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Water Potential

Water Potential. AP Biology – Chapter 7 & Lab 4. Learning Targets. I can explain what water potential is and how it is impacted by Ψs and Ψp . I can design an experiment to determine the concentration of solutes in a living cell. Ψ = Ψs + Ψp Ψ s= - iCRT. Plants & Water Potential.

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Water Potential

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  1. Water Potential AP Biology – Chapter 7 & Lab 4

  2. Learning Targets • I can explain what water potential is and how it is impacted by Ψs and Ψp. • I can design an experiment to determine the concentration of solutes in a living cell. • Ψ =Ψs +Ψp • Ψ s= -iCRT

  3. Plants & Water Potential • Plants can use the potential energy in water to perform work • Ex: Tomato plant retains turgor pressure – cell pushes against wall due to uptake of water

  4. Water Potential • The combined effects of: • Solute concentration • Physical pressure (cell wall) • To calculate water potential:

  5. Pressure Potential p • p is the physical pressure on a solution • p can be negative  transpiration in the xylem tissue of the plant (water tension) • p can be positive  water in living plant cells is under positive pressure (turgid) • Pressure potential does not exist in animal cells, what structure provides this in plant cells? • Answer: cell wall

  6. Solute Potential s • s is the solute concentration • Also known as osmotic potential because solutes affect the direction of osmosis • Solutes bind to water reducing the number of free water molecules  lowers water’s ability to do work • s of pure water is 0 • s of any solution at atmospheric pressure is always negative – why? • Answer: less free water molecules to do work

  7. Calculating Solute Potential • Use the equation: s = -iCRT • i= ionization constant (# of particles/ions a molecule makes in water) • C = molar concentration • R = pressure constant (0.0831 liter bar/mole K) • T = temperature in degrees Kelvin (273 + C)

  8. Practice • Calculate the solute potential of a 0.1M NaCl solution at 25C. If the concentration of NaCl inside the plant cell is 0.15M, which way will the water diffuse if the cell is placed into the 0.1M NaCl solution. • What must the turgor pressure be if there is no net diffusion between the solution and the cell?

  9. Hints & Reminders • Remember that water always moves from high  low • Water moves from hypotonic  hypertonic (high   low ) • Solute is related to osmotic pressure • Pressure is related to pressure potential • When working problems, p = 0for animal cells and open beakers

  10. How will you determine the solute potential of y0ur potato cells?

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