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IB Assessment Statements
• 9.2.5 Define Transpiration• 9.2.6 Explain how water is carried by the
transpirational stream, including structure of xylem vessels, transpirational pull, cohesion, adhesion, and evaporation.
TranspirationTranspiration is the loss of water form the leaves and stem of
plants.
Pathway of Water in Plant Transport
1. Soil ---> Epidermis of Roots Hair Cell2. Epidermis of Roots Hair Cell Cortex Cells3. Cortex Cell - Endodermis Cells (with a
Casparian strip)4. Endodermis Cells Vascular Tissue (xylem
Cells)5. Xylem Cells Leaves6. Leaves Stomata or Leaves to chloroplasts
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Root Functions1. Soil ---> Epidermis of Roots Hair Cell• The high concentration of mineral ions in the
plant cells causes water molecules to move into the plant by osmosis.
Epidermis root hairs
Cortex cells
2. Epidermis of Roots Hair Cell Cortex Cells
Water movement across the cortex cell is by two pathways both involving a water potential gradient. 1. The cortex cell cytoplasm has a solute concentration
gradient. This moves water symplastically from cell to cell by osmosis (b,c,d,h in above diagram).
2. The Apoplastic pathway moves water by capillary action of mass flow through the connecting cellulose cell wall (e, f in above diagram)
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Root Functions3.Cortex Cell - Endodermis Cells (with a Casparian strip)• Endodermis cells are surrounded on four sides
by a waterproof strip called a Casparian strip.
Casparian strip
Casparian stripEndodermisCortex Cells
3.Cortex Cell - Endodermis Cells (with a Casparian strip)
• The casparian strip of the endodermis is a barrier to the movement of water of minerals by the apoplastic pathway.
• All solute and water must move through the plasma membrane of the endodermal cells before entering the xylem.
4.Endodermis Cells Vascular Tissue (xylem Cells)
• The cellulose cell wall of the endodermis contains a strip (casparian strip) of a waxy water repellant substance called suberin.
• The suberin prevents water and dissolved minerals from passing into the xylem by the apoplastic pathway.
• Therefore water solution must pass through the plasma membrane of the endodermis. The endodermis plasma membrane can then selectively control mineral uptake and rate of uptake.
• Minerals are actively loaded into the xylem which in turn causes water to enter the xylem vessel via osmosis.
• Pressure within the xylem increases forcing water upward (Root Pressure). This is probably not a major factor in transpiration of large plants.
5. Xylem Cells Leaves
Xylem Facts• Xylem vessels form a continuous
pipe from the root up through the stem along petioles to the leaf.
• Xylem cells are produced from the division of the cambium and then differentiation into xylem
• The cytoplasm full breaks down and the end wall break down to form the pipeline. Xylem cells (tracheid & Vessel elements) are DEAD.
• Vessel element cells have thick lignified secondary walls.
5. Xylem Cells Leaves
Xylem Phloem
Tracheid
Vessel element
Companion cell
Sieve tube element
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5. Xylem Cells Leaves
5. Xylem Cells Leaves
Cohesion and Adhesion• Water molecules are weakly
attracted to each other by hydrogen bonds (Cohesion). This action extends down the xylem creating a 'suction' effect.
• There is also adhesion between water molecules and the xylem vessels
5. Xylem Cells Leaves
Capillary Action• The cohesion and
adhesion act together to maintain the water column all the way up from the root to the stomata
• The tendency of water to rise in a thin tube due to adhesion & cohesion is called capillary action
6. Leave --> Stomata• The cohesion and adhesion act together
to maintain the water column all the way up from the root to the stomata. (a)
• Water evaporates out of the leave via the stomata (b,c)
• The rapid loss of water from the leaf pulls the water column stressing the cohesion and adhesion between water molecules. This creates 'tension' within the xylem vessel sufficient to cause the walls of the xylem to be bent inwards.
• The movement of water is an example of mass flow due to a negative pressure potential. Stomata
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6. Leave --> Stomata•The movement of water out of the leaf “pulls” water upward through the vascular system all the way from the roots.•This process is known as transpirational pull.
Watch the below animations about transpiration
Click on the below links to access the animations on transpiration.• http
://www.youtube.com/watch?v=QYbg4lQ-iaU&feature=related
• http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/animation_-_water_uptake.html
Summary of concepts
Summary of concepts
IB Assessment Statement
• 9.2.7 State that guard cells can regulate transpiration by opening and closing stomata
• 9.2.8 State that the hormone abscisic acid causes the closing of stomata
Regulation of Stomata
• 9.2.7 Guard cells and transpiration regulation.
• Stomata ( singular Stoma ) are pores in the lower epidermis of leaves.
• Each stomata is formed by two specialised Guard Cells.
• During the day the pore opens to allow carbon dioxide to enter for photosynthesis. However the plant will experience water loss. If the water loss is too severe the stoma will close.
• During the night plants cannot photosynthesis and so the plant closes the pores thereby conserving water.
Regulation of Stomata
Abscisic hormone and guard cells.• Plants have a mechanism which
closes the stoma at night. However when a plant is suffering water stress (lack of water) there is another mechanism to close the stoma.
• (a) dehydrated (low water potential) of the mesophyll cell causes them to release abscisic acid.
• (b) Abscisic acid stimulates the stoma to close..
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Regulation of Stomata
Abscisic Acid (ABA) – • Is a chemical found in plants that causes the guard cells to close the stomata when there is not a lot of water available for the plant
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Regulation of Stomata
•When water pressure within guard cells is high, the stoma open.
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Regulation of Stomata
•When water pressure within guard cells decreases due to abscisic acid (ABA) –
, the stoma closes
IB ASSESSMENT STATEMENT
• 9.2.9• Explain how abiotic factors light, temperature,
wind and humidity affect the rate of transpiration in a typical terrestrial plant.
Abiotic Factors that affect Transpiration Rate
• Humidity• Light • Wind• Temperature• Soil water• Carbon dioxide
