The next chapter

PLANT

TRANSPORT

Learning outcome

b) Define the term transpiration and explain why it

is an inevitable consequence of gaseous

exchange

Transpiration

• It is an important process

• Driving force of water movement in the plants

• Water transport pathway takes place in the roots, stem and leaves.

• Sets the water potential gradient across the plant (bottom to top)

• The importance demands us to know more about the notion of transpiration– Gaseous exchange

– Factors affecting transpiration

– Experiment pertaining the rate of transpiration

Transpiration

Definition

– Loss of water vapour from the surfaces of the leaf,

particularly the stoma

Transpiration

Why transpiration is the driving force of water

movement in a plant?

1) Water loss from the leaves kept the leaves dry

2) So more water enter the leaves from the stem (vessels)

3) The stem will eventually get drier

4) So more water enter the stem from the root tissues

5) And the roots will get drier, thus creates steep water potential gradient that will drive more water to diffuse into the root cells from the soil via osmosis.

Transpiration

Explain how the mechanisms of transpiration

becomes the inevitable consequence of gaseous

exchange?

Transpiration

• Gasses e.g. CO2 and O2

are often exchanged in the

leaves because of

photosynthesis.

• Photosynthetic cells

require CO2 to make

sugars while other

respiring cells require O2

to make ATP.

Transpiration

Transpiration

Mechanisms of transpiration

• Evaporation happens in the cell wall (forms

water vapour)

• Water pathway in the leaves begins when water

exits the vessels and enters the mesophyll cells.

• Symplast and apoplast pathway involved.

• Water vapour diffuse through the stoma into the

environment when the stoma opens.

• Q – how does the stoma opens?

Transpiration

How does the stoma opens?

Light activates proton pump proton pump actively pumps H+ out of the cell K+ enters to balance the neutrality water potential in the cell decreases water enters via osmosis = turgid cell

Microfibril to prevent elongation of the cell

Thick inner cell wall to prevent sideway expansion

So stoma opens

Transpiration

Finally… how is transpiration an inevitable

consequence of gaseous exchange

• CO2 needed for photosynthesis

• When the stoma opens……

• Large surface area of mesophyll cell

• Moist surface of mesophyll cell

Transpiration

What are factors affecting transpiration?

• Light -- ?

• Wind -- ?

• Temperature -- ?

• Humidity -- ?

*You must be able to explain why these can affect the rate of transpiration*

Learning outcome

d) Describe how to investigate experimentally the

factors that affect transpiration rate

Transpiration

Potometer is used to measure the rate of transpiration

Q – What is the assumption made when this apparatus is used?

Q – Why this apparatus cannot measure the actual transpiration rate of an actual plant?

Q – What are the precautions taken while carrying out this experiment?

Learning outcome

d) e) and f) will be covered in the practical.

Learning outcome

g) Explain the movement of water between plant

cells, and between them and their environment, in

terms of water potential (no calculations involving

water potential will be set);

Osmosis between cells and

environment / cell-to-cell

Answer the following… True/False

1) Water enter the cell

2) Water leave the cell

3) Net movement of water is from the cell to surrounding solution

Learning outcome

h) Describe the pathways and explain the

mechanisms by which water is transported from

soil to xylem and from roots to leaves

Root pressure

• Caused by active transport of ions into root cells,

especially ones near the xylem.

• Water gets drawn naturally into the xylem via

osmosis.

• But first water must get into the root tissues via

osmosis from the soil.

• This will cause the root cortex to be very turgid.

• This creates a pressure that pushes water up

the xylem vessels. This is called root pressure

Root pressure

Root pressure

• Water must diffuse from one cell to another in the root cortex.

• This refers back to symplast and apoplast.

• Apoplast pathway must be stopped.

• This pathway is unrestricted and whatever water carries along might end up taken by the xylem

• The Casparian strip within the endodermis cell layer functions to block apoplast pathway because it has suberin, a highly impermeable water substance.

Cohesion-Adhesion Force

• Mass flow of water

• Hydrogen bond

• Continuous column

• Water sticking onto plant surfaces

• Movement due to difference in hydrostatic

pressure

• Against gravity

Learning outcome

i) Outline the roles of nitrate ions and magnesium ions in plants.

Nitrate ions – synthesis of amino acids, proteins (for growth of vegetative tissues) and nucleic acids (Cell Division or Cellular/Tissue Repair)

Magnesium ions – synthesis of chlorophyll (light dependent reaction of photosynthesis)

Learning outcome

j) Describe how the leaves of xerophytic plants are

adapted to reduce water loss by transpiration

Learning outcome

k) Explain translocation as an energy requiring

process transporting assimilates, especially

sucrose, between the leaves (sources) and other

parts of the plant (sinks)

l) Explain the translocation of sucrose using the

mass flow hypothesis

Translocation

• Movement of sugar in phloem is bidirectional,

whereas water in xylem is unidirectional.

• In phloem, most common transported sugars are

non-reducing sugars, such as sucrose. Why

sucrose? It is because not only it is soluble but it

is also less reactive to the cells compared to

other sugars, e.g. glucose.

Translocation – the mass flow model

• Source – Any exporting region that produces

sugars above and beyond its own needs. E.g.

leaves, mesophyll cells.

• Sink – Any area that does not produce enough

sugar to meet its own needs. E.g. fruit, roots

Translocation – Active loading of

sucrose

• The loading of sucrose into the sieve tube

element is an active transport

• This requires the active proton pump and a

secondary co-transport.

• The companion cells are required because the

STE themselves lack the nuclear ability to

provide necessities for the transport.

Phloem loading uses a proton/sucrose co-transport protein.

Mass-flow

• Phloem loading leads to a buildup of sugars (the phloem cells becomes very negative in water potential)

• In response, water enters sieve elements from xylem via osmosis

• Thus phloem turgor pressure increases

Mass flow

• In sink tissue…

– Phloem unloading leads to

lower sugar concentration

(the phloem cells become

hypotonic)

– Water leaves the phloem

and enters sink sieve

elements and xylem (via

osmosis)

– Thus phloem turgor

pressure decreases

Fig. 10.10

Phloem solution moves along a gradient of hydrostatic pressure generated by a flow of water between source and sink ends of the pathway

Translocation at sink

• Unloading of sucrose is also an active process

• Water diffuse out of the STE and turgor pressure

decreases.