IB Assessment Statements Define Transpiration Explain how water is carried by the transpirational stream, including structure of xylem vessels, transpirational pull, cohesion, adhesion, and evaporation.

Transpiration Transpiration is the loss of water form the leaves and stem of plants.

Pathway of Water in Plant Transport 1.Soil ---> Epidermis of Roots Hair Cell 2. Epidermis of Roots Hair Cell  Cortex Cells 3.Cortex Cell -  Endodermis Cells (with a Casparian strip) 4.Endodermis Cells  Vascular Tissue (xylem Cells) 5.Xylem Cells  Leaves 6.Leaves  Stomata or Leaves to chloroplasts

Copyright Pearson Prentice Hall Root Functions 1.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)

Copyright Pearson Prentice Hall Root Functions 3.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 Endodermis Cortex 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

Copyright Pearson Prentice Hall 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

Copyright Pearson Prentice Hall 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. iaU&feature=related iaU&feature=related hill.com/sites/ /student_view0/ch apter38/animation_-_water_uptake.html hill.com/sites/ /student_view0/ch apter38/animation_-_water_uptake.html

Summary of concepts

IB Assessment Statement State that guard cells can regulate transpiration by opening and closing stomata State that the hormone abscisic acid causes the closing of stomata

Regulation of Stomata 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..

Copyright Pearson Prentice Hall 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

Copyright Pearson Prentice Hall Regulation of Stomata When water pressure within guard cells is high, the stoma open.

Copyright Pearson Prentice Hall Regulation of Stomata When water pressure within guard cells decreases due to abscisic acid (ABA) –, the stoma closes

IB ASSESSMENT STATEMENT 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