1. Transport in Vascular Plants I AL Biology

2. Key Concepts
Concept.1: Physical forces drive the transport of materials in plants over a range of distances
Concept.2: Roots absorb water and minerals from the soil
Concept.3: Water and minerals ascend from roots to shoots through xylem (木質部)
Concept.4: Stomata help regulate the rate of transpiration (蒸散作用)

3. Non-vascular plants Vs Vascular plants
The evolutionary journey onto land involved the differentiation of the plant body into roots and shoots

4. Vascular tissue
Transports nutrients throughout a plant; such transport may occur over long distances
Figure 36.1

5. Anatomy of a woody stem

6. Transport in vascular plants occurs on three scales
Concept 1: Physical forces drive the transport of materials in plants over a range of distances
Transport in vascular plants occurs on three scales
Transport of water and solutes by individual cells, such as root hairs
Short-distance transport of substances from cell to cell at the levels of tissues and organs
Long-distance transport within xylem and phloem at the level of the whole plant

7. A variety of physical processes
are involved in the different types of transport . 5 4
CO2 O2
Light
H2O
Sugar 3 6 . 2 7 1 O2
H2O
CO2
Minerals
Figure 36.2

8. A variety of physical processes
Are involved in the different types of transport
Sugars are produced by
photosynthesis in the leaves. 5
Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon for
photosynthesis. Some O2 produced by photosynthesis is used in cellular respiration. 4
CO2 O2
Light
H2O
Sugar
Transpiration, the loss of water
from leaves (mostly through
stomata), creates a force within
leaves that pulls xylem sap upward. 3
Sugars are transported as
phloem sap to roots and other
parts of the plant. 6
Water and minerals are
transported upward from
roots to shoots as xylem sap. 2
Roots exchange gases
with the air spaces of soil,
taking in O2 and discharging
CO2. In cellular respiration,
O2 supports the breakdown
of sugars. 7
Roots absorb water
and dissolved minerals
from the soil. 1 O2
H2O
CO2
Minerals
Figure 36.2

9. Selective Permeability of Membranes: A Review
The selective permeability of a plant cell’s plasma membrane
Controls the movement of solutes into and out of the cell
Specific transport proteins
Enable plant cells to maintain an internal environment different from their surroundings

10. Three Major Compartments of Vacuolated Plant Cells
Transport is also regulated
By the compartmental structure of plant cells
Compartmentation
Cell wall
Cytosol
Vacuole

11. Cell compartments
The plasma membrane controls the traffic of molecules into and out of the two major compartments, the cell wall and the cytosol
The vacuolar membrane regulates transport between the cytosol and the vacuole
Three compartments
Cell wall
Transport proteins in the plasma membrane regulate traffic of molecules between the cytosol and the cell wall.
Transport proteins in
the vacuolar membrane regulate traffic of molecules between the cytosol and the vacuole.
Cytosol
Vacuole
1. Vacuolar membrane
(tonoplast) (液胞膜)
3. Plasmodesma
(細胞質連絡絲)
2. Plasma
membrane
Cell compartments. The cell wall, cytosol, and vacuole are the three main
compartments of most mature plant cells.
(a)
Figure 36.8a

12. In most plant tissues, the cell walls and cytosol are continuous from cell to cell
The cytoplasmic continuum - is called the symplast
The apoplast - is the continuum of cell walls plus water-filled extracellular spaces

13. Tissue compartments (穿越細胞膜途徑) (質外體) (共質體) (共質體途徑) (質外體途徑)
Key
Symplast
Apoplast
The symplast is the
continuum of
cytosol connected
by plasmodesmata.
The apoplast is
the continuum
of cell walls and
extracellular
spaces.
1. Vacuolar route
2. Symplastic route
3. Apoplastic route
Transport routes between cells. At the tissue level, there are three passages:
the Vacuolar, symplastic, and apoplastic routes. Substances may transfer
from one route to another.
(b)
(共質體途徑)
(質外體途徑)
(質外體)
(共質體)
(穿越細胞膜途徑)
Figure 36.8b

14. Functions of the Symplast and Apoplast in Transport
Water and minerals can travel through a plant by one of three routes
Via the symplast (共質體/合胞體)
Along the apoplast (離質體/非原質體/質外體)
Both --- that is: Out of one cell, across a cell wall, and into another cell

