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### Transcript of Teacher: What is the formula for water? Student: H, I, J, K, L, M, N, O Teacher: That's not what I...

• Teacher: What is the formula for water? Student: H, I, J, K, L, M, N, O Teacher: That's not what I taught you. Student: But you said the formula for water was...H to O.Water cycling in ecosystems

• Whats water potential?Water potentials () are a way of measuring the free-energy (G) of water. Water will flow spontaneously from a high potential to a low potential, like a ball rolling down a hill.What forces cause water to move?PressuresGravityForces created by organisms (turgor pressure)Osmotic gradients (concentration of solutes)

• Water Cycle:These forces also operate in the hydrologic (water) cycle. Remember in the hydrologic cycle that water runs downhill (likewise it falls from the sky, to get into the sky it must be acted on by the sun and evaporated, thus needing energy input to power the cycle)

• Whats the deal with water potential?Plants need water (as a solvent, chemical reactant in hydrolysis, for cell shape, to counteract lost water from transpiration-necessary for CO2 uptake and photosynthesis)The water potential at any given point, , is the combined effect of all the factors that make water move.Water potential is usually negative so it moves from -1 to -4 (high to low).

• What factors affect water potential Its concentration (chemical potential): water will diffuse from a dilute solution to concentrated solution (osmosis). Its pressure (mechanical potential): water will from from a high pressure system (hose-pipe) to a low pressure system (vacuum). Its height (gravitational potential): water will flow downhill. Its charge (electrical potential): water is uncharged, so we can ignore this.

These factors sum together to give the overall free energy of a mass of water.

• Plant-available water: Difference between field capacity and permanent wilting pointCoarse FineParticle size

• Components of Water PotentialPressure potential (p).Difference between pressure of the water and that of the atmosphere. Most healthy cells exhibit a slightly + pressure potential due to the outward push of water (within vacuoles & cytoplasm) on the membrane or cell wall= turgor pressureOsmotic pressure (solute potential) (s).Affect of concentrations of solutes inside vs outside the cell that influence osmosisMatric potential (in soil) Affect of adhesion sticking water to soil surfaces. Measures how likely it is water becomes unstuckVapor potentialGravitational potential

• Pressure PotentialTurgid cell (left) have a high positive pressure potential (the water presses out on the cell wall, and the wall presses back on the water with an equal but opposite force).Cells that are flaccid or plasmolysed (right) have a pressure potential of zero.We assume a pressure potential of zero when measuring water potential in our experiment.

• Osmotic/ Solute Potentials = C R T C, concentration of solutes (M). R, Universal gas constant (8.314472 J K1 mol1). T, absolute temperature (K).Higher temp means more rapid molecular movt. More rapid osmosis.Considered primary factor affecting our experiment, often critical in living systems, Solute potential of pure water is 0

• Matric PotentialNot used in our study b/c these interactions are more common in soilsAdhesive forces between water and soil particles influence the readiness of water to leave soil pores.As soil becomes drier matric potentials increase & water is less likely to flow into roots.

• Gravitational Potential and Vapor potentialTheir effects are negligible at the cellular level within a small plant.Gravitational potential explains why condensed water falls as rain or precipitation.Higher up you go, the higher the gravitational potentialCan also be a factor in moving water up tall treesVapor potential -dont worry about it, except that it becomes more important when explaining water movement from leaves--> atmosphere

• HomeostasisIn plant cells (and animals) equilibrium is achieved when water potential in/out of cell are same:Concentration of solutes inside and out of cell are equal (facilitated largely by osmosis)Pressures are same (of water inside and outside cell)For working, growing plants most cells are in dynamic (every changing) equilibriumin this way they can change with environmental conditions and move water and nutrients as needed.

• Water moves along a pressure gradient

• Transpiration is a major driving forceDry warm air has low water potential & pulls water from cells with higher potential.Water is lost under these conditions when pores on the leaf surface are open for gas exchange for photosynthesisWater moves in continuous column from soil particles to leaf cellsPlant spends no energy in transporting water Passive transport driven be transpiration

• What about Capillary Action?Cohesive and adhesive forces of capillary action help HOLD water in plant tissuesCan only move water against gravity ~1/2 meter on their own.Capillary action also explains how water is held in soils

• What about vessel size?Transpiration causes the difference in water potentials necessary to move large quantities of water from root to leaf in large plants.Flow rate increases in proportion to capillary radius (larger is faster)45 meters per hour vs. 6 per hourLarger vessels = larger risk. Water columns break as they carry larger volumes. The drier the plant becomes the more frequent the breaks.Most plant have large and small vessels (xylem)

• Root PressureDifferences in concentrations of salts in soil or sugars in roots can influence water flow. In spring, sugar concentration is high in roots. This difference in concentration increases osmotic potential in cells.Water can then push into the roots and fill empty vessels from last season or new growing tissue. The vessels must be full before cohesion and transpiration can cause a steady pull of water up the plant.

• Water potential and water flow in plants in a nutshellWater moves from one part of the plant to another down a water potential gradient. Different components of are important at different stages.

Soil to roots: matric potential. Roots to stems: pressure potential. Stems to cells: osmotic potential. Cells to stomata: vapor potential.Where did our lab focus?

• Surface soil water Yt = - 0.8 MPaWater in the soil-plant-atmosphere continuum J = L (Dyt/l)Water moves along a gradient of decreasing water potentialWhat happens at night?Stomata close which term does this affect?How would water potential gradient respond?

• Relatable topicsYou should be able to use water potential in discussions regarding:Photosynthesis and transpirationThe water cycleThe chemical importance of water to life on earthPlant physiologyCell homeostasis and water balanceMembrane function and cell wall functionPlant evolution and adaptations of higher (vascular plants)Form and function of membranes and vacuoles Kinetics, free energy changeDiffusion/ Osmosis