Chapter 36:

Resource Acquisition &

Transport in Vascular Plants

2. Transport of Water & Minerals

1. Overview of Transport in Plants

3. Transport of Sugars

1. Overview of

Transport in Plants

H2O

H2O and

minerals

CO2

O2

O2

CO2

LightSugar

Resources

Needed by

Plants

Resources Needed by Plants

CO2 – carbon source used during photosynthesis

of sugars and other organic molecules

O2 – required for the synthesis of ATP by aerobic

respiration

Sunlight – source of energy for photosynthesis

Water – obtained primarily from the soil

Minerals & other Nutrients – obtained primarily

from the soil

Leaf Arrangement (Phyllotaxy)

Ground area

covered by plant

Plant A

Leaf area = 40%

of ground area

(leaf area index = 0.4)

Plant B

Leaf area = 80%

of ground area

(leaf area index = 0.8)

Leaf arrangement and orientation evolved to:

• maximize light absorption

• reduce self shading (blocking light to lower leaves)

• avoid damage from intense light

Leaf area index

represents % of

ground area

covered by plant:

• commonly >1 due to

multiple layers of

leaves

• plants self-prune

structures that don’t

receive enough light

More on Leaf Arrangement…

• light absorption tends to correlate with water loss

Plants must balance light absorption and

water loss:

• greater surface area for light absorption = greater

surface area for water loss

• leaf shape & arrangement reflect a balance

between the two:

• harsh light & low moisture

= smaller, vertical leaves

• low light & high moisture =

larger, horizontal leaves

3 Modes of Transport

Cell wall

Cytosol

Apoplastic route

Symplastic route

Transmembrane route

Plasmodesma

Plasma membrane

Key

Apoplast

Symplast

Apoplastic – through extracellular spaces

Symplastic – through cytosol, plasmodesmata

Transmembrane – across multiple plasma membranes

Solute Transport Across

Plant Cell MembranesCYTOPLASM EXTRACELLULAR

FLUID

Proton pump

Hydrogenion

H+/sucrosecotransporter

H+/NO3−

cotransporter

Sucrose(neutral solute)

Potassium ion

Ion channel

(b) H+ and cotransport of neutral solutes

(a) H+ and membrane potential

(c) H+ and cotransport of ions (d) Ion channels

H+

H+

H+

H+

H+

H+

H+

H+

ATP

H+

+

+

+

+

−

−

−

−

+

+

+

+

−

−

−

−

−

+

+

−

+

+

+

+

+

−

−

−

−

− +

−

+

+

+

+

+

−

−

−

− +

−

H+

H+

H+

H+

H+

H+

H+

H+

H+H+

H+

H+

H+

H+H+

H+

H+

H+

H+

H+

H+

S

S

S

K+

K+

K+

K+

K+

K+

K+

Nitrate

−

Potassium ion

Ion channel

Ion channels

+

+

+

+

−

−

−

− +

−

K+

K+

K+

K+

K+

K+

K+

Transport Through Ion Channels• plants have gated ion channels that, when opened,

allow ions to flow down the electrochemical gradient

H+ ions are pumped by active transport to create an

electrochemical gradient (membrane potential)…

The Role of H+ in Cotransport

CYTOPLASM EXTRACELLULARFLUID

Proton pump

Hydrogenion

H+ and membrane potential

H+

H+

H+

H+

H+

H+

H+

H+

ATP

+

+

+

+

−

−

−

−

−

+

H+/sucrosecotransporter

Sucrose(neutral solute)

+

+

+

+

−

−

−

−

− +

−

+

H+

H+

H+

H+H+

H+

H+

H+

H+H+

H+S

S

S

H+ and cotransport of neutral solutes

H+ flow down its electrochemical gradient can be

coupled to the active transport (movement from low to

high conc.) of neutral solutes such as sugars…

H+/NO3−

cotransporter

H+ and cotransport of ions

H+

+

+

+

+

−

−

−

−

− +

−

+H+

H+

H+H+

H+

H+

H+

H+

H+

H+

Nitrate

…or the active transport of ions such as nitrate (NO3-)

Osmosis is the diffusion of water across a cell

membrane.

