Investigating Water Potential (AQA A-Level Biology): Revision Notes
Investigating Water Potential
Purpose and Principle
This practical investigates water potential in plