Investigating the Effect of Osmosis on Plant Tissue

Objectives

• Describe how to investigate the effect of osmosis on plant tissue.
• Calculate percentage change.

Key Concepts

• Osmosis: Diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
  • In water, plant cells expand as water moves in by osmosis.
  • In a concentrated solution, plant cells shrink as water moves out by osmosis.

Investigating Osmosis

Materials:

• Potato (or other vegetables like beetroot or parsnip)
• Cork borer
• Scalpel
• Ruler
• Balance
• Test tubes
• Sugar solutions (0.5 molar and higher concentrations)
• Distilled water
• Paper towels

Steps:

1. Prepare Potato Cylinders

   • Peel the potato to remove the skin, which can affect osmosis.
   • Use a cork borer to produce three cylinders of the same diameter.
   • Trim cylinders to a uniform length (around 3 cm is recommended).

2. Initial Measurements

   • Measure the length and mass of each cylinder.

3. Setup Osmosis Experiment

   • Place each cylinder into separate test tubes.
   • Add 10 cm³ of 0.5 molar sugar solution to the first test tube.
   • Add 10 cm³ of a higher concentration sugar solution to the second test tube.
   • Add 10 cm³ of distilled water to the third test tube.

4. Incubation

   • Leave the potato cylinders overnight to allow osmosis to occur.

5. Post-Experiment Measurements

   • Remove cylinders and gently roll them on paper towels to remove surface moisture.
   • Measure the final length and mass of the cylinders.

Calculating Percentage Change

• Formula:

  [ \text{Percentage Change} = \frac{\text{Change in Value}}{\text{Original Value}} \times 100 ]

• Example Calculation:

  • Starting mass: 1.56 grams; Increase: 0.25 grams
  • Change = 0.25 grams
  • Percentage Change = ( \frac{0.25}{1.56} \times 100 = +16.03% )

• Decrease Example:

  • Starting mass: 1.32 grams; Decrease: 0.19 grams
  • Change = -0.19 grams
  • Percentage Change = ( \frac{-0.19}{1.32} \times 100 = -14.39% )

Analyzing Results

• Plot a graph of percentage changes in mass or length against the sugar solution concentration.
• Graph Interpretation:
  • In water: Potato cylinder gains mass (water enters by osmosis).
  • In concentrated solution: Potato cylinder loses mass (water exits by osmosis).
  • Where the line crosses the x-axis: No change in mass; indicates equal concentration inside and outside the cell.

Conclusion

• Understanding the process of osmosis and ability to calculate percentage change are critical in analyzing the effects of osmosis on plant tissues.