The Transport of Water

The net direction of osmosis (water movement) in

plants depends on 2 factors:

• differences in the concentration of water & solutes across the

membrane (water diffuses from high to low concentration)

• differences in pressure (water moves from high to low

pressure)

The combination of these 2 factors (concentration &

pressure) is called water potential.

Initial flaccid cell:

Initial flaccid cell:

Final plasmolyzed cell at osmotic

equilibrium with its surroundings:

Final turgid cell at osmotic

equilibrium with its surroundings:

(a) Initial conditions: cellular ψ > environmental ψ

(b) Initial conditions: cellular ψ < environmental ψ

ψP = 0

ψS = −0.7

ψ = −0.7 MPa

ψP = 0

ψS = −0.9

ψ = −0.9 MPa

ψP = 0

ψS = −0.7

ψ = −0.7 MPa

ψP = 0.7

ψS = −0.7

ψ = 0 MPa

ψP = 0

ψS = −0.9

ψ = −0.9 MPa

ψP = 0

ψS = 0

ψ = 0 MPa

Environment

0.4 M sucrose solution:

Environment

Pure water:

…more on

Water Potential

Water potential (y) =

the sum of solute

potential (yS) and

pressure potential (yP)

y = yS + yP

For pure water yS = 0,

the more solutes the

more negative the yS

yP can be + or – in

relation to atmospheric

pressure

Turgor Pressure in Plants

The protoplast (interior part) of plant cells normally has

a positive yP due to osmosis, a pressure called turgor

pressure which keeps cells turgid (opposite of flaccid).

Normal plant

with turgid cellsWilted plant

with flaccid cells

In extracellular compartments such as xylem, yP is

negative which aids in the movement of fluid up from

the root system.

Rate of osmosis is

increased by

aquaporins.

2. Transport of Water & Minerals

in Xylem

From Root Hairs to XylemCasparian strip

Pathway along

apoplast

Endodermal cell

Pathway

through

symplast

Water

moves

upward

in vascular

cylinderPlasmodesmata

Plasma

membrane

Casparian strip

Vessels

(xylem)

Apoplastic

route

Symplastic

route

Root

hair

Endodermis Vascular

cylinder

(stele)

Epidermis

Cortex

Apoplastic

route

Symplastic

route

Transmembrane

route

The endodermis: controlled entry

to the vascular cylinder (stele) Transport in the xylem

1

1

2

3

23

4

4

4

5

5

5

Water & Mineral Uptake by Roots

The transport of water, minerals and other nutrients

via xylem vessels begins at the interface of the root

tip & root hair epidermis and the surrounding soil.

• the root epidermal cells are permeable to the aqueous

soil solution which freely passes along the cell walls

(apoplastic route) to the root cortex

• once this material reaches the endodermis, water and

desired solutes are transported across the endodermal

cells to the vascular cylinder (stele)

• once in the stele, water and mineral nutrients enter the

tracheids and vessel elements of the xylem as xylem

sap to be transported throughout the plant

How is Xylem Moved “Up”?

Xylem sap moves upward in the plant due to

a combination of the following:

ROOT PRESSURE (a minor factor)

• active transport of ions into the roots lowers

the water potential resulting in water flowing in

due to osmosis

TRANSPIRATION (the major factor)

• loss of water through the stomata of leaves

• adhesion of water to xylem vessels & cohesion

of water molecules to each other “pull” water

up to replace water lost through transpiration

Source of “Pull” in Transpiration

Cuticle

Upper

epidermis

Mesophyll

Lower

epidermis

Cuticle

Xylem

Airspace

Stoma

Water from xylem pulled

into cells and air spaces.

Microfibrils

in cell wall of

mesophyll cell

Microfibril

(cross section)Water

film

Air-water

interface

Increased surface

tension pulls

water from cells

and air spaces.

Air-water

interface

retreats.

Water vapor

replaced

from water

film.

