Water Potential Osmosis & Plant cells

Plants & water potential Plants can use the potential energy in water to perform work. Tomato plant regains turgor pressure – cell pushes against wall due to uptake of water

Plants & water potential The combined effects of 1.) solute concentration 2.) physical pressure (cell wall) can be measured as Water Potential   is measured in kilopascals (kPa) or bars 1 Mpa = 10 atmospheres of pressure or 10 bars

Water Potential Ψ = Ψp + Ψs Water potential is determined by solute potential and pressure potential. Ψ = Ψp + Ψs Water moves from regions of high water potential to regions of low water potential.

Water Potential: an artificial model Water flows from “hypo” to “hyper” Or from high  on left to low  on right

Pressure Potential – the sum of all pressure on water. Turgor pressure – force caused by cell membrane pushing against cell wall. Wall pressure – an equal and opposite force exerted by cell wall. Other pressures – tension, cohesion, atmospheric, root, etc. When working problems, use zero for pressure potential in animal cells & open beakers.

Ψs Solute (osmotic) potential Pure water has a solute potential (Ψs) of zero. Solute potential can never be positive. Adding more solute is a negative experience; the solute potential becomes negative.

Ψs Solute (osmotic) potential Once you know the solute concentration, you can calculate solute potential using the following formula: Solute potential (ΨS ) = –iCRT

Solute potentialΨs = − iCRT i = The ionization constant for NaCl this would be 2; for sucrose or glucose, this number is 1 C = Molar concentration R = Pressure constant = 0.0831 liter bar/mole K T = Temperature in degrees Kelvin = 273 + °C of solution

The pressure potential of a solution open to the air is zero. Water potential The pressure potential of a solution open to the air is zero. Figure: 32.1c Caption: (c) If wall pressure is very high, it can counteract the tendency of water to move via osmosis as in the example given here.

Practice Problem What is the water potential of a cell with a solute potential of -0.67 kPa and a pressure potential of 0.43 kPa?

Practice Problem The molar concentration of a sugar solution in an open beaker has been determined to be 0.5M. Calculate the solute potential at 24°C. Round your answer to the nearest hundredth.