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Chapter 36: Transport in Vascular Plants. 1. Where does transport occur in plants? Start with water…. H 2 O. H 2 O. Minerals. Figure 36.2 An overview of transport in a vascular plant. CO 2. O 2. H 2 O. H 2 O. Minerals. Figure 36.2 An overview of transport in a vascular plant.

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slide1

Chapter 36: Transport in Vascular Plants

1. Where does transport occur in plants? Start with water….

slide6

Chapter 36: Transport in Vascular Plants

  • Where does transport occur in plants? Start with water….
  • How are solutes transported between cells?
figure 36 3 proton pumps provide energy for solute transport

CYTOPLASM

EXTRACELLULAR FLUID

+

H+

+

ATP

H+

+

H+

Proton pump generates

membrane potential

and H+ gradient.

H+

H+

H+

H+

+

H+

+

Figure 36.3 Proton pumps provide energy for solute transport
figure 36 4 solute transport in plant cells

+

EXTRACELLULAR FLUID

CYTOPLASM

+

K+

Cations ( for

example) are driven into the cell by themembrane potential.

K+

+

K+

K+

K+

K+

K+

+

K+

+

Transport protein

(a) Membrane potential and cation uptake

+

H+

H+

NO3 –

Cell accumulates

anions (NO3 –, for

example) by

coupling their transport to theinward diffusion of H+ through a

cotransporter.

NO3–

+

H+

+

H+

H+

H+

H+

H+

NO3–

+

NO3 –

NO3 –

+

H+

NO3–

H+

H+

H+

+

(b) Cotransport of anions

+

H+

H+

H+

S

Plant cells can

also accumulate a

neutral solute,

such as sucrose

( ), by

cotransporting

down the

steep proton

gradient.

+

H+

+

H+

H+

S

S

H+

H+

H+

H+

S

S

S

+

H+

+

H+

S

H+

+

(c) Cotransport of a neutral solute

Figure 36.4 Solute transport in plant cells
slide9

Chapter 36: Transport in Vascular Plants

  • Where does transport occur in plants? Start with water….
  • How are solutes transported between cells?
  • What influences the movement of water?
    • Ψ = Ψs + Ψp
    • Water moves from HIGH  low (more  less)
fig 36 5 water potential and water movement an artificial model

(b)

(a)

(c)

(d)

0.1 M

solution

Purewater

H2O

H2O

H2O

H2O

YP = 0

YP = –0.30

YP = 0

YP = 0.30

YP = 0.23

YS = –0.23

YS = –0.23

YS = –0.23

YS = 0

YS = –0.23

Y = –0.30 MPa

Y = –0.23 MPa

Y = 0.07 MPa

Y = 0 MPa

Y = 0 MPa

Y = 0 MPa

Y= –0.23 MPa

Y = 0 MPa

Fig. 36.5 Water potential and water movement: an artificial model

Ψ = Ψs + Ψp

+ solute decreases Ψs

Water goes from high  low

+ pressure counteracts Ψs

More pressure forces water across membrane

(-) pressure also moves water

slide11

Chapter 36: Transport in Vascular Plants

  • Where does transport occur in plants? Start with water….
  • How are solutes transported between cells?
  • What influences the movement of water?
  • What does this mean for plant cells?
figure 36 6 water relations in plant cells

Initial flaccid cell:

p = 0

s = –0.7

0.4 M sucrose solution:

Distilled water:

= –0.7 MPa

p = 0

p = 0

Turgid cell

at osmotic

equilibrium

with its

surroundings

Plasmolyzed

cell at osmotic

equilibrium

with its

surroundings

s = 0

s = –0.9

= 0 MPa

= –0.9 MPa

p = 0

p = 0.7

s = –0.9

s = –0.7

= 0 MPa

= –0.9 MPa

Initial conditions: cellular  > environmental . The cell

loses water and plasmolyzes. After plasmolysis is complete,

the water potentials of the cell and its surroundings are the

same.

(a)

(b)

Initial conditions: cellular  < environmental . There

is a net uptake of water by osmosis, causing the cell to

become turgid. When this tendency for water to enter is

offset by the back pressure of the elastic wall, water

potentials are equal for the cell and its surroundings.

(The volume change of the cell is exaggerated in this

diagram.)

Figure 36.6 Water relations in plant cells

Plasmolysis – shrinking of a plant cell away from its cell wall due to water loss

Turgid – plant cell full of water due to its high solute concentration

(turgor pressure)

Aquaporins allow water to move quickly across a membrane

slide13

Chapter 36: Transport in Vascular Plants

  • Where does transport occur in plants? Start with water….
  • How are solutes transported between cells?
  • What influences the movement of water?
  • What does this mean for plant cells?
  • What are the transport routes dissolved substances can take between cells?
fig 36 8 cell compartments and routes for short distance transport

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

Plasmodesma

Vacuolar membrane

(tonoplast)

Plasma membrane

(a)

Cell compartments. The cell wall, cytosol, and vacuole are the three main

compartments of most mature plant cells.