15. Bulk Flow in Long-Distance Transport
In bulk flow (巨流)
Movement of fluid (sap) in the xylem and phloem is driven by pressure differences at opposite ends of the xylem vessels and phloem sieve tubes

16. Absorption of water and minerals from the soil
Concept 2: Roots absorb water and minerals from the soil
Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and where root hairs are located
Root hairs account for much of the surface area of roots

17. Mycorrhizae
Most plants form mutually beneficial relationships with fungi, which facilitate the absorption of water and minerals from the soil
Roots and fungi form mycorrhizae, symbiotic structures consisting of plant roots united with fungal hyphae
2.5 mm
Figure 36.10

18. The extensive surface area of cortical cell
Once soil solution enters the roots
The extensive surface area of cortical cell membranes enhances uptake of water and selected minerals

19. Lateral transport of minerals and water in roots

20. The Central Role of Proton Pumps
Proton pumps in plant cells
Create a hydrogen ion gradient that is a form of potential energy that can be harnessed to do work
Contribute to a voltage known as a membrane potential
CYTOPLASM
EXTRACELLULAR FLUID
ATP H+
Proton pump generates
membrane potential
and H+ gradient. – +
Figure 36.3

21. (a) Membrane potential and cation uptake
Plant cells use energy stored in the proton gradient and membrane potential
To drive the transport of many different solutes +
CYTOPLASM
EXTRACELLULAR FLUID
Cations ( , for
example) are driven into the cell by the membrane potential.
Transport protein K+ –
(a) Membrane potential and cation uptake
Figure 36.4a

22. (b) Cotransport of anions
In the mechanism called cotransport
A transport protein couples the passage of one solute to the passage of another
Figure 36.4b H+
NO3–
NO3 – – +
(b) Cotransport of anions
of through a
cotransporter.
Cell accumulates
anions ( , for
example) by
coupling their transport to the inward diffusion

23. (c) Contransport of a neutral solute
The “coat-tail” effect of cotransport
Is also responsible for the uptake of the sugar sucrose by plant cells H+ S
Plant cells can
also accumulate a
neutral solute,
such as sucrose
( ), by
cotransporting
down the
steep proton
gradient. – +
Figure 36.4c
(c) Contransport of a neutral solute

24. The Endodermis: A Selective Sentry
Is the innermost layer of cells in the root cortex
Surrounds the vascular cylinder and functions as the last checkpoint for the selective passage of minerals from the cortex into the vascular tissue

25. Effects of Differences in Water Potential
To survive
Plants must balance water uptake and loss
Osmosis
Determines the net uptake or water loss by a cell

26. Water potential
Is a measurement that combines the effects of solute concentration and pressure
Determines the direction of movement of water
Water
Flows from regions of high water potential to regions of low water potential

27. How Solutes and Pressure Affect Water Potential
Both pressure and solute concentration
Affect water potential
The solute potential of a solution
Is proportional to the number of dissolved molecules
Pressure potential
Is the physical pressure on a solution

28. Quantitative Analysis of Water Potential
The addition of solutes
Reduces water potential
0.1 M
solution
H2O
Pure
water
P = 0
S = 0.23
 = 0.23 MPa
 = 0 MPa
(a)
Figure 36.5a

29. Application of Positive physical pressure
Increases water potential
H2O
P =
S = 0.23
 = 0 MPa
 = 0 MPa
(b)
H2O
P =
S = 0.23
 = MPa
 = 0 MPa
(c)
Figure 36.5b, c

30. Negative pressure Decreases water potential Figure 36.5d (d) H2O
 = 0.23 MPa
(d)
P = 0.30
S = 0
 = 0.30 MPa
Figure 36.5d

31. Ascend from roots to shoots through the xylem
Concept 3: Water and minerals ascend from roots to shoots through the xylem
Plants lose an enormous amount of water through transpiration, the loss of water vapor from leaves and other aerial parts of the plant
The transpired water must be replaced by water transported up from the roots

32. The Ascent of Xylem Sap Xylem sap
Rises to heights of more than 100 m in the tallest plants
Water-conducting cells

33. Pushing Xylem Sap: Root Pressure
At night, when transpiration is very low
Root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential
Water flows in to the xylem from the root cortex
Generating root pressure

34. Guttation- a demonstration of root pressure
Root pressure sometimes results in guttation, the exudation of water droplets on tips of grass blades or the leaf margins of some small, herbaceous plants
Figure 36.11

35. Root pressure
In most plants, root pressure is not the major mechanism driving the ascent of xylem sap.
At most, root pressure can force water upward only a few meters, and many plants generate no root pressure at all.
For the most part, xylem sap is not pushed from below by root pressure but pulled upward by the leaves themselves.

36. Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism
Water is pulled upward by negative pressure in the xylem
Transpirational Pull
starts with water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata

37. Water Movement from the Leaf to the Atmosphere
Transpiration = the evaporation of water from leaf surfaces

38. Transpiration produces negative pressure (tension) in the leaf
Which exerts a pulling force on water in the xylem, pulling water into the leaf

39. Cohesion and Adhesion in the Ascent of Xylem Sap
Outside air Y
= –100.0 MPa
Leaf Y (air spaces)
= –7.0 MPa
Leaf Y (cell walls)
= –1.0 MPa
Trunk xylem Y
= – 0.8 MPa
Water potential gradient
Root xylem Y
= – 0.6 MPa
Soil Y
= – 0.3 MPa
Mesophyll
cells
Stoma
Water
molecule
Atmosphere
Transpiration
Adhesion
Cell
wall
Cohesion, by
hydrogen
bonding
Root
hair
Soil
particle
Cohesion
and adhesion
in the xylem
Water uptake
from soil
Figure 36.13
The transpiration pull on xylem sap
Is transmitted all the way from the leaves to the root tips and even into the soil solution
Is facilitated by cohesion and adhesion

40. Water-conducting cells of xylem

41. The Driving Force for Water Loss Is the Difference in Water Vapor Concentration
Transpiration from the leaf depends on
1) difference in water vapor concentration between the leaf air spaces and
the external air: cwv(leaf) – cwv(air)
2) the diffusional resistance (r) of this pathway

42. The Plant – Soil – Atmosphere Continuum
Movement of water from soil through plant to atmosphere involves different mechanisms of transport:
In the vapor phase, water moves by diffusion until it reaches outside air (and convection, a form of bulk flow, becomes dominant)
In xylem, water moves by bulk flow in response to a pressure gradient (ΔΨp)
For water transport across membranes, water potential difference across membrane is driving force (osmosis, e.g. when cells absorb water and roots transport water from soil to xylem)
In all cases: water moves toward regions of low water potential (or free energy)

43. Water moves toward regions of low water potential
Overview of water potential and its components at various points in the transport pathway from soil through plant to atmosphere

44. Maple tree Transpiration: 200 liters/day
75 cm/min

45. Stomatal Control -- Water conservation vs Leaf Photosynthesis
Problem:
Plants have to take up CO2 from atmosphere, but simultaneously need to
limit water loss – transpiration is a necessary evil
Cuticle protects from desiccation
However, plants cannot prevent outward diffusion of water without
simultaneously excluding CO2 from leaf and concentration gradient for
CO2 uptake is much smaller than concentration gradient that drives water
loss
Solution:

46. Stomatal Control -- Water conservation vs Leaf Photosynthesis
Problem:
Plants have to take up CO2 from atmosphere, but simultaneously need to
limit water loss – transpiration is a necessary evil
Cuticle protects from desiccation
However, plants cannot prevent outward diffusion of water without
simultaneously excluding CO2 from leaf and concentration gradient for
CO2 uptake is much smaller than concentration gradient that drives water
loss
Solution: Temporal regulation of stomatal apertures
Closed at night: no photosynthesis → no demand for CO2 → stomatal
aperture kept small → preventing unnecessary loss of water
Open during day: sunny morning; water abundant, light favors
photosynthesis → large demand for CO2 → stomata wide open →
decreased stomatal resistance to CO2 diffusion → water loss by
transpiration also substantial, but water supply is plentiful, i.e. plant trades
water for the product of photosynthesis needed for growth 46

47. Reference: The Transpiration Ratio -- Measures the Relationship between Water Loss and Carbon Gain
Transpiration ratio = the effectiveness of plants in moderating water loss while allowing sufficient CO2 uptake for photosynthesis
Transpiration ratio = Amount of water lost by transpiration Amount of CO2 fixed by photosynthesis
Also termed Water Use Efficiency (WUE) = inverse of transpiration ratio (e.g. plant with transpiration ratio of 500 has a WUE of 1/500 = 0.002)
Generally, H2O efflux is more than CO2 influx; this is due to:
Concentration gradient driving water loss is ~ 50 times larger than driving the influx of CO2 (due to low concentration of CO2 in air) and the relatively higher water vapor in leaf
CO2 diffuses ~ 1.6 times more slowly through air than water does (CO2 molecule is larger than H2O and has a smaller diffusion coefficient)
CO2 uptake must cross the plasma membrane, cytoplasm, chloroplast envelope before it is assimilated in the chloroplast → these membranes add to the resistance of CO2 diffusion pathway

