpiration (Ts) Evaporation of water from aerial plant parts.
Major force to pull xylem sap up tall trees.

47. TCTM Theory Transpiration Cohesion Tension Mechanism
Explanation for how plants get water to the tops of trees.

48. How does TCTM work?
Water evaporates from leaves, especially from the cell walls of the spongy mesophyll.
Reason: water potential of the air is usually much less than that of the cells.

50. As water evaporates:
Cohesion: water molecules sticking together by H bonds.
Adhesion: water molecules sticking to other materials (cell walls etc.).

51. Result
The loss of water from the leaves creates “tension” or negative pressure between the air and the water in the plant.

52. Tension causes:
Xylem sap to move to replace the water lost from the mesophyll cells.

53. Xylem Sap
Is “pulled” by the resulting tension all the way down the plant to the roots and soil.

54. Ts Summary
Xylem sap moves along a continual chain of water potential from: air leaf stem roots soil

56. Comments
Tension is a negative pressure which causes a decreased in the size of xylem cells.
Xylem cells would collapse without secondary cell walls.

57. Factors that Affect Transpiration Rate
1. Environmental
2. Plant Structures
Multiple Layer Epidermis
Stomatal Crypt

58. Environmental Factors
1. Humidity
2. Temperature
3. Light
4. Soil Water Content
5. Wind

59. Plant Structure Factors
1. Cuticle
2. Stomate Number
3. Hairs

60. Stomates Openings in the epidermis that allow water and gas exchange.
Controlled by Guard Cells.
Control rate of Ts and Ps.

61. Guard Cells Turgid: Swell - open stomata.
Flaccid: Shrink - close stomata.
Size of the cells is a result of turgor pressure changes.

62. Turgid - Open
Flaccid - Closed

63. Turgor Pressure of Guard cells
Controlled by K+ concentrations.

64. Comment
Plant must balance loss of water by transpiration with CO2 uptake for Ps.

65. Phloem Transport Moves sugars (food). Transported in live cells.
Ex: Sieve & Companion Cells

66. Source - Sink Transport
Model for movement of phloem sap from a Source to a Sink.

67. Source Sugar production site
Ex: Ps Starch breakdown in a storage area.

68. Sink
Sugar uptake site.
Ex: Growing areas Storage areas Fruits and seeds

69. Comment
The same organ can serve as a source or a sink depending on the season.

70. Result
Phloem transport can go in two directions even in the same vascular bundle.

71. Xylem Transport: In Contrast to Phloem
Usually unidirectional.
Endodermis prevents back flow.
Dead cells.

72. Phloem Loading at the Source:
1. Diffusion
2. Transfer Cells
3. Active Transport

73. Phloem Loading

74. Result Sugar loaded into phloem. Water potential (yp) decreases.
Bulk flow is created.

75. Bulk Flow Movement of water into phloem.
Pressure forces phloem sap to move toward the sink.

76. At the Sink: Sugar is removed. Water potential is raised.
Water moves out of phloem over to xylem.

78. Phloem: summary Source - builds pressure. Sink - reduces pressure.
Pressure caused by:
Sugar content changes
Water potential changes

79. Comment
Plants move materials without "moving" parts, unlike animals.

80. Summary Know various ways plants use to move materials.
Know how Ts works and the factors that affect Ts.
Know how phloem transport works.