1. Chapter 36
Transport in Plants

2. Pathways for Survival
For vascular plants
The evolutionary journey onto land involved the differentiation of the plant body into roots and shoots

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

4. Transport occurs on 3 scales
Transport of water and solutes by individual cells, such as root hairs
Short-distance transport of substances from cell to cell
Long-distance transport within xylem and phloem

5. Types of transport CO2 O2 Light H2O Sugar O2 H2O CO2 Figure 36.2
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

6. Central Role of Proton Pumps
Create a H+ (proton) gradient PE that can be harnessed to do work
 membrane potential
CYTOPLASM
EXTRACELLULAR FLUID
ATP H+
Proton pump generates
membrane potential
and H+ gradient. – +
Figure 36.3

7. (b) Cotransport of anions
Coupled transport
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

8. (c) Contransport of a neutral solute
“coattail” effect of cotransport
Responsible for the uptake of 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

9. Effects of Differences in Water Potential
Plants must balance water uptake and loss
Osmosis
Determines uptake or loss
Affected by solute conc. & pressure

10. Water potential (y) Water Solute concentration and pressure
 Direction of movement of water
Water
Flows f/ high y to low y

11. Solute potential Pressure potential
Proportional to # of dissolved molecules
Pressure potential

12. Addition of solutes Reduces y Figure 36.5a (a) P = 0 S = 0.23
0.1 M
solution
H2O
Pure
waer
P = 0
S = 0.23
 = 0.23 MPa
 = 0 MPa
(a)
Figure 36.5a

13. Application of pressure
Increases y
H2O
P =
S = 0.23
 = 0 MPa
 = 0 MPa
(b)
H2O
P =
S = 0.23
 = MPa
 = 0 MPa
(c)
Figure 36.5b, c

14. Negative pressure Decreases y Figure 36.5d (d) H2O P = 0.30 P = 0
 = 0.23 MPa
(d)
P = 0.30
S = 0
 = 0.30 MPa
Figure 36.5d

15. y Flaccid cells in higher solute concentration
Affects uptake and loss of water by plant cells
Flaccid cells in higher solute concentration
Cells lose water  plasmolyzed
Figure 36.6a
0.4 M sucrose solution:
Initial flaccid cell:
Plasmolyzed cell
at osmotic equilibrium
with its surroundings
P = 0
S = 0.7
S = 0.9
 = 0.9 MPa
 = 0.7 MPa

16. If Flaccid cell solution w/ low conc
Cell gains water  turgid
Distilled water:
Initial flaccid cell:
Turgid cell
at osmotic equilibrium
with its surroundings
P = 0
S = 0.7
S = 0
P = 0.7
Figure 36.6b
 = 0.7 MPa
 = 0 MPa
 = 0 MPa

17. Turgor loss (wilting)
Reversed when the plant is watered
Figure 36.7

18. Aquaporin Proteins
Transport proteins in the cell membrane that allow the passage of water
Do not affect y

19. Plasma membrane
Controls traffic of molecules into and out of the protoplast

20. Vacuolar membrane
Regulates transport between the cytosol and the vacuole
Transport proteins in
the plasma membrane
regulate traffic of
molecules between
the cytosol and the
cell wall.
the vacuolar
membrane regulate
traffic of molecules
between the cytosol
and the vacuole.
Plasmodesma
Vacuolar membrane
(tonoplast)
Plasma membrane
Cell wall
Cytosol
Vacuole
Cell compartments. The cell wall, cytosol, and vacuole are the three main
compartments of most mature plant cells.
(a)
Figure 36.8a

21. Cell walls and cytosol continuous from cell to cell
Cytoplasmic continuum
 symplast
Extracellular continuum
 apoplast

22. Symplastic route Apoplastic route
Key
Symplast
Apoplast
The symplast is the
continuum of
cytosol connected
by plasmodesmata.
The apoplast is
the continuum
of cell walls and
extracellular
spaces.
Transmembrane route
Symplastic route
Apoplastic route
Transport routes between cells. At the tissue level, there are three passages:
the transmembrane, symplastic, and apoplastic routes. Substances may transfer
from one route to another.
(b)
Figure 36.8b

23. Bulk Flow in Long-Distance Transport
Movement of fluid in the xylem and phloem is driven by pressure differences

24. Water and mineral salts f/ soil epidermis of roots shoot system

25. Lateral transport Figure 36.9 Casparian strip Endodermal cell1 2 3
Uptake of soil solution by the
hydrophilic walls of root hairs
provides access to the apoplast.
Water and minerals can then
soak into the cortex along
this matrix of walls.
Minerals and water that cross
the plasma membranes of root
hairs enter the symplast.
As soil solution moves along
the apoplast, some water and
minerals are transported into
the protoplasts of cells of the
epidermis and cortex and then
move inward via the symplast.
Within the transverse and radial walls of each endodermal cell is the
Casparian strip, a belt of waxy material (purple band) that blocks the
passage of water and dissolved minerals. Only minerals already in
the symplast or entering that pathway by crossing the plasma
membrane of an endodermal cell can detour around the Casparian
strip and pass into the vascular cylinder.
Endodermal cells and also parenchyma cells within the
vascular cylinder discharge water and minerals into their
walls (apoplast). The xylem vessels transport the water
and minerals upward into the shoot system.
Casparian strip
Pathway along
apoplast
Pathway
through
symplast
Plasma
membrane
Apoplastic
route
Symplastic
Root
hair
Epidermis
Cortex
Endodermis
Vascular cylinder
Vessels
(xylem)
Endodermal cell 4 5

26. Root hairs account for much of the surface area of roots

27. Mycorrhizae
Mutually beneficial relationships (symbiosis) with fungal hyphae, facilitate the absorption of water and minerals
2.5 mm
Figure 36.10

