Download G3 – Transport (Plants)

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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?
Microfibril to prevent
elongation of the cell
Thick inner cell wall to
prevent sideway
expansion
So 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
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 cotransport 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
Phloem solution moves along a gradient of
hydrostatic pressure generated by a flow of water
between source and sink ends of the pathway
Fig. 10.10
Translocation at sink
• Unloading of sucrose is also an active process
• Water diffuse out of the STE and turgor pressure
decreases.