IB Biology 2015 Curriculum

1. 9.1: Transport in the xylem of plants
http://131.229.88.77/microscopy/Portfolios/Arlene/Portf
olio4_files/Portfolio2.html
Orange xylem (magnified 2500x)
Essential Idea: Structure and function are correlated in the xylem of
plants

2. Understandings
Statement Guidance
9.2 U.1 Transpiration is the inevitable consequence of gas
exchange in the leaf
9.2 U.2 Plants transport water from the roots to the leaves to
replace losses from transpiration
9.2 U.3 The cohesive property of water and the structure of
the xylem vessels allow transport under tension
9.2 U.4 The adhesive property of water and evaporation
generate tension forces in leaf cell walls
9.2 U.5 Active uptake of mineral ions in the roots causes
absorption of water by osmosis.

3. Applications and Skills
Statement Guidance
9.1 A.1 Adaptations of plants in deserts and in saline soils for
water conservation.
9.1 A.2 Models of water transport in xylem using simple
apparatus including blotting or filter paper, porous pots
and capillary tubing.
9.1 S.1 Drawing the structure of primary xylem vessels in
sections of stems based on microscope images
9.1 S.2 Measurement of transpiration rates using potometers.
(Practical 7)
9.1 S.3 Design of an experiment to test hypotheses about the
effect of temperature or humidity on transpiration
rates.

4. http://www.greenpeace.org/international/Global/international/photos/forests/2010/7/01amazon-clouds.jpg
9.1 U.1 Transpiration is the inevitable consequence of gas exchange in
the leaf

5. 9.1 U.1 Transpiration is the inevitable consequence of gas
exchange in the leaf
• Exchange of these H2O and CO2
gases must take place in order
to sustain photosynthesis
• Stomata – pores
• Transpiration – loss of water
vapor from the leaves and stems
of plants
• Guard cells – found in pairs (1 in
either side stoma), control
stoma and can adjust from wide
open to fully closed
• Liverworts – exception of
stomata
Click4biology.com

6. 9.1 U.1 Transpiration is the inevitable consequence of gas exchange in
the leaf
• Stomata ( singular Stoma ) are
pores in the lower epidermis
formed by two specialized Guard
Cells.
• The epidermis and its waxy
cuticle is impermeable to carbon
dioxide and water.
• If the water loss is too severe the
stoma will close. This triggers
mesophyll cells to release abscisic
acid (hormone). Which stimulates
the stoma to close.
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7. 9.1 U.2 Plants transport water from the roots to the leaves to replace
losses from transpiration
• Water evaporates into the air
• The water loss from the leaf draws
new water vapor from the spongy
mesophyll (symplastic & apoplastic
movement) into air space.
• In turn the water molecules of
the Mesophyll space draw water
molecules from the end of the xylem.
• Water molecules are weakly attracted
to each other by hydrogen bonds
(Cohesion). Therefore this action
extends down the xylem creating a
'suction' effect.

8. 9.1 U.3 The cohesive property of water and the structure of the
xylem vessels allow transport under tension
Plants are however more
'architectural' in their
Structure with adaptations
which provide support for a
static structure, much in the
same way as seen in
buildings.
Thickening of the
cellulose cell wall
and lignin rings

9. 9.1 U.3 The cohesive property of water and the structure of
the xylem vessels allow transport under tension
• In the diagram to the above
left the xylem shows a
cylinder of cellulose cell
wall with annular
lignification in rings.
• The photograph to the left
show the thickening of the
cellulose walls of the xylem.
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10. 9.1 U.4 The adhesive property of water and evaporation generate
tension forces in leaf cell walls

11. 9.1 U.5 Active uptake of mineral ions in the roots causes
absorption of water by osmosis.
Pathways for water movement:
(a) Water enters epidermal cell cytoplasm by osmosis. The solute concentration is
lower than that of soil water due to the active transport of minerals from the soil
water to the cytoplasm.
Symplastic Pathway (b) to (c): water moves along a solute concentration gradient.
There are small cytoplasmic connections between plant cells called
plasmodesmata. In effect making one large continuous cytoplasm.
Apoplastic Pathway (d) to (e):water moves by capillarity through the cellulose cell
walls. Hydrogen bonding maintains a cohesion between water molecules which
also adhere to the cellulose fibers.
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12. 9.1 U.5 Active uptake of mineral ions in the roots causes absorption of
water by osmosis.
Loading water into the xylem:
• Concentration of mineral
ions in the root 100 times
greater than water soil
• Active transport, protein
pumps
• Minerals are actively loaded
into the xylem in the roots
which in turn causes water
to enter the xylem vessel.
• Chloride for example is
actively pumped creating a
water potential gradient
that moving water passively
into the xylem.
• Pressure within the xylem
increases forcing water
upward (Root Pressure).Click4biology.com

13. 9.1 U.5 Active uptake of mineral ions in the roots causes absorption of
water by osmosis.
• Fungus grows on the surface
of the roots (sometimes in the
cells of the root) to overcome
the problem of ions slow
movement as they bind to the
surface of soil particles.
http://www.sanniesshop.com/images/symbiosis-3.jpg
http://planthealthproducts.com/wp-content/uploads/2013/04/mycroot.gif

14. 9.1 A.1 Adaptations of plants in deserts and in saline soils for
water conservation.
• Plants adapted to reduce water
loss in dry environments.
Examples of such water stress
habitats include:
• Desert (high temp, low
precipitation)
• High Altitude & High
Latitude (low
precipitation
• Tundra where water is
locked up as snow or ice.
• Areas with sandy soil
which causes water to
rapidly drain.
• Shorelines that contain
areas of high salt levels

15. 9.1 A.1 Adaptations of plants in deserts and in saline soils for
water conservation.
Waxy Leaves:
•The leaves of these plant
have waxy cuticle on both
the upper and lower
epidermis
•The waxy repels water loss
through the upper and lower
epidermal cells. If an
epidermal cell has no cuticle
water will rapidly be lost as
the cellulose cell wall is not a
barrier to water loss.

