1. Chapter 36:
Transport in Plants

2. Plants
Leaves  roots may be 100m apart.

3. Question ?
How do plants move materials from one organ to the other ?

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

5. 3 Levels of Plant Transport
A) Cellular Transport
The transport of solutes and water across cell membranes.
Types of transport:
Passive Transport
Diffusion and Osmosis.
Requires no cellular energy.
Materials diffuse down concentration gradients.
Problem: very slow
Mechanisms
Transport Proteins
Ex: Carrier Proteins Selective Channels
Potassium Channel
Found in most plant cell membranes.
Allow K+ but not Na+ to pass.
Often “gated” to respond to environmental stimuli.
B. Active Transport
C. Water Transport

6. 2. Active Transport Requires cell energy.
Moves solutes against a concentration gradient.
Ex: Proton Pump (another example of Chemiosmosis)
Uses ATP to move H+ out of cells.
H+ creates a membrane potential.
H+ allows cotransport.
Membrane Potentials
Allow cations to moved into the cell.
Ex: Ca+2, Mg+2
Cotransport
Couples H+ with anions to move both into cell.
Ex: NO3-

7. Summary

8. 3. Water Transport y = yr + yp
Osmosis - water moves from high concentration to low concentration.
Water Potential
The potential energy of water to move from one location to another.
Abbreviated as y
Has two components:
Pressure potential: yr
Solute potential: yp
y = yr + yp

9. 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.

11. Bulk Flow The movement of water between two locations due to pressure.
Much faster than osmosis.
Tension (negative pressure).
May cause bulk flow against the diffusion gradient.

12. Tension
Is a very important force to "pull" water from one location to another.
Plant Vacuoles
Create Turgor Pressure against the cell wall.
Affect water potential by controlling water concentrations inside cells.

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

14. Proton Pumps Drives solutes inside the vacuole.
Lowers water potential (yp ) inside the vacuole.
Result
Water moves into the vacuole.
Vacuole swells.
Turgor pressure increases.

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

17. Turgid
Flaccid

18. Short Distance Transport
Aquaporins
Water specific facilitated diffusion transport channels.
Help water move more rapidly through lipid bilayers.
Short Distance Transport
1. Transmembrane route
2. Symplast route
3. Apoplast route

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

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

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

22. Point
Movement of materials can take place by all 3 routes.

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

25. Start - Roots
Absorb water.
Take up minerals.

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

27. Root Cortex Very spongy. Apoplast route very common. Problem
Can't control uptake of materials if the apoplast route is used.

28. Solution
Endodermis with its Casparian Strip.

29. Casparian Strip Waxy layer of suberin.
Creates a barrier between the cortex and the stele.
Forces materials from apoplast into endodermis symplast.
Result
Plant can now control movement of materials into the stele.

30. Casparian Strip
Endodermis

31. 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.

32. Mycorrhizae

33. 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.

34. Xylem Sap Transport Methods
1. Root Pressure
2. Transpiration (Ts)
Root Pressure
Root cells load minerals into xylem.
Water potential (yp) is lowered.
Water flows into xylem.
Result
Volume of water in xylem increases
Xylem sap is pushed up the xylem tissues creating root pressure.

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

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

38. TCTM Theory
Transpiration
Cohesion
Tension
Mechanism

39. 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.

41. As water evaporates:
Cohesion: water molecules sticking together by H bonds.
Adhesion: water molecules sticking to other materials (cell walls etc.).
Result
The loss of water from the leaves creates “tension” or negative pressure between the air and the water in the plant.

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

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

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

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

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

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

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

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

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

52. Turgid - Open
Flaccid - Closed

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

54. To Open Stomata: 1. K+ enters the guard cells.
2. Water potential lowered.
3. Water enters guard cells.
4. Turgor pressure increases.
5. Guard cells swell and Stomata opens.

55. To Close Stomata: 1. K+ leaves guard cells.
2. Water leaves guard cells.
3. Turgor pressure decreases.
4. Guard cells shrink and Stomata close.

56. K+ Movement Regulated by proton pumps and K+ channels. Controlled by:
Light (Blue)
CO2 concentrations
Abscisic Acid (water stress)

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

58. Adaptations for Balance
C4 Ps
CAM Ps

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

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

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

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

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

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

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

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

67. Phloem Loading

68. Transfer Cells
Modified cell with ingrowths of cell wall to provide more surface area for sugar diffusion.

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

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

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

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

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

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