13.8 Movement of water
up stems
Learning outcomes
Students should be able to understand the following:
 What transpiration is
 The process by which water moves through a leaf
 How water moves up the xylem
Candidates should be able to:
 Describe the roles of root pressure and cohesion
tension in moving water through the xylem
How do tall trees get enough water?
Redwoods are the tallest species
of tree in the world.
The tallest redwood ever
measured was 120 metres tall.
That is six times the height of the
Angel of the North!
How does a plant this size get
water from its roots to the
branches at the top?
The water will travel up the
stem, or trunk, of the tree.
What experiment could show
that water travels up stems?
Movement of water up a stem
Transpiration
 The loss of water from the leaves of a plant is called
transpiration, and the resulting flow of water through
the plant is called the transpiration stream. The
transpiration stream is important because:
 it carries water for photosynthesis to the palisade cells
in the leaves
 the water carries essential mineral salts in solution
 evaporation from the leaves has a cooling effect
Water movement out of stomata
cu ticle
u p p er e p id erm is ce lls
p a lisa d e m e so p hyll cells
v e in
xylem
sp o n gy m e so p hyll cells
phloem
su b -sto m a tal a ir sp a ce
sheath
lo w e r e p id e rm is ce lls
g u ard ce lls
sto m a
Opening and closing of stoma
Transport of water up the xylem
There are 3 pathways:
 Root pressure (-minor)
 Capillarity (-minor)
 Cohesion tension (major)
 Root pressure is caused by the
mineral ions which are actively
transported into xylem vessels in
the root by endodermal cells. This
makes the water potential of the
xylem more negative and causes
water to enter the xylem by
osmosis.
Capillarity is responsible for some water creeping
up the xylem vessels simply because they are very
narrow.
Cohesion-tension theory
 The mass flow of water through the xylem relies on 2
important properties of water:
 Cohesion – the water molecules tend to stick together
 Adhesion – the water molecules also tend to stick to
the inside of the xylem vessels.
Cohesion-tension theory
Putting it all together
 Transport in xylem
 transpiration – pathways
Private life of plants - xylem, phloem.wmv
Extension and Homework
1. AQA AS Biology textbook pg 195
Summary question 1
Application questions 1-5
2. Complete the exam style question about
transpiration and cohesion-tension
Mark scheme - exam style question
(a) Suitable accepted evidence, 1 mark for evidence and 1 mark for
explanation
EITHER
(only) upward pressure could force liquid water out of leaves;
OR
Sap exuding from a cut, rooted stem;
(only) upward force could make this happen;
(b)
(i)
(ii)
2
(Note: max. two for any component)
Evaporation from leaves during daytime only/mainly;
tension/negative pressure (on water) in xylem creates inward pull
(on walls of xylem vessel);
xylem vessels become narrower;
due to adhesion of water molecules (to walls of xylem vessels);
root pressure gives outward force/push on walls of xylem vessels;
tree would become wider/stay same diameter;
xylem vessels become wider/stay same diameter;
max 3
[5]
Learning outcomes
Students should be able to understand the following:
 What transpiration is
 The process by which water moves through a leaf
 How water moves up the xylem
Candidates should be able to:
 Describe the roles of root pressure and cohesion
tension in moving water through the xylem