16. 9.1 A.1 Adaptations of plants in deserts and in saline soils for water
conservation.
Firs and Pines:
•Confers have their
distribution extended beyond
the northern forests. Plants in
effect experience water
availability more typical of
desert environments.
•Needles as leaves to reduce
surface area.
•Thick waxy cuticle
•Sunken stomata to limit
exposer.
•No lower epidermis.
http://fc08.deviantart.net/fs71/i/2014/127/1/c/pine_needles_by_neelfyn-d7hj7z4.jpg

17. 9.1 A.1 Adaptations of plants in deserts and in saline soils for water
conservation.
Succulent
•The leaves have been
reduced to needles to
reduce transpiration.
•The stem is fleshy in
which the water is
stored.
•The stem becomes the
main photosynthetic
tissue.
http://upload.wikimedia.org/wikipedia/commons/c/c9/Echinocactus_grusonii_kew.jpg

18. 9.1 A.1 Adaptations of plants in deserts and in saline soils for
water conservation
• Species of grass occupying sand
dunes habitat.
• Thick waxy upper epidermis
extends
• Leaf rolls up placing, containing
hairs. The stomata in an enclosed
space not exposed to the wind.
• The groove formed by the rolled
leaf also acts as a channel for rain
water to drain directly to the
specific root of the grass stem.
http://upload.wikimedia.org/wikipedia/commons/a/a2/
AmericanMarramGrassKohlerAndraeStateParkLake
Michigan.jpg
click4biology

19. 9.1 A.2 Models of water transport in xylem using simple apparatus
including blotting or filter paper, porous pots and capillary tubing.

20. 9.1 S.1 Drawing the structure of primary xylem vessels in sections of
stems based on microscope images
https://pw-biology-2012.wikispaces.com/file/view/Xylem.png/354386126/800x479/Xylem.png
https://classconnection.s3.amazonaws.com/263/flashcards/1226263/jpg/6959590336_b28341ed7a_z1354494841351.jpg

21. Transpiration
•Transpiration is the loss of water from
a plant by evaporation
•Water can only evaporate from the
plant if the water potential is lower in
the air surrounding the plant
•Most transpiration occurs via the
leaves
•Most of this transpiration is via the
stomata.
9.1 S.2 Measurement of transpiration rates using potometers. (Practical 7)
In the plant: factors affecting the rate of
transpiration
1.Leaf surface area
2.Thickness of epidermis and cuticle
3.Stomatal frequency
4.Stomatal size
5.Stomatal position

22. A SimpleA Simple
PotometerPotometer
1’’’’’’’’2’’’’’’’’3’’’’’’’’4’’’’’’’’5’’’’’’’’6’’’’’’’’7’’’’’’’’8’’’’’’’’9’’’’’’’’10’’’’’’’’11’’’’’’’’12’’’’’’’’13’’’’
Air tight seals
Plastic tubing
Graduated scale
Capillary tube
Leafy shoot cut
under water
Water evaporatesWater evaporates
from the plantfrom the plant
Movement of meniscus is
measured over time
9.1 S.2 Measurement of transpiration rates using potometers. (Practical 7)

23. 1’’’’’’’’2’’’’’’’’3’’’’’’’’4’’’’’’’’5’’’’’’’’6’’’’’’’’7’’’’’’’’8’’’’’’’’9’’’’’’’’10’’’’’’’’11’’’’’’’’12’’’’’’’’13’’’’
The rate of water loss from
the shoot can be measured
under different
environmental conditions
volume of water taken upvolume of water taken up
in given timein given time
Limitations
•measures water uptake
•cutting plant shoot may damage plant
•plant has no roots so no resistance to water being pulled up
Water is pulled upWater is pulled up
through the plantthrough the plant
9.1 S.2 Measurement of transpiration rates using potometers. (Practical 7)

24. 6 Environmental Factors Affecting Transpiration6 Environmental Factors Affecting Transpiration
1. Relative humidity:- air inside leaf is saturated
(RH=100%). The lower the relative humidity
outside the leaf the faster the rate of
transpiration as the Ψ gradient is steeper
2. Air Movement:- increase air movement
increases the rate of transpiration as it moves
the saturated air from around the leaf so the Ψ
gradient is steeper.
3. Temperature:- increase in temperature
increases the rate of transpiration as higher
temperature
• Provides the latent heat of vaporisation
• Increases the kinetic energy so faster
diffusion
• Warms the air so lowers the Ψ of the air, so
Ψ gradient is steeper
9.1 S.2 Measurement of transpiration rates using potometers. (Practical 7)
4. Atmospheric pressure:- decrease in atmospheric
pressure increases the rate of transpiration.
5. Water supply:- transpiration rate is lower if there
is little water available as transpiration
depends on the mesophyll cell walls being wet
(dry cell walls have a lower Ψ). When cells
are flaccid the stomata close.
6. Light intensity :- greater light intensity increases
the rate of transpiration because it causes the
stomata to open, so increasing evaporation
through the stomata.

25. 9.1 S.3 Design of an experiment to test hypotheses about the effect of
temperature or humidity on transpiration rates.