Copy down the diagram, what do you think might be written on the end of
the lines?
Objectives
Outcomes
Define transpiration
3: Describe transpiration as a
process.
Explain how water is carried by
the transpiration stream including
structure of xylem vessels,
transpiration pull, adhesion,
cohesion, evaporation.
5: Explain the process of
transpiration in plants.
7: Analyse methods which plants
use in order to control transpiration.
State that guard cells can
regulate transpiration by opening
and closing stomata
State that abscisic acid causes
this.
Key terms: transpiration, evaporation, cohesion, vapour.
Task
Outcomes
Define transpiration.
3: Describe transpiration as a
process.
The movement of water from
roots to the leaves due to the
loss of water vapour from the
leaves and stems of plants.
5: Explain the process of
transpiration in plants.
7: Analyse methods which plants
use in order to control transpiration.
Key terms: transpiration, evaporation, cohesion, vapour.
Outcomes
3: Describe
transpiration as a
process.
5: Explain the
process of
transpiration in
plants.
7: Analyse methods
which plants use in
order to control
transpiration.
Key terms: transpiration, evaporation, cohesion, vapour.
Outcomes
3: Describe
transpiration as a
process.
5: Explain the
process of
transpiration in
plants.
7: Analyse methods
which plants use in
order to control
transpiration.
Key terms: transpiration, evaporation, cohesion, vapour.
But why does it happen?
Outcomes
3: Describe transpiration as a
process.
5: Explain the process of
transpiration in plants.
7: Analyse methods which plants
use in order to control transpiration.
Key terms: transpiration, evaporation, cohesion, vapour.
But why does it happen?
Key terms: transpiration, evaporation, cohesion, vapour.
The transpiration stream occurs in xylem vessels and the movement of
water is passive. Mature xylem vessels are long dead structures made up
of cells arranged from end to end. The cell walls between the adjacent
xylem cells are broken down and the cytoplasmic content dies to form a
continuous tube. The cells also lack a plasma membrane which allows
water to enter the vessels freely. In addition, they also contain pores in
the outer cell walls which allows the movement of water out of the
vessels and into the surrounding cells of leaves. The outer cell walls
contain thickenings which resemble spirals or rings impregnated with
lignin which makes the vessels strong and able to withstand low
pressures. Low pressure (suction) is created in the xylem vessels when
water is pulled out of the transpiration stream via evaporation of water
vapour from the spongy mesophyll cell walls in the leaves. Heat from the
environment is necessary as it provides the energy required for the
evaporation of water.
Key terms: transpiration, evaporation, cohesion, vapour.
Key terms: transpiration, evaporation, cohesion, vapour.
Outcomes
• Water molecules have dipoles which cause
an attraction between them.
• Water is ‘pulled’ up the xylem vessels by
transpiration. When this happens, the pull is
transmitted all the way down the water
column, pulling all of the water molecules up
the vessel.
• For this to work, the xylem vessel must be a
continuous column of water i.e. contain no
bubbles.
3: Describe
transpiration as a
process.
5: Explain the
process of
transpiration in
plants.
7: Analyse methods
which plants use in
order to control
transpiration.
Key terms: transpiration, evaporation, cohesion, vapour.
Outcomes
• Mass flow of water in xylem vessels –
pressure lowered as water leaves vessels.
3: Describe
Water moves up from the roots where the
transpiration as a
pressure is higher.
process.
• Diffusion of water through cells from xylem.
5: Explain the
• Water vapour diffuses through leaf air
process of
spaces.
transpiration in
• If the concentration of water vapour in the
plants.
leaf is higher than outside, water vapour
7: Analyse methods
will diffuse out of the leaf through the
which plants use in
stomata.
order to control
• Capillary action (water molecules attracted
transpiration.
to polar molecules in xylem walls
Key terms: (adhesion)).
transpiration, evaporation, cohesion, vapour.
• Cohesion – tension theory.
Explain using diagrams and
written explanation the process
of transpiration in angiosperms.
Outcomes
3: Describe transpiration
as a process.
5: Explain the process
of transpiration in plants.
7: Analyse methods
which plants use in
order to control
transpiration.
Key terms: transpiration, evaporation, cohesion, vapour.
So how is water loss reduced?
Outcomes
3: Describe transpiration
as a process.
5: Explain the process
of transpiration in plants.
7: Analyse methods
which plants use in
order to control
transpiration.
Key terms: transpiration, evaporation, cohesion, vapour.
Outcomes
• Stomata can be closed to prevent water
loss.
• Guard cells curve apart when turgid.
• When flaccid, the edges of the cells lie
close together.
• However, this drastically slows
transpiration and means that no carbon
dioxide can enter the leaf (= no
photosynthesis).
3: Describe
transpiration as a
process.
5: Explain the
process of
transpiration in
plants.
7: Analyse methods
which plants use in
order to control
transpiration.
Key terms: transpiration, evaporation, cohesion, vapour.
Outcomes
3: Describe
transpiration as a
process.
5: Explain the
process of
transpiration in
plants.
The plant hormone
abscisic acid causes
the closing of stomata.
7: Analyse methods
which plants use in
order to control
transpiration.
Key terms: transpiration, evaporation, cohesion, vapour.
Outcomes
3: Describe
transpiration as a
process.
5: Explain the
process of
transpiration in
plants.
7: Analyse methods
which plants use in
order to control
transpiration.
Key terms: transpiration, evaporation, cohesion, vapour.
Key terms: transpiration, evaporation, cohesion, vapour.
Set up a potometer
Outcomes
3: Describe transpiration
as a process.
5: Explain the process
of transpiration in plants.
7: Analyse methods
which plants use in
order to control
transpiration.
Key terms: transpiration, evaporation, cohesion, vapour.