investigate the effects of immersing plant tissues in solutions of different water potentials, using the results to estimate the water potential of the tissues

Movement into and out of Cells

Objective

To investigate the effects of immersing plant tissues in solutions of different water potentials and, from the observed changes, estimate the water potential of the tissues.

Key Concepts

• Water potential (Ψ) – the potential energy of water per unit volume relative to pure water at the same temperature and pressure.
• Formula: \$\Psi = \Psis + \Psip\$

  • Ψ_s = solute (osmotic) potential (negative for solutions containing solutes).
  • Ψ_p = pressure potential (positive when turgor pressure is present).

• Osmosis – the net movement of water across a semi‑permeable membrane from a region of higher water potential to lower water potential.
• Turgor pressure – the pressure exerted by the cell contents against the cell wall when the cell is fully hydrated.
• Plasmolysis – the shrinkage of the protoplast away from the cell wall when the cell loses water to a hypertonic solution.

Experimental Design

The experiment uses sections of fresh potato (or onion epidermis) placed in a series of sucrose solutions of known concentrations. By observing the direction and magnitude of water movement, the water potential of the tissue can be inferred.

Materials

• Fresh potato tuber (or onion epidermal strips)
• Sucrose solutions: 0 %, 0.2 %, 0.4 %, 0.6 %, 0.8 %, 1.0 % (w/v)
• Distilled water
• Electronic balance (±0.01 g)
• Cylindrical cut‑off pieces (≈1 cm³) – 6 per solution
• Labelled test tubes or beakers
• Timer or stopwatch
• Ruler (for length measurements, if using epidermal strips)

Procedure

1. Prepare sucrose solutions of the required concentrations and label each container.
2. Cut six uniform pieces of potato (or strips of onion epidermis). Record the initial mass of each piece (to the nearest 0.01 g) and label them (A–F).
3. Place one piece in each solution, ensuring it is fully submerged.
4. Leave the samples for a fixed period (e.g., 30 min) at room temperature (≈20 °C).
5. After the incubation, gently blot each piece to remove surface liquid and record the final mass.
6. Calculate the percentage change in mass for each sample:

   \$\% \Delta m = \frac{m{\text{final}} - m{\text{initial}}}{m_{\text{initial}}}\times 100\$

7. Plot % Δm against sucrose concentration to locate the concentration at which there is no net change in mass (the isotonic point).
8. Convert the isotonic sucrose concentration to its solute potential using:

   \$\Psi_s = -iCRT\$

   where *i* = 1 (non‑dissociating), *C* = molarity, *R* = 0.0831 L·bar·K⁻¹·mol⁻¹, *T* = temperature in Kelvin.

9. Since at the isotonic point Ψ_p ≈ 0, the water potential of the tissue equals the solute potential of the external solution:

   \$\Psi{\text{tissue}} = \Psis (\text{isotonic solution})\$

Data Table

Analysis

Interpret the results as follows:

• If % Δm is positive, water moved into the tissue (tissue water potential < solution water potential).
• If % Δm is negative, water moved out of the tissue (tissue water potential > solution water potential).
• The concentration at which % Δm ≈ 0 is the isotonic point; this gives the water potential of the tissue.

Sample Calculation

Assume the isotonic concentration is found to be 0.5 % w/v sucrose.

Convert to molarity (M):

\$\text{Molar mass of sucrose} = 342\ \text{g mol}^{-1}\$

\$C = \frac{0.5\ \text{g 100 mL}^{-1}}{342\ \text{g mol}^{-1}} \times 10 = 0.0146\ \text{mol L}^{-1}\$

Calculate solute potential at 20 °C (293 K):

\$\Psi_s = - (1)(0.0146\ \text{mol L}^{-1})(0.0831\ \text{L·bar·K}^{-1}\text{mol}^{-1})(293\ \text{K})\$

\$\Psi_s \approx -0.36\ \text{MPa}\$

Thus, the estimated water potential of the potato tissue is \$-0.36\ \text{MPa}\$.

Safety and Precautions

• Handle sharp knives with care when cutting plant material.
• Label all solutions clearly to avoid mixing up concentrations.
• Dispose of sucrose solutions according to local laboratory waste guidelines.
• Wear lab coat and safety glasses throughout the experiment.

Extension Questions

1. How would the results differ if you used a solute that dissociates (e.g., NaCl) instead of sucrose?
2. Explain why the pressure potential of a fully turgid cell is not zero, and how this affects the calculation of Ψ.
3. Design an experiment to compare the water potential of two different plant species using the same method.
4. Discuss how temperature influences solute potential and the interpretation of your data.