10 - Transport in plants

Basic plant needs

co2

light

h2o

o2

minerals and nutrients

why do you need co2

light

h2o

to make glucose

water transports through ___

xylem

Sugar transport through ____

phloem

3 hypotheis of how Water movement through xylem

1. Capillary action 2. Root osmotic push 3. Transpiration

what is Capillary action

H2O moves upward in a small tube through adhesion to tube walls and cohesion btw H2O molecules

adhesion

the action or process of adhering to a surface or object.

with the stem

cohesion

with other water molecular

what is stronger adhesion or cohesion

adhesion

what distance is capillary action

limit is 1 m

capillary action relation to width and distance

thinner tube = higher rise

Osmosis (osmotic pressure)

H2O moves through semipermeable membrane (e.g. cell wall) from low concentration to high concentration (solution) until concentrations on both membrane sides are equal

it pushes water up

High solute concentration in root hairs

Plant actively pumps ions into root hair (=epidermal hair) cells

High ion concentration → greater osmotic pressure in root hairs than

surrounding soil H2O → water moves into cells by osmosis

distance of osmotic push

limit is 3 m

Transpiration

Transpiration pull is the primary force driving the upward movement of water through the xylem in plants.

It is generated by the loss of water vapor from the stomata of leaves due to transpiration.

How Transpiration Pull Works

• Water Evaporation: Water evaporates from the mesophyll cells of leaves into the air spaces and exits through the stomata.

• Negative Pressure Formation: As water leaves the cells, it creates a lower water potential in the leaf, pulling water from nearby cells.

• Cohesion and Adhesion: Due to the cohesive property of water (hydrogen bonding), water molecules form a continuous column in the xylem. Adhesion helps water molecules stick to xylem walls, preventing them from breaking apart.

• Upward Water Movement: The tension created in the leaves pulls water upward from the roots, overcoming gravity.

• Root Water Absorption: To maintain the continuous flow, water is absorbed from the soil through root hairs.

Uptake from soil into root hairs

Water enters root hairs from soil by osmotic pressure gradient (more solute int he roots )

Water moves into xylem by osmosis and transpirational pull (from the leaves)

Total plant water transport

Water loss from leaves draws water up from roots

Cohesion holds together string of water

Adhesion helps with the push a little bit

do you also need transpiration for smaller plants

yes

2 ways water enters roots

apoplastic ransport

symplastic transport

Apoplastic transport:

through cell walls; faster, less resistance to H2O flow (inbetweeb )

Symplastic transport:

through cells and plasmodesmata (of parenchyma)

Casparian strip:

impassable for apoplastic H2O

Endodermis with Casparian strip

“control” layer to what substances can permeate into xylem

suberin

waxy barrier to apoplastic movement

outside the endodermis

cortex

inside endodermis

xylem

ATP-dependent control in endodermis

atp is needed from apoplastic flow to symplastic flow and then back into apoplast

what type of transport can cross the endodermis

symplastic

Total plant water transport

Water loss from leaves draws water up from roots

Photosynthesis – transpiration compromise

For PS, plants need to open stomata → access to CO2 (CO2 , H20, light)

>90% of H20 transported through xylem: lost to transpiration through stomata

→ problem if H2O cannot be replaced from soil (heat, drought)

Mechanism of stoma opening & closing

Osmosis though active regulation of K+ concentration (ATP-driven pump)

Source in terms for phloem

Tissues that make or dispense food

sink in terms for phloem

Tissues that require carbon for energy & for biosynthesis

can a tissue be both source and sink

yes

eg. root

sugar Source

• Seed endosperm

• Photosynthetic leaves

• Mature roots

sugar sink

• Shoot or root meristems

• Developing seeds

• Developing flowers

• Developing roots

how is phloem flow driven

Phloem flow driven by pressure gradient → flow from high to low sugar concentration

how is phloem driven enrgy required

Energy required to establish pressure gradient (uploading and unloading of phloem), but no energy required along pathway

At source end of pathway

Active sugar uploading (ATP pump) into sieve tube members cells from source through companion cells

→ increase of osmotic pressure in phloem cells

→ H2O enters along osmotic pressure into phloem cells (from xylem)

→ pressure gradient increases, forcing liquid toward lower sugar concentration through continuous sieve tubes

→ With continued active sugar loading, liquid flows from high pressure (source) to low pressure (sinks)

At sink end of pathway

Active unloading (ATP pump) of sugars from sieve cells

→ Decrease of pressure in sieve cells

→ Water flows out of sieve tube members into xylem (pressure difference and transpirational pul

1. In order to utilize osmotic pressure, what do plants do?

Plants modify their internal solute concentration to be higher than the one of the 

surrounding soil water

1. What is the semipermeable membrane which involved in water uptake through roots?

Cell wall of all root hairs on roots

In order to pass the Casparian strip, symplastic water needs to be uploaded into the apoplast

false

water transport through the root cortex

The largest bulk of the water taken up through root hairs moves through the apoplast of the cortex

Once the apoplastically transported water and minerals hit the endodermis, this route of water transport is no longer possible

In symplastic transport, water flows through the cytosol of parenchyma cells

Which plant organ can be both sink and source for phloem flow, depending on the season?’

roots

What are the forces and drivers behind the capillary action?

Adhesion and cohesion and Width of tube or the capillary

1. Where does the force originate to drive transpirational pull?

leaves

A hydrostatic pressure caused by a difference in the amounts of solutes between solutions that are separated by a semi-permeable membrane

Osmotic pull

If the concentration of potassium is higher in guard cells than in the surrounding epidermal cells, water will flow from epidermal cells into guard cells and hence the stoma will close

false

What factors need to be in place for transpirational pull to work?

Cohesion between water molecules

Evaporation of water in leaves

Solar power

Stomatal opening

The flow of dissolved sugar through phloem is reversible. Which cell type is in control which direction it is going?

Companion cells

In leaves, water needs to flow from phloem into neighboring xylem

false

Similarly as with the control of osmotic pressure in roots, the maintenance of the appropriate osmotic pressure gradient across epidermal and guard cells is ATP-driven

true

1. In this particular season, the phloem flow is bottom-up. When is this happening

in the spring

1. If a plant is water-stressed in the summer and closes its stomata, can phloem still transport dissolved sugars top down?

No, because phloem needs water to flow from xylem near the source into phloem near the source