28. The Endodermis: A Selective Sentry
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

29. Waxy Casparian strip of the endodermal wall blocks apoplastic transfer

30. Water and minerals ascend from roots to shoots through the xylem
Plants lose an enormous amount of water through transpiration, loss of water f/ leaves

31. Xylem sap
Rises to heights of more than 100 m in the tallest plants

32. 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 from the root cortex
Generating root pressure

33. Root pressure results in guttation, (exudation) of water f/ tips of leaf
Figure 36.11

34. Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism
Water is pulled upward by negative pressure in the xylem
Based on cohesion of water molecules which is based on hydrogen bonding

35. Water vapor in the airspaces of a leaf
Transpirational Pull
Water vapor in the airspaces of a leaf
Diffuses down its water potential gradient and exits the leaf via stomata

36. Transpiration produces negative pressure (tension) in the leaf
Pulls on water in the xylem, pulling water into the leaf
Evaporation causes the air-water interface to retreat farther into
the cell wall and become more curved as the rate of transpiration
increases. As the interface becomes more curved, the water film’s
pressure becomes more negative. This negative pressure, or tension,
pulls water from the xylem, where the pressure is greater.
Cuticle
Upper
epidermis
Mesophyll
Lower
Water vapor
CO2 O2
Xylem
Stoma
Evaporation
At first, the water vapor lost by
transpiration is replaced by
evaporation from the water film
that coats mesophyll cells.
In transpiration, water vapor (shown as blue dots) diffuses from the moist air spaces of the leaf to the drier air outside via stomata.
Airspace
Cytoplasm
Cell wall
Vacuole
Water film
Low rate of
transpiration
High rate of
Air-water
interface
Y = –0.15 MPa
Y = –10.00 MPa 3 1 2
Figure 36.12
Air- space

37. Cohesion and Adhesion in the Ascent of Xylem Sap
Transpirational pull on xylem sap
Transmitted f/ leaves to roots and even into the soil solution
Facilitated by cohesion and adhesion

38. Water potential gradient
Ascent of xylem sap
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

39. Stomata help regulate the rate of transpiration

40. Stomata regulate transpiration rate, also:
Photosynthetic rate
Water loss rate
20 µm
Figure 36.14

41. Stomata
About 90% of the water a plant loses through stomata

42. Stomata flanked by guard cells
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

43. Chgs. in turgor pressure open and close stomata
From uptake and loss of K+ by the guard cells
H2O K+
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.
(b)
Figure 36.15b

44. Xerophytes Plants adapted to arid climates Leaf modifications
 reduce transpiration

45. Upper epidermal tissue
Stomata of xerophytes concentrated on the lower leaf surface, often located in depressions
Lower epidermal
tissue
Trichomes
(“hairs”)
Cuticle
Upper epidermal tissue
Stomata
100 m
Figure 36.16

46. Translocation
Transport of organic nutrients (e.g. sugar) in the plant

47. Sugar Movement Phloem sap Aqueous sucrose soln.
Travels from a sugar source to a sugar sink

48. Sugar source Sugar sink Sugar producer, e.g. leaves
Consumer or storer of sugar, e.g.tuber or bulb

49. Sugar loaded into sieve-tube members  
Figure 36.17a
Mesophyll cell
Cell walls (apoplast)
Plasma membrane
Plasmodesmata
Companion
(transfer) cell
Sieve-tube
member
Phloem
parenchyma cell
Bundle-
sheath cell
Sucrose manufactured in mesophyll cells can
travel via the symplast (blue arrows) to
sieve-tube members. In some species, sucrose
exits the symplast (red arrow) near sieve
tubes and is actively accumulated from the
apoplast by sieve-tube members and their
companion cells.
(a)

50. Phloem loading requires active transport
Proton pumping and cotransport of sucrose and H+
A chemiosmotic mechanism is responsible for
the active transport of sucrose into companion cells
and sieve-tube members. Proton pumps generate
an H+ gradient, which drives sucrose accumulation
with the help of a cotransport protein that couples
sucrose transport to the diffusion of H+ back into the cell.
(b)
High H+ concentration
Cotransporter
Proton
pump
ATP
Key
Sucrose
Apoplast
Symplast H+
Low H+ concentration S
Figure 36.17b

51. Pressure Flow
Sap moves through a sieve tube by bulk flow driven by positive pressure
Vessel
(xylem)
H2O
Sieve tube
phloem)
Source cell
(leaf)
Sucrose
Sink cell
(storage
root) 1
Loading of sugar (green dots) into the sieve tube at the source reduces water potential inside the
sieve-tube members. This causes the tube to take up water by osmosis. 2 4 3
This uptake of water generates a positive pressure that forces the sap to flow along
the tube.
The pressure is relieved by the unloading of sugar and the consequent loss of water from the tube at the sink.
In the case of leaf-to-root
translocation, xylem recycles water from sink
to source.
Transpiration stream
Pressure flow
Figure 36.18

52. Pressure flow explains why phloem sap always flows from source to sink
Aphid feeding
Stylet in sieve-tube
member
Severed stylet
exuding sap
Sieve-
Tube
EXPERIMENT
RESULTS
CONCLUSION
Sap droplet
Stylet
Sap
droplet
25 m
Sieve- tube member
To test the pressure flow hypothesis,researchers used aphids that feed on phloem sap. An aphid probes with a hypodermic- like mouthpart called a stylet that penetrates a sieve-tube member. As sieve-tube pressure force-feeds aphids, they can be severed from their
stylets, which serve as taps exuding sap for hours. Researchers measured the flow and sugar concentration of sap from stylets at different
points between a source and sink.
The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration.
The results of such experiments support the pressure flow hypothesis.
Figure 36.19