Water Movement Within a Plant

1. Water Movement Within a Plant AP Biology Unit 5

2. d- d+ d- d+ d+ d+ Hydrogen bond Review: Properties of Water • Water is a polar molecule • Water molecules can form a “column” • due to hydrogen bonds between them • Cohesion = Water is attracted to other water molecules • Adhesion = Water is attracted to other polar molecules

3. Question… • Why do plants wilt? • When there is enough water  it flows into plant cells  cells are plump with water (turgid) • If there isn’t enough water  it flows out of the cell  cells are limp (flaccid)

4. Movement Into Roots • Water is absorbed into the roots through osmosis • Osmosis = diffusion of water • Osmosis is a passive process– no energy required • Water potential (ψ) determines the direction that osmosis will occur

5. Water Potential • Pure H2O has ψ = 0 • Water potential is a combination of the pressure from solutes (ψs)and the physical pressure (ψp). • The more solutes there are in a solution, the more negative the water potential will be.

6. Water Potential • Water always moves from areas of higher water potential (less solutes) to lower water potential (more solutes). • Moves from less negative ψ to more negative ψ. No net water movement

7. Pathways of Water • Apoplast Pathway = water travels between cells (outside of them) • Symplast Pathway = water travels through cells (inside them to get from cell to cell)

8. Water Movement in Roots • Water and ions are able to freely diffuse up to a point in the root cells • To get into the stele (where the vascular tissue is located), the water and ions must pass through the cells of the endodermis (symplast pathway).

9. Casparian Strip • Blocks the water from crossing the endodermis through apoplast pathway • control what enters the vascular tissue • Prevents water and solutes from “leaking” out into the soil

10. Water Transport in Plant • Water is transported through the plant in the xylem • Cells that make up the xylem: • Tracheids– found in all plants • Vessel Elements – found in some plants

11. Formation of xylem • Cell dies. • Cell contents disintegrate. • Water can move through these hollow cells with little resistance.

12. Capillary Action • movement of H2O up a very narrow tube • Does account for a little water movement up the xylem, but not much (about 40 cm) Image obtained without permission from http://ianrpubs.unl.edu/fieldcrops/graphics/soilh2o5.gif

13. Transpiration Pull • Accounts for most of the water movement up the xylem. • As H2O evaporates from the stomata (=transpiration), water from the xylem moves into the leaf to take its place. • Because of the hydrogen bonds between water molecules, water is “pulled” up the xylem.

14. Tension-Cohesion-Evaporation Model

15. Question… • When you receive a bouquet of flowers, why is it important to cut them under water? • Transpiration is still continuing, so if you cut it under water, it will ensure that water (not air) gets into the xylem. • Air bubble in xylem would disrupt the column of water.

16. Transpiration Rate • Factors that affect transpiration rate include: light, humidity, temperature. • The plant might also close its stomata to limit transpiration • We’ll look at this in more detail in the upcoming dry lab.

17. Stomata and Guard Cells • Stoma is Greek for "mouth" (plural = stomata) • Function of Stoma = to allow gas exchange with the outside environment (CO2 and O2) • Guard cells regulate the opening of the stomata

18. Stomatal Opening and Closing • When guard cells are full of water (plump)  stretch away from each other  stoma is open • When guard cells don’t have much water in them (limp)  don’t pull away from each other  stoma is closed

19. Role of K+ • K+ ions (potassium) control the opening of stoma by changing the water content in the guard cells. • K+ in : H20 follows by osmosis  stoma open • K+ out: H20 follows by osmosis  stoma closed

20. Question… • What conditions favor stomatal opening? • Plentiful water • Moderate temperatures

21. Hormonal Control of Stomata • Abscissic Acid (plant hormone) acts on guard cells to close stomata. • “stress” hormone in plants • Farmers can use it as an anti-transpirant (reduces water loss and the need for irrigation). • There are genetically engineered plants that have a mutant era gene—this makes them highly sensitive to abscissic acid.