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 (Cambridge 4.2)

Learning Objectives

• Investigate how plant tissues respond when immersed in solutions of differing water potential.
• Use the observed mass changes to estimate the water potential (Ψ) of the tissue.
• Relate water‑potential concepts to the structure of the plasma membrane and to other syllabus topics (4.1, 7, 8).

Key Concepts

• Water potential (Ψ) – the potential energy of water per unit volume relative to pure water at the same temperature and pressure.
  

  Ψ = Ψs + Ψp

  • Ψ_s (solute or osmotic potential) – always ≤ 0. Calculated with the van’t Hoff equation.
  • Ψ_p (pressure potential) – ≥ 0 for turgid cells; may be zero for plasmolysed cells or for the isotonic point in the potato assay.

• Osmosis – 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 (Ψ_p > 0).
• Plasmolysis – shrinkage of the protoplast away from the cell wall when water leaves a cell placed in a hypertonic solution.

Plasma‑Membrane Context (link to Topic 4.1)

The plasma membrane is a fluid‑mosaic structure composed of a phospholipid bilayer with embedded proteins. This architecture underpins the six transport processes listed below:

Six Transport Processes (Cambridge 4.2)

Experimental Design – Estimating the Water Potential of Plant Tissue

The classic “potato (or onion epidermis) in sucrose solutions” experiment provides a quantitative estimate of Ψ for living tissue.

Materials – Plant‑Tissue Part

• 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 pieces (≈1 cm³) – 6 per solution
• Labelled test tubes or beakers
• Timer or stopwatch
• Paper towels (for blotting)

Procedure – Plant‑Tissue Part

1. Prepare the sucrose solutions and label each container clearly.
2. Cut six uniform potato cylinders (≈1 cm³). Record the initial mass of each piece to 0.01 g and label them A–F.
3. Place one piece in each solution, ensuring it is completely submerged.
4. Leave the samples for 30 min at room temperature (≈20 °C).
5. After incubation, gently blot each piece with a paper towel to remove surface liquid and record the final mass.
6. Calculate the percentage change in mass:

   
   %Δm = [(mfinal – minitial) / minitial] × 100

7. Plot %Δm against sucrose concentration. The concentration where %Δm ≈ 0 is the isotonic point.
8. Convert the isotonic sucrose concentration to solute potential using the van’t Hoff equation:

   
   Ψs = – i C R T

   • i = 1 (sucrose does not dissociate)
   • C = molarity of the isotonic solution (mol L⁻¹)
   • R = 0.0831 L·MPa K⁻¹ mol⁻¹
   • T = temperature in Kelvin (K)

9. At the isotonic point the tissue and external solution have the same water potential, so Ψ_p of the tissue is effectively zero. Therefore:

   
   Ψtissue = Ψs(isotonic)

Data Table – Plant Tissue

Analysis – Plant Tissue

• Positive %Δm → water entered the tissue (tissue Ψ < solution Ψ).
• Negative %Δm → water left the tissue (tissue Ψ > solution Ψ).
• The concentration where %Δm ≈ 0 gives the isotonic point and therefore the water potential of the tissue.
• Remember that Ψ_p is assumed ≈ 0 only at the isotonic point; in living cells under normal conditions Ψ_p > 0 and contributes to the total Ψ.

Sample Calculation

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

1. Convert to molarity:
   

   0.5 g sucrose per 100 mL → 0.5 g / 342 g mol⁻¹ = 0.00146 mol per 0.1 L → C = 0.0146 M

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

   Ψs = – (1)(0.0146)(0.0831)(293) ≈ –0.36 MPa

3. Thus, Ψtissue ≈ –0.36 MPa (since Ψ_p ≈ 0 at the isotonic point).

Practical Extension – Non‑Living Materials (Diffusion & Osmosis)

To demonstrate that diffusion and osmosis are purely physical processes, repeat a simplified version using agar discs (or dialysis tubing) instead of living tissue.

Materials – Extension

• Agar powder (1 % w/v solution)
• Petri dishes
• Dialysis tubing (MWCO ≈ 12 kDa) – optional
• Sucrose solutions of the same concentrations as above
• Ruler (mm) or digital caliper
• Digital camera for time‑lapse (optional)

Procedure – Extension

1. Prepare a 1 % agar solution, pour into a shallow tray and allow it to set.
2. Using a cork borer, cut three discs (10 mm diameter, 2 mm thick).
3. Place each disc in a separate beaker containing 0 %, 0.4 % and 0.8 % sucrose solutions.
4. Leave for 30 min at room temperature.
5. Remove the discs, blot dry, and measure the change in thickness (or diameter). Record the data.
6. Plot change in size against sucrose concentration. The point of no size change corresponds to the isotonic solution for the agar.

Link to Plant‑Tissue Experiment

• Agar is a non‑living, semi‑permeable matrix; the same water‑potential principles apply.
• Comparing the isotonic points of agar and potato highlights the additional solutes (e.g., sugars, ions) already present inside living cells.

Surface‑Area‑to‑Volume Ratio Activity

SA:V explains why small cells reach equilibrium faster than large cells – a key point for AO2.

Materials

• Three cubes of agar (or modelling clay) with edge lengths 1 cm, 2 cm, and 3 cm.
• Ruler or caliper.

Calculations

1. For each cube calculate surface area (SA) = 6 × edge² and volume (V) = edge³.
2. Determine the ratio SA : V (expressed as cm⁻¹).
3. Tabulate the results and comment on the trend.

Example Table

As size increases, SA:V decreases, meaning larger cells have proportionally less membrane surface for water exchange and therefore equilibrate more slowly.

Safety and Precautions

• Use a sharp knife or scalpel carefully; cut away from the body.
• Label all solutions clearly to avoid mixing concentrations.
• Dispose of sucrose solutions according to local waste‑disposal regulations.
• Wear a lab coat, safety glasses and gloves where appropriate.
• If dialysis tubing is used, handle with clean gloves to prevent contamination.

Cross‑Topic Links (Syllabus Integration)

• Topic 4.1 – Fluid‑Mosaic Model: The transport processes described rely on membrane proteins (channels, carriers, pumps) embedded in the fluid bilayer.
• Topic 7 – Transport in Plants: Water‑potential gradients drive xylem ascent; the same Ψ concepts are used in the root‑uptake experiment.
• Topic 8 – Transport in Animals: Osmosis and solute potential also determine blood plasma osmolality and kidney function.

Extension / Critical Thinking Questions

1. How would the isotonic concentration change if NaCl (which dissociates into two ions) were used instead of sucrose? Explain using the van’t Hoff factor (i = 2 for NaCl).
2. Why is the pressure potential of a fully turgid cell not zero? How does this affect the calculation of the cell’s total water potential?
3. Design an experiment to compare the water potential of two different plant species (e.g., spinach vs. carrot) using the potato‑type method. Include how you would control for tissue size and temperature.
4. Discuss how temperature influences solute potential and how you would correct your calculations if the experiment were performed at 30 °C.
5. Using the SA:V activity, predict which of the three agar cubes would reach isotonic equilibrium fastest and justify your answer.

Suggested Diagram (AO1 – Interpretation)

Include a labelled cross‑section of a plant cell showing:

• Cell wall, plasma membrane, vacuole, and cytoplasm.
• Arrows indicating water movement into the cell in a hypotonic solution (turgor) and out of the cell in a hypertonic solution (plasmolysis).
• Labels for Ψ_s, Ψ_p, and total Ψ.

Students may sketch this diagram themselves or use a textbook illustration for reference.