RP 3

• 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?

  By plotting a calibration curve of percentage change in mass against concentration and finding the x-intercept where the plant tissue is isotonic to the sucrose solution.
• What occurs when plant tissue is placed in a hypotonic solution?

  Water moves into the plant tissue by osmosis, causing the plant tissue to increase in mass.
• What occurs when plant tissue is placed in a hypertonic solution?

  Water moves out of the plant tissue by osmosis, causing the plant tissue to decrease in mass.
• Why are the potato discs left in solution for 20 minutes?

  To allow time for osmosis until the plant tissue reaches equilibrium with its surrounding solution.
• What determines water potential?

  The concentration of solutes; higher solute concentration results in lower water potential.
• Outline the procedure of investigating osmosis using potato tissue.

  1. Make a simple dilution of 1M sucrose to produce 5 concentrations and add 5 cm³ to 5 different test tubes.
  2. Cut a potato into equal sized chips and weigh them.
  3. Place a chip in each test tube and leave for 20 minutes.
  4. Take out, dab the excess water, and weigh them again.
  5. Calculate the percentage change in mass.
• Why is the percentage change used rather than the actual change in mass?

  Because potato chips may not all have the same starting mass, and percentage change allows for comparison.
• What is indicated by the x-intercept of the calibration curve?

  The concentration that is isotonic to the solution tested.
• Explain the change in mass in the potato chips.

  Potato chips with a concentration lower than the sucrose solution lose mass due to a net movement of water out of the cells, while those with a higher concentration gain mass due to a net movement of water into the cells.
• Why are the potato chips dabbed dry after removing them from the sucrose solution?

  To remove any excess water clinging to their surface.
• What are the controlled variables of the osmosis practical with potato tissue?

  • Volume of sucrose solution
  • Size of potato chips
  • Length of time left in solution
  • Dab each potato disc with paper towels