Transport in Plants

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
Cellular Transport

• The transport of solutes and water across cell
  membranes.

• Types of transport
• 1. Passive Transport
• 2. Active Transport
• 3. Water Transport

6
1. Passive Transport

• Diffusion and Osmosis.
• Requires no cellular energy.
• Materials diffuse down concentration gradients.

7
Problems

• Usually very slow.
• How can diffusion be assisted?

8
Mechanisms

• Transport Proteins
• Ex Carrier Proteins Selective Channels

9
Potassium Channel

• Found in most plant cell membranes.
• Allow K but not Na to pass.
• Often gated to respond to environmental stimuli.

10
2. Active Transport

• Requires cell energy.
• Moves solutes against a concentration gradient.
• Ex Proton Pumps

11
Proton Pump

• Uses ATP to move H out of cells.
• H creates a membrane potential.
• H allows cotransport.

12
Membrane Potentials

• Allow cations to moved into the cell.
• Ex Ca2, Mg2

13
Cotransport

• Couples H with anions to move both into cell.
• Ex NO3-

14
Summary
15
Comment

• Proton pump is another example of Chemiosmosis.

16
3. Water Transport

• Osmosis - water moves from high concentration to
  low concentration.

17
Water Potential

• The potential energy of water to move from one
  location to another.
• Abbreviated as y

18
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.

19
Water Potential

• Has two components
• Pressure potential yr
• Solute potential yp
• y yr yp

20
Comment

• See the Ts lab handout for more on water
  potential.

21
(No Transcript)
22
Bulk Flow

• The movement of water between two locations due
  to pressure.

23
Bulk Flow

• Much faster than osmosis.
• Tension (negative pressure).
• May cause bulk flow against the diffusion
  gradient.

24
Tension

• Is a very important force to "pull" water from
  one location to another.

25
Plant Vacuoles

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

26
Tonoplast

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

27
Proton Pumps

• Drives solutes inside the vacuole.
• Lowers water potential (yp ) inside the vacuole.

28
Result

• Water moves into the vacuole.
• Vacuole swells.
• Turgor pressure increases.

29
(No Transcript)
30
Turgor Pressure

• Important for non-woody plant support.
• Wilting
• Loss of turgor pressure.
• Loss of water from cells.

31
Flaccid
Turgid
32
Aquaporins

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

33
Short Distance Transport

• 1. Transmembrane route
• 2. Symplast route
• 3. Apoplast route

34
1. Transmembrane

• Materials cross from cell to cell by crossing
  each cell's membranes and cell walls.

35
2. Symplast

• The continuum of cytoplasm by plasmodesmata
  bridges between cells.

36
3. Apoplast

• Extracellular pathway around and between cell
  walls.

37
Point

• Movement of materials can take place by all 3
  routes.

38
(No Transcript)
39
Long Distance Transport

• Problem diffusion is too slow for long
  distances.
• Answer tension and bulk flow methods.

40
Start - Roots

• Absorb water.
• Take up minerals.

41
Root Hairs

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

42
Root Cortex

• Very spongy.
• Apoplast route very common.

43
Problem

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

44
Solution

• Endodermis with its Casparian Strip.

45
Casparian Strip

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

46
Casparian Strip
Endodermis
47
Result

• Plant can now control movement of materials into
  the stele.

48
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.

49
Mycorrhizae
50
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.

51
Xylem Sap Transport Methods

• 1. Root Pressure
• 2. Transpiration (Ts)

52
Root Pressure

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

53
Result

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

54
Comments

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

55
Transpiration (Ts)

• Evaporation of water from aerial plant parts.
• Major force to pull xylem sap up tall trees.

56
Movie
57
TCTM Theory

• Transpiration
• Cohesion
• Tension
• Mechanism

58
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.

59
(No Transcript)
60
As water evaporates

• Cohesion water molecules sticking together by H
  bonds.
• Adhesion water molecules sticking to other
  materials (cell walls etc.).

61
Result

• The loss of water from the leaves creates
  tension or negative pressure between the air
  and the water in the plant.

62
Tension causes

• Xylem sap to move to replace the water lost from
  the mesophyll cells.

63
Xylem Sap

• Is pulled by the resulting tension all the way
  down the plant to the roots and soil.

64
Summary

• Xylem sap moves along a continual chain of water
  potential from
  air? leaf? stem? roots? soil

65
(No Transcript)
66
Comments

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

67
Factors that Affect Transpiration Rate

• 1. Environmental
• 2. Plant Structures

Multiple Layer Epidermis
Stomatal Crypt
68
Environmental Factors

• 1. Humidity
• 2. Temperature
• 3. Light
• 4. Soil Water Content
• 5. Wind

69
Plant Structure Factors

• 1. Cuticle
• 2. Stomate Number
• 3. Hairs

70
Stomates

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

71
Guard Cells

• Turgid Swell - open stomata.
• Flaccid Shrink - close stomata.
• Size of the cells is a result of turgor pressure
  changes.

72
Turgid - Open
Flaccid - Closed
73
Turgor Pressure of Guard cells

• Controlled by K concentrations.

74
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.

75
To Close Stomata

• 1. K leaves guard cells.
• 2. Water leaves guard cells.
• 3. Turgor pressure decreases.
• 4. Guard cells shrink and Stomata close.

76
K Movement

• Regulated by proton pumps and K channels.
• Controlled by
• Light (Blue)
• CO2 concentrations
• Abscisic Acid (water stress)

77
Comment

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

78
Adaptations for Balance

• C4 Ps
• CAM Ps

79
Phloem Transport

• Moves sugars (food).
• Transported in live cells.
• Ex Sieve Companion Cells

80
Source - Sink Transport

• Model for movement of phloem sap from a Source to
  a Sink.

81
Source

• Sugar production site
• Ex Ps Starch breakdown in a
  storage area.

82
Sink

• Sugar uptake site.
• Ex Growing areas Storage areas
  Fruits and seeds

83
Comment

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

84
Result

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

85
Xylem Transport In Contrast to Phloem

• Usually unidirectional.
• Endodermis prevents back flow.
• Dead cells.

86
Phloem Loading at the Source

• 1. Diffusion
• 2. Transfer Cells
• 3. Active Transport

87
Phloem Loading
88
Transfer Cells

• Modified cell with ingrowths of cell wall to
  provide more surface area for sugar diffusion.

89
Result

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

90
Bulk Flow

• Movement of water into phloem.
• Pressure forces phloem sap to move toward the
  sink.

91
At the Sink

• Sugar is removed.
• Water potential is raised.
• Water moves out of phloem over to xylem.

92
(No Transcript)
93
Phloem summary

• Source - builds pressure.
• Sink - reduces pressure.
• Pressure caused by
• Sugar content changes
• Water potential changes

94
Comment

• Plants move materials without "moving" parts,
  unlike animals.

95
Summary

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

96
Ts Lab Report

• Use data to prepare graph as directed by Analysis
  of Results.
• Answer questions 1-6 about the Ts graph.
• Answer questions 1- 4 about the celery.