Water vapor diffuses outside

via stomata.1

2

3

4

5

Diffusion of water vapor out of stomata starts the “pull”

which creates a negative water potential drawing water up:

Transpiration

Xylem

cells

Xylem sap

Mesophyll cells

Stoma

Water molecule

Atmosphere

Transpiration

Adhesion by

hydrogen bonding

Cell

wall

Cohesion

by hydrogen

bondingCohesion and

adhesion in

the xylem

Water molecule

Root hair

Soil particle

Water

Water uptake from soil

Wa

ter

po

ten

tia

l g

rad

ien

t

Outside air ψ

= −100.0 MPa

Leaf ψ (air spaces)

= −7.0 MPa

Leaf ψ (cell walls)

= −1.0 MPa

Trunk xylem ψ

= −0.8 MPa

Trunk xylem ψ

= −0.6 MPa

Soil ψ

= −0.3 MPa

Guard Cell Control of Stomata

• when guard cells are turgid, they bend and as a

result open the stomata

• when guard cells are more flaccid the stomata are

closed

Guard cells turgid/

Stoma open

Guard cells flaccid/

Stoma closed

Guard cellVacuole

Radially oriented

cellulose microfibrils

Cell

wall

Changes in guard cell shape and stomatal

opening and closing (surface view)

Regulation of Guard Cells

Guard cell turgidity is controlled by K+ ions which

move in response to changes in membrane

potential due to active transport of H+:

• pumping H+ out of guard cells

Role of potassium ions (K+) in stomatal

opening and closing

K+

H2O H2O

H2OH2O

H2O

H2O

H2O

H2O

H2O

H2O

Guard cells turgid/

Stoma open

Guard cells flaccid/

Stoma closed

• H+ pumped out of guard

cells lowers the

membrane potential

(more negative) drawing

K+ ions into the cell

• the intracellular

increase in K+ lowers

the water potential and

water flows in

Plants open stomata by

pumping H+ in response

to light and low CO2

(provided there is

enough water)

3. Transport of Sugars

Sugar Translocation via Phloem

The transport of photosynthetic products, a process

called translocation, proceeds through phloem

vessels in a direction opposite to that of xylem sap.

• photosynthetic products such as sucrose are produced

in photosynthetic organs such as leaves

• they are transported in phloem sap to sites of sugar use

or storage – sugar sinks

• e.g., fruits, tubers, growing shoot and root tips

• the transfer of sugars to phloem sieve tube elements or

companion cells occurs through both symblastic and

apoplastic routes…

Loading Sugars into Phloem

Sieve Tube ElementsApoplast

Symplast

Mesophyll cell

Cell walls (apoplast)

Plasma

membrane

Plasmodesmata

Companion

(transfer) cell

Sieve-tube

element

Mesophyll cell

Bundle-

sheath cell

Phloem

parenchyma cell

High H+ concentration

Proton

pump

Low H+ concentration

Cotransporter

Sucrose

H+

H+ H+

S

S

ATP

(b) A chemiosmotic mechanism is

responsible for the active transport

of sucrose.

(a) Sucrose manufactured in mesophyll cells

can travel via the symplast (blue arrows)

to sieve-tube elements.

• transport from apoplast to

sieve tube element symplast

involves cotransport with H+

Bulk Flow of Phloem Sap

Vessel

(xylem)

Sieve

tube

(phloem)

Source cell

(leaf)

Loading of sugar

decreases water

potential Sucrose

Sink cell

(storage

root)

Sucrose

Uptake of water

increases pressure

Unloading of sugar

and loss of water

relieves the pressure

Recycling of water

H2O

H2O

H2O

Bu

lk f

low

by n

eg

ati

ve p

ressu

re

Bu

lk f

low

by p

os

itiv

e p

res

su

re

1

1

2

2

3

34

4

• unlike xylem sap,

phloem sap flows

toward sugar

sinks due to

positive pressure

• this results in a net

diffusion of sugar

and movement of

water towards the

sinks

• sugar concentration

decreases near the

sugar sinks due to

usage for energy or

addition to polymers

such as starch