Root Hair Cells
Look at the following pictures. What
adaptations do roots have to allow for
absorption of water?
Root Hair Cells
Water Movement
 Mass flow of water in xylem vessels – pressure lowered
as water leaves vessels. Water moves up from the roots
where the pressure is higher.
Diffusion of water through cells from xylem.
Water vapour diffuses through leaf air spaces.
If the concentration of water vapour in the leaf is
higher than outside, water vapour will diffuse out of
the leaf through the stomata.
Capillary action (water molecules attracted to polar
molecules in xylem walls (adhesion)).
Cohesion – tension theory.
Cohesion-Tension Theory
 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.
Cross Section of A Leaf
Opening/ Closing Stomata
 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
Stomata Diagrams
Animation link
Factors Affecting Water Loss
 Diffusion rate is affected by:
 Surface area (the higher the surface area, the greater the
rate of diffusion).
 Difference in concentration (the greater the
concentration gradient, the higher the rate of diffusion).
 Length of the diffusion path (the greater the length, the
lower the rate of diffusion).
Preventing Excess Water Loss
 Cuticle (waxy layer) on leaf which is impermeable to
Most stomata found on underside of leaf as it is cooler
in dicotyledons.
Thick leaves = reduced water loss.
Spines/ hairs increasing boundary layer (undisturbed
layer of air).
Stomata closed at certain times of the day.
Stomata may be sunken and found in pits.
Xerophytes – Marram Grass
• Found on sand dunes.
• When dry, leaves roll up, so
stomata open to an enclosed
• Water vapour accumulates
in this space = reduced
diffusion gradiant.
• Spines increase width of
boundary layer.
Xerophytes Cont.
 Some xerophytes may have large numbers of stomata.
 Xerophytes cells may have extra support to prevent
cells collapsing when they dry out.
 Extensive root system.
 Leaves may have evolved to become spines, with water
being stored in the stem e.g. cacti.
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