prac 3 water potential

• what is the purpose of calibration curves?

  they are used to determine the concentration of an unknown sample by comparing it to a set of standard values with known concentrations
• how is a calibration curve used to find the concentration of plant tissue?

  plot a curve of percentage change in mass against concentration.

  find the x intercept where the plant tissue is isotonic to the sucrose solution
• what happens when a plant tissue is placed in a hypotonic solution?

  water moves into the plant tissue by osmosis
• what happens when plant tissue is placed in a hypertonic solution?

  water moves out of the plant tissue by osmosis
• why are the potatoes left in solution for 20 mins

  to allow time for osmosis so the plant tissue reaches equilibrium
• what is water potential determined by?

  concentration of solutes

  higher the solute concentration the lower the water potential
• what is the method of investigations

  make simple salt dilutions to make 5 concentrations

  at 5 cm3 to 5 test tubes

  cut a core of potatoes and dab dry

  place each core in a test tube and leave for 20 minutes

  take out and remove excess water

  calculate percentage change in mass
• why is percentage change used ?

  cores may not have the same initial mass

  allows comparison
• what does the x intercept on the curve mean?

  the concentration that is isotonic to the tested solution
• explain the change in mass of the cores

  the cores with a lower concentrations than the solution lose mass because water leaves cells

  cores with a higher concentration than solution gain mass because water moves into cells
• what are the controlled variables?

  volume of sucrose solution

  size of cores

  time left in solution