13-1 Taiz. Water is needed for optimal corn yieldVegetative and reproductive growth depend on water. Why?Germination: Water needs Outline1. Water is the single most important molecule in biology.. Why? Unique properties of water2. How does water move?A. Water moves in 3 ways: diffusion, bulk flow, osmosis.b. Water moves from a region of high water potential to a region of low water potential.3. Cell expansion is driven by turgor pressure.4. How is water absorbed by roots?5. How is water transported up to the leaves?What do seeds need to germinate?1.2.3.Why?%Water lettuce?carrotseed (nut)WATER IS THE SINGLE MOST IMPORTANT MOLECULE IN BIOLOGY. WHY?• Good Solvent • Solvent and reactant for biochemical reactions• Medium used for transport of all ions and metabolites• Incompressible, so its uptake leads to cell expansion• Transparent, so it enables light to penetrate cells• Well suited for temperature regulation•What properties make water such an amazing molecule?3-3 Taiz. Polarity of water leads to H bonding.23-4. H bondingResult of H-bonding: High melting pointHigh boiling pointHigh specific heat of water High specific heat of vaporization•Cohesion – intermolecular attractionresults in high surface tension•Adhesion- attraction of water to solid phase (cell wall), results in capillary rise•Tensile strength of water is high.3.5 (A) Shape of a droplet placed on a solid surface; (B) CapillarityPositive pressure builds up when you push the syringe.Negative pressure results when you pull the syringe.Water is a universal solvent. It forms electrostatic interactions with ions and keeps ions in solution [Farabee, ch. 4]Water needs for germination: 1. Put metabolites in solution2. Hydrate enzymes so they fold to active state3. Breakdown stored food4. Generate energy for synthesis of new cells5. Provide turgor pressure for cell expansion3HOW DOES WATER MOVE ? (into & within seed and seedling)In which direction?Water movement is passive. i.e. energetically downhill.Water movement is down conc. gradient and pressure gradienta. Three ways water move:1) Diffusion- movement down a conc. gradient2) Bulk Flow: movement down a pressuregradient3) Osmosis: movement across a membrane and down both conc. and pressuregradient3-7. Taiz diffusionb) What determines the direction of water movement?The direction depends on the driving force.Water moves from a region of highwater potential to a region of lowwater potential until the water potential of the two sides are equal. (equilibrium)Water potential indicates the free energy of water and the potential to do work. The total driving force of water, the water potential, depends on the sum of the pressure potential + the osmotic (or solute) potential.ψw= ψP+ ψsExampleSee example 3.93-10. Taiz. Small increases in volume cause large changes in turgor pressure in a cell. Hydrated cell has + turgor pressure3-6. How does water cross cell membranes?Water can cross membranes via I) lipid bilayer,ii) Protein water channel4Why is hydrostatic pressure necessary?1. Positive Pressure causes wall to stretch.Therefore cells can grow.Cell expansion is driven by turgor pressure.2. Turgor pressure increases mechanical rigidity of cells, and gives plants support.Water potential is a measure of the water status of a plant. I.e. is plant suffering from water deficit or not?3-12 Taiz. Sensitivity of various processes to water potential of plantsCell growth is sensitive to water stress.