Water Potential Tutorial

Water potential is the frequency of water moving from one place to another. In order for water to displace another object, water must be moving, meaning force must be applied. It is caused by osmosis, gravity, and surface tension. Moreover, water potential is represented by Greek letter Ψ (Psi). Since water potential is the movement of water, it is found within plants, animals, and soil.

In an open container, the water potential is the same as the solute potential because the pressure potential in an open space is zero. Therefore, the water potential formula is:

Ψ = Ψp + Ψs

Water potential = pressure potential + solute potential

Water will ALWAYS move from a high potential to low potential. Hypotonic solutions have greater water potential than hypertonic solutions. Water moves from hypo to hypertonic. Pure water is the standard for measuring water potential. 1 bar pressure = 1 atmosphere. Pressure raises water potential. Solute potential of any solution at atmospheric pressure is always negative (-) because it means that less free water molecules to do the work.

Example #1

If a plant cell’s pressure potential is 2 bars and its solute potential is -3.5 bars, what is the resulting water potential?

Ψ = Ψp + Ψs

2 bars + (-3.5 bars) = -1.5 bars

Example #2

If a plant cell’s water potential is 10 bars and its solute potential is -5 bars, what is the plant’s pressure potential?

Pressure potential (Ψp) = water potential (Ψp) – solute potential (Ψs)

15 bars = 10 bars - (-5 bars)

Practice Problems!

  1. Scientists are trying to determine under what conditions a plant can survive. They collect the following data and would like to know the water potential of the plant cell. The solute potential is -0.6 Mpa (megapascals) and the pressure potential is -1.0 MPa. What is the water potential? Round to nearest hundredths.
  2. You measure the total water potential of a cell and find it to be -0.24 kPa (kilopascals). If the pressure potential of the same cell is 0.46 kPa, what is the solute potential of that cell? Round to the nearest hundredth.
  3. A plant cell with a solute potential of -7.5 bars keeps a constant volume when immersed in an open-beaker solution that has a pressure potential of -4 bars. What is the cell's water potential? Round to the nearest tenth.
  1. A cell having a solute potential of -0.35 kPa is placed in a solution of pure water, what will be its pressure potential at equilibrium? Round to the nearest hundredth.
  1. What is the water potential of a cell with a solute potential of -0.67 kPa and a pressure potential of 0.43 kPa? Round to the nearest hundredth.
  1. A plant cell with a Ψs of -9 bars keeps a constant volume when immersed in an open-beaker solution that has a Ψp of -4 bars. What is the cell’s Ψ? Round to the nearest hundredth.

7. Mrs. Willis calculated a cell’s total water potential to be -0.2kPa. She also discovered that the cell’s pressure potential is 0.45kPa. If the information recorded above is true, what is the solute potential of the cell?

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