Key

Symplast

Apoplast

Transmembrane route

Apoplast

The apoplast is

the continuum

of cell walls and

extracellular

spaces.

The symplast is the

continuum of

cytosol connected

by plasmodesmata.

Symplast

Symplastic route

Apoplastic route

(b)

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.

Fig 36.8 Cell compartments and routes for short-distance transport

How does water get into the plant?

figure 36 9 lateral transport of minerals and water in roots

Casparian strip

Endodermis

Pathway along

apoplast

Pathway

through

symplast

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.

1

Casparian strip

Plasma

membrane

Minerals and water that cross

the plasma membranes of root

hairs enter the symplast.

2

Apoplastic

route

1

Vessels

(xylem)

2

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.

3

3

Root

hair

Symplastic

route

Epidermis

Endodermis

Vascular cylinder

Cortex

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.

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.

5

5

4

4

Figure 36.9 Lateral transport of minerals and water in roots
  • Why is the Casparian strip so important?
  • forces dissolved substances across a selectively permeable membrane
  • Keeps unwanted & unrecognized substances OUT of the plant
slide16

Chapter 36: Transport in Vascular Plants

  • Where does transport occur in plants? Start with water….
  • How are solutes transported between cells?
  • What influences the movement of water?
  • What does this mean for plant cells?
  • What are the transport routes dissolved substances can take between cells?
  • What is the mutualistic relationship between plant roots and
  • another biological organism?
slide18

Chapter 36: Transport in Vascular Plants

  • Where does transport occur in plants? Start with water….
  • How are solutes transported between cells?
  • What influences the movement of water?
  • What does this mean for plant cells?
  • What are the transport routes dissolved substances can take between cells?
  • What is the mutualistic relationship we discussed between plant roots
  • another biological organism?
  • How is xylem sap transported? (How can it defy gravity?)
    • Cohesion – water’s ability to stick to itself via hydrogen bonds
    • Adhesion – water’s ability to stick to other polar substances via H-bonds
    • WHY??
      • electronegative oxygen creates polar covalent bond in water
figure 36 13 ascent of xylem sap

Xylem

sap

Outside air Y

= –100.0 MPa

Mesophyll

cells

Stoma

Leaf Y (air spaces)

= –7.0MPa

Water

molecule

Transpiration

Atmosphere

Leaf Y (cell walls)

= –1.0 MPa

Xylem

cells

Adhesion

Cell

wall

Water potential gradient

Trunk xylem Y

= – 0.8 MPa

Cohesion,

by

hydrogen

bonding

Cohesion

and adhesion

in the xylem

Water

molecule

Root xylem Y

= – 0.6 MPa

Root

hair

Soil Y

= – 0.3 MPa

Soil

particle

Water uptake

from soil

Water

Figure 36.13 Ascent of xylem sap

Transpiration – loss of water vapor through leaves that pulls water up from roots

What controls the loss of water?

Stomata

fig 36 14 open stomata left and closed stomata colorized sem

20 µm

Fig. 36.14 Open stomata (left) and closed stomata (colorized SEM)

What controls the opening & closing of the stomata?

- K+ in the guard cells

figure 36 15 the mechanism of stomatal opening and closing

Cells turgid/Stoma open

(a)

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.

Radially oriented

cellulose microfibrils

Cell

wall

Vacuole

Guard cell

(b)

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

Figure 36.15 The mechanism of stomatal opening and closing

Cells flaccid/Stoma closed

slide22

Chapter 36: Transport in Vascular Plants

  • Where does transport occur in plants? Start with water….
  • How are solutes transported between cells?
  • What influences the movement of water?
  • What does this mean for plant cells?
  • What are the transport routes dissolved substances can take between cells?
  • What is the mutualistic relationship we discussed between plant roots
  • another biological organism?
  • How is xylem sap transported? (How can it defy gravity?)
  • How is phloem sap transported?
figure 36 17 loading of sucrose into phloem

High H+ concentration

Cotransporter

Sieve-tube

member

Companion

(transfer) cell

Mesophyll cell

H+

Proton

pump

Cell walls (apoplast)

S

Plasma membrane

Plasmodesmata

Key

ATP

Sucrose

H+

H+

Apoplast

S

Phloem

parenchyma cell

Bundle-

sheath cell

Low H+ concentration

Symplast

Mesophyll cell

(a)

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.

(b)

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.

Figure 36.17 Loading of sucrose into phloem
figure 36 18 pressure flow in a sieve tube

Vessel

(xylem)

Sieve tube

(phloem)

Source cell

(leaf)

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.

Sucrose

1

1

H2O

H2O

2

2

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.

Transpiration stream

Pressure flow

In the case of

leaf-to-root

translocation,

xylem recycles

water from sink

to source.

Sink cell

(storage

Root)

4

4

3

3

Sucrose

H2O

Figure 36.18 Pressure flow in a sieve tube