Transpiration and Translocation

Transport in Plants

Mass Flow Hypothesis

- Water potential is lowered in the sieve tube element due to entry of sucrose.

- Water moves in by osmosis from the xylem.

- Increases hydrostatic pressure.

Using the information in your notes summarise the loading of sucrose into the sieve tube element

Source to Sink

Companion Cell

Mesophyll

Cell

Sieve Tube Element

Sucrose moves from source to sink by mass flow through the phloem.

Diffusion

Sucrose

Co-transporter protein

Sucrose

and

H+ ions

Facillitated diffusion

ADP + Pi

H+ ions

Active Transport

ATP

Lesson objectives:

The mass flow hypothesis of movement from the source to the sink

Some words you need to know

- Sucrose diffuses out at sink.

- Hydrostatic pressure lower at sink, as water also moves out.

- Hydrostatic pressure gradient produces flow along phloem.

- This is MASS FLOW

LOADING OF SUCROSE AT SOURCE

Some words you

need to know

Sucrose diffuses from the companion cell to the sieve tube element through the plasmodesmata.

Translocation = transport of assimilates e.g. sugars in the phloem tissue.

COPY OUT: Assimilates = organic molecules such as sucrose, that have been made in the plant.

Directions: up, down & across.

Source = releases sucrose into the phloem

Sink = removes sucrose from the phloem

To describe the factors affecting the rate of transpiration.

To describe and explain the process of translocation.

In summary, sucrose in solution moves from high pressure

at the source to low pressure at the sink.

Sucrose moves out into the tissues by diffusion and active transport. Here it is converted to:

a)    glucose to be used in respiration

b)   a storage compound e.g. starch

Translocation

UNLOADING OF SUCROSE

FROM PHLOEM

Sucrose in solution moves in the phloem by MASS FLOW.

Activity

The process of sucrose being loaded in the phloem tissue.

LOADING OF SUCROSE AT SOURCE

H+ diffuse back into the cell down a concentration gradient using a carrier protein.

The same carrier proteins also carry sucrose molecules with them back into the companion cell. (Yes at the same time!)

It is called a CO-TRANSPORTER protein.

Translocation occurs in the

phloem sieve tube cells.

Factors affecting the Rate of Transpiration

Cut and stick the statements, on the Mechanism of Translocation (page 18), in order.

It moves from source - leaves to sink –seeds, roots, fruit, flowers, meristems.

Starter Activity

Factors affecting the rate of

transpiration

Match the factor with its explanation on page 13 in your Transport in Plants booklet.

Use numbers to match the factor with its explanation.

• Sucrose is loaded into the companion by an ACTIVE PROCESS.
• Hydrogen ions (H+) are actively pumped out of the companion cell
• This requires ATP
• There is a higher concentration of H+ outside the companion cell

Anything that increases the water vapour potential gradient, between the inside and the outside of the leaf, will increase the water loss.

These can either be to do with the plant or its environment.

LOADING OF SUCROSE AT SOURCE

If it is windy...

If there are greater number of leaves...

If the temperature is high...

If the relative

humidity is high...

A: ...then there will be a low water vapour potential gradient maintained, as the wind will carry water vapour, outside the leaf, away.

B: ...then there will be a high water vapour potential gradient maintained, as the wind will carry water vapour, outside the leaf, away.

C: ...then there will be a low water vapour potential gradient maintained, as the wind will carry water vapour, inside the leaf, away.

A: ...there will be a smaller surface area over which water vapour can be lost.

B: ...there will be a larger surface area over which water vapour can be retained.

C: ...there will be a larger surface area over which water vapour can be lost.

A: ...then there will be an increase the rate of evaporation and an increase in the rate of diffusion through stomata.

B: ...then there will be an increase the rate of evaporation and a decrease in the rate of diffusion through stomata.

C: ...then there will be an decrease the rate of evaporation and an increase in the rate of diffusion through stomata.

A: ...there will be a bigger water vapour diffusion gradient and an increase in rate of water loss.

B: ...there will be a smaller water vapour diffusion gradient and an increase in rate of water loss.

C: ...there will be a smaller water vapour diffusion gradient and a decrease in rate of water loss.

If the water availability is low...

If there is a waxy cuticle...

If the position of the stomata is on lower surface...

If there is light...

A: ...then the stomata open to allow gaseous exchange for photosynthesis.

B: ...then the stomata open to allow gaseous exchange for respiration.

C: ...then the stomata close to allow gaseous exchange for photosynthesis.

A: ...then the plant cannot replace the water vapour that transpires and the stomata becomes turgid to reduce water loss.

B: ...then the plant cannot replace the water vapour that transpires and the stomata becomes flaccid to reduce water loss.

C: ...then the plant cannot replace the water vapour that transpires and the stomata becomes turgid to increase water loss.

A: ...then the water vapour loss is faster.

B: ...then the water molecule loss is slower.

C: ...then the water vapour loss is slower.

A: ...Waxy cuticle reduces evaporation from the leaf surface.

B: ...Waxy cuticle prevents evapouration from the leaf surface.

C: ...Waxy cuticle reduces evaporation from the stomata.

Plenary

Answer the past paper questions on translocation.