48. Stomata: Major Pathways for Water Loss
About 90% of the water a plant loses
Escapes through stomata

49. Stomata Stomata from sedge (Carex) Cytosol and vacuole Pore
Heavily thickened
guard cell wall
Guard cells
Subsidiary cells
Stoma from a grass
Epidermal cell

50. Stomata Stomata from onion epidermis
Outside surface of the leaf with stomatal pore inserted into the cuticle
Guard cells facing the stomatal cavity, toward the inside of the leaf
Stomatal pore
Guard cell

51. The cell walls of guard cells have specialized features
Two main types of guard cells
- kidney-shaped stomata: for dicots, monocots, mosses,
ferns, gymnosperms
- grass-like stomata: grasses and a few other monocots
(e.g. palms)
Kidney-shaped stomata
Grass-like stomata

52. An increase in guard cell turgor pressure opens the stomata
- guard cells function as multisensory hydraulic valves
- guard cells sense changes in the environment: light intensity, light quality,
temperature, leaf water status, intracellular CO2
- this process requires ion uptake and other metabolic changes
in the guard cells (discussed later)
- due to this ion uptake → Ψs decreases → Ψw decreases → water moves
into guard cells → Ψp (turgor pressure) increases → cell volume increases
→ opening of stomata (due to differential thickening of guard cell walls)

53. The cell walls of guard cells have specialized features
Atmosphere
Portions of the guard cell wall are substantially thickened (up to 5 um across)
Alignment of cellulose microfibrils are essential for opening and closing
Pore
Nucleus
Plastid
Vacuole
Substomatal cavity
Inner cell wall

54. Special alignment of cellulose microfibres in guard cells
Each stoma is flanked by guard cells
Which control the diameter of the stoma by changing shape
Cells flaccid/Stoma closed
Cells turgid/Stoma open
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
Guard cell
Changes in guard cell shape and stomatal opening
and closing (surface view). Guard cells of a typical
angiosperm are illustrated in their turgid (stoma open)
and flaccid (stoma closed) states. The pair of guard
cells buckle outward when turgid. Cellulose microfibrils
in the walls resist stretching and compression in the
direction parallel to the microfibrils. Thus, the radial
orientation of the microfibrils causes the cells to increase
in length more than width when turgor increases.
The two guard cells are attached at their tips, so the
increase in length causes buckling.
(a)
Figure 36.15a

55. Changes in turgor pressure that open and close stomata
Result primarily from the reversible uptake and loss of potassium ions by the guard cells
Role of potassium in stomatal opening and closing.
The transport of K+ (potassium ions, symbolized
here as red dots) across the plasma membrane and
vacuolar membrane causes the turgor changes of
guard cells.
H2O
H2O
H2O
H2O
H2O K+
H2O
H2O
H2O
H2O
H2O

56. The stomata of xerophytes
Are concentrated on the lower leaf surface
Are often located in depressions (sunken) that shelter the pores from the dry wind
Upper
epidermal
tissue
Cuticle
Trichomes
(“hairs”)
Lower
epidermal
tissue
100 m
Stomata
Figure 36.16

57. Effects of Transpiration on Wilting and Leaf Temperature
Plants lose a large amount of water by transpiration
If the lost water is not replaced by absorption through the roots
The plant will lose water and wilt

58. Turgor loss in plants causes wilting
Turgor and wilting
Turgor loss in plants causes wilting
Which can be reversed when the plant is watered
Figure 36.7

59. Evaporative cooling?
Transpiration also results in evaporative cooling
Which can lower the temperature of a leaf and prevent the denaturation of various enzymes involved in photosynthesis and other metabolic processes
But recent findings cast doubt on the actual significance of the cooling effects

60. Water and Plants – Summary
water is the essential medium of life
plants gain energy from sunlight and need to have open pathway for CO2
- plants have large surface area that is not differentially permeable to CO2 vs. water vapor → conflict: need for water conservation and need for CO2 assimilation
- the need to resolve this conflict determines structure of plants: ?

61. Environmental Factors affecting transpiration
Light: stimulates the stomata to open allowing gas exchange for photosynthesis → transpiration increases (plants may lose water during the day and wilt)
Temperature: High temperature increases the rate of evaporation of water from the spongy cells, and reduces air humidity, so transpiration increases.
Humidity: High humidity means a higher water potential in the air, so a lower water potential gradient between the leaf and the air, so less evaporation.
Wind: Blows away saturated air from around stomata, replacing it with drier air, so increasing the water potential gradient and increasing transpiration. 61

62. Factors affecting transpiration
Plant factors
Leaf structure
Leaf area
Shoot-Root ratio
Adaptations to dry habitats
Plants in different habitats are adapted to cope with different problems of water availability.
Mesophytes plants adapted to a habitat with adequate water
Xerophytes plants adapted to a dry habitat
Halophytes plants adapted to a salty habitat
Hydrophytes plants adapted to a freshwater habitat 62

63. Structure of plants in relation to the conflict of water conservation and Carbon assimilation
- extensive root system to extract water from the soil
- a low-resistance pathway through the xylem vessels and tracheids to bring water into the leaves
- a hydrophobic cuticle covering the plant to reduce
evaporation
- stomata to allow gas exchange
- guard cells to regulate stomatal aperture (opening)

64. Marram grass

65. Marram grass –a xerophyte

66. Rolled leaves of marram grass

67. Internal Factors affecting transpiration
Some adaptations of xerophytes are:
Adaptation
How it works
Example
stops uncontrolled evaporation through leaf cells
most dicots
less area for evaporation
conifer needles, cactus spines
more humid air on lower surface, so less evaporation
reduce water loss at certain times of year
deciduous plants
maintains humid air around stomata
marram grass, pine
marram grass, couch grass
marram grass,
stores water
cacti
maximize water uptake 67 67

68. Internal Factors affecting transpiration
Some adaptations of xerophytes are:
Adaptation
How it works
Example
thick cuticle
stops uncontrolled evaporation through leaf cells
most dicots
small leaf surface area
less area for evaporation
conifer needles, cactus spines
stomata on lower surface of leaf only
more humid air on lower surface, so less evaporation
shedding leaves in dry/cold season
reduce water loss at certain times of year
deciduous plants
sunken stomata
maintains humid air around stomata
marram grass, pine
stomatal hairs
marram grass, couch grass
folded leaves
marram grass,
succulent leaves and stem
stores water
cacti
extensive roots
maximize water uptake 68 68

69. Xylem Sap Ascent by Bulk Flow: A Review
The mechanism of transpiration depends on the generation of negative pressure (tension) in the leaf due to unique physical properties of water.
As water transpires from the leaf, water coating the mesophyll cells replaces water lost from the air spaces..
Adhesion to the wall and surface tension causes the surface of the water film to form a meniscus, “pulling on” the water by adhesive and cohesive forces.

70. adhesion and surface tension lowers the water potential
The tension generated by adhesion and surface tension lowers the water potential, drawing water from where its potential is higher to where it is lower.
Mesophyll cells will lose water to the surface film lining the air spaces, which in turn loses water by transpiration.
The water lost via the stomata is replaced by water pulled out of the leaf xylem.

71. Transpiration pull on xylem sap is transmitted all the way….
The transpirational pull on xylem sap is transmitted all the way from the leaves to the root tips and even into the soil solution.
Cohesion of water due to hydrogen bonding makes it possible to pull a column of sap from above without the water separating.
Helping to fight gravity is the strong adhesion of water molecules to the hydrophilic walls of the xylem cells.
The very small diameter of the tracheids and vessel elements exposes a large proportion of the water to the hydrophilic walls.

72. tension within the xylem….
The upward pull on the cohesive sap creates tension within the xylem
This tension can actually cause a decrease in the diameter of a tree on a warm day.
Transpiration puts the xylem under tension all the way down to the root tips, lowering the water potential in the root xylem and pulling water from the soil.

73. an unbroken chain of water molecules….
Transpirational pull extends down to the roots only through an unbroken chain of water molecules
Cavitation, the formation of water vapor pockets in the xylem vessel, breaks the chain.
This occurs when xylem sap freezes in water.
Small plants use root pressure to refill xylem vessels in spring, but trees cannot push water to the top and a vessel with a water vapor pocket can never function as a water pipe again.
The transpirational stream can detour around the water vapor pocket, and secondary growth adds a new layer of xylem vessels each year.
The older xylem supports the tree.