Chapter 2 Water Metabolism

1. Chapter 2 Water Metabolism

2. Plant Water Relations • For every g of organic matter made by the plant, approximately 500 g of water is transpired by the plant. • Leaves loose up to ~ 100% of their water/hr.

4. Amounts of water in plants • Living plants =~ 80 to 95% water. • Vegetables =~ 85 to 95% water • Sapwood =~ 35 to 75% water • Seeds =~ 5 to 15% water

5. Roles of Water in Plant cells • Medium for biochemical reactions. • Physical support. • • Turgor pressure. • Thermal regulation. • Transport of water, nutrients and other molecules.

6. Status of water in plants • bound water • free water • Cytoplasm • Protein = 60% • Hydrophilic • Hydrophobic

7. How Does Water Move? • – Passive Processes • Diffusion - down a concentration gradient. • Bulk Flow - down a pressure-driven gradient. • Osmosis - down both a concentration & pressure driven gradient (= water potential). • • Across a selectively semi-permeable membrane.

8. Diffusion • Diffusion - directed movement - high to low concentration (higher to lower free energy). • Random thermal motion. • Significant role in: • • short-distance movement

9. Bulk Flow = Mass Flow • The concerted movement of groups of molecules by mass, most often in response to a pressure gradient. • Examples: • • Water moving through the xylem. • • A river flowing, rain falling. • Controled by : • • Aquaporin (水孔蛋白）

11. A B A. Diffusion B．Bulk flow

12. Osmosis • Movement of water across a selectively semi-permeable membrane in response to chemical and pressure gradients (together = water potential). • • Selectively semi-permeable membrane • prevents exchange of solutes, but • allows passage of water.

14. Concept • Water Potential • Water potential = the free energy associated with water = potential for doing work. • Water moves down a water potential gradient (from higher to lower potentials) = gives up energy as it moves.

15. Water Potential • Water potential is not an absolute value. It is relative to a reference state. • • Reference state = pure water at ambient • temperature and pressure. • The water potential within a cell is usually less than the reference state.

16. Plant cell—osmotic system • The Plant Cell • • Cell wall –permeable • • Membrane and protoplasm – semi-permeable.

17. If a cell is placed in a: • Hypotonic Solution (lower solute concentration) – water rushes in and generates turgor pressure. • Hypertonic Solution (higher solute concentration) - water flows out of the cell. • Isotonic Solution (equal solute concentration) - equilibrium.

18. Concepts • Plasmolysis (质壁分离)- protoplast shrinks away from the cell wall. • • Turgor pressure = 0

19. Water Potential of the plant cell

21. Osmotic Potential • Ψs = - RTCs • Ψs = osmotic potential. • R = the gas constant (0.00832). • T = temperature. • Cs = solute concentration (mol L-1). dissolved solutes Ψs

23. Pressure Potential • Cell expands with water and pushes the cell membrane against the rigid cell wall => pressure. • • Turgor pressure = positive hydrostatic • pressure inside cells pressing against the • cell wall. • Water continues to enter the cell until pressure counteracts the negative osmotic pressure.

26. Water Movements in Cells • Water moves in and out of cells because of differences in water potential between the cell and the surrounding solution. • Water moves from cell to cell in the same manner.

27. Water movement from cell to cell

29. Root Water Absorption Root Hairs • Thin-walled outgrowths of epidermal cells. • Provide a large root surface area and close contact between root & soil. • Greatly increase contact of root with soil H2O. • Small diameter permits them to penetrate capillary spaces.

30. Vertical section of root tip

31. Transversal section

32. Root Water Absorption • Pathways • Apoplast • Transmembrane • Symplast • Driving force • Root pressure • Transpirational pull • Soil factors affecting root water absorption • Available water • Aeration conditions • Temperature • Solute concentration

33. 3 Pathways: Water Movement Inside the Root • Via the Apoplast – e.g. through cell walls and intercellular spaces. • Via the Symplast – e.g. passing from cell to cell via the plasmodesma. • Via Transmembrane Pathway – e.g. through membranes (cell membrane and vacuole membrane).

34. Apoplast • Symplast • Apoplast and Symplast pathway

35. Transmembrane Pathway

36. Driving force for root water absorption Active absorption • Root Pressure • Roots actively absorb and transport ions into the xylem. • As solutes build up, water potential decreases and water flows into the cells => increases pressure = Root pressure.

37. Bleeding because of root pressure

38. Driving force for root water absorption Water potential gradient Transpirational pull :Passive absorption • Transpiration:Evaporated Water moves from the leaf to the outside. • Primarily through the stomata. • Water potential gradient from leaf to root

39. Effects of Soil on Root Water Absorption: Available water • Sand: low • • Large Diameter • = 20-2000 µm. • • Low Surface Area • = <1-10 m2. • • Large spaces • between particles • =water drains. • • Lower Field • Capacity. • Clay: high • • Small Diameter • =<2 µm. • • Large Surface Area • =100-1000 m2. • • Small spaces • between particles • =holds water. • • Greater Field • Capacity.

40. Effects of Soil on Root Water Absorption: Aeration conditions • CO2 • • anaerobic respiration • • alcoholism • • water absorption inhibited

41. Low T • • Viscidity • • Respiration • • Growth • • Enzyme • High T • • Aging • • Enzyme Effects of Soil on Root Water Absorption: Temperature

42. Low solute • • Water potential • High solute • • Water potential water absorption inhibited Effects of Soil on Root Water Absorption: Solute concentration

44. Transpiration

45. Absorbed water by plant Metabolism Loss 95%—99% 1％—5％ Through: 1) Liquid state (e.g. bleeding, guttation） 2) Evaporated state (transpiration) Major way

46. Transpiration (蒸腾作用) • The loss of evaporated water from the plant to the outside air via diffusion. • • Most is lost through the stomata (>90%). • • But also through the cuticle (表皮), lenticels (皮孔), etc (5-10%).

47. Significance of transpiration • Responsible for the movement of water and nutrients through the plant. • Cooling of the leaves. • Improving gas exchange.

48. Dicot Monocot

49. Guard Cells • Take up water (cells swell) = opens pore. • Lose water – closes pore.

50. Mechanisms for stomatal movement • Starch–sugar interconversion • Potassium ion uptake • Malate production