Water Potential, Osmosis, and Plant Cells: A Comprehensive Guide, Study notes of Chemistry

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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

Calculating Water Potential

P

S Or Water = pressure + solute Potential potential potential

Solute Potential 

S  Solute potential is also called the osmotic potential because solutes affect the direction of osmosis.   S of any solution at atmospheric pressure is always negative – why?  Answer = less free water molecules to do work

Pressure Potential 

P

P is the physical pressure on a solution.

P can be negative  transpiration in the xylem tissue of a plant (water tension)

P can be positive  water in living plant cells is under positive pressure (turgid)

Standard for measuring 

 Pure water is the standard.  Pure water in an open container has a water potential of zero at one atmosphere of pressure.

Water Potential: an artificial model  (b) adding +0.23 pressure with plunger  no net flow of water  (c) applying +0.30 pressure increases water potential solution now has  of +0.  Water moves right to left

 (d) negative pressure or tension using plunger decreases water potential on the left.  Water moves from right to left

Water Potential: an artificial model

Water relations in plant cells

 (a) Flaccid cell placed in hypertonic solution Water potential is out of cell  plasmolysis

Calculating Solute potential

 Need solute concentration  Use the equation

S = - iCRT i = # particles molecule makes in water C = Molar concentration R = pressure constant 0.0831 liter bar mole o K T = temperature in degrees Kelvin = 273 + o C

Hints & reminders

1. Remember water always moves from [hi] to [lo].
2. Water moves from hypo  hypertonic.
3. [Solute] is related to osmotic pressure. Pressure is related to pressure potential.
4. Pressure raises water potential.
5. When working problems, use zero for pressure potential in animal cells & open beakers.
6. 1 bar of pressure = 1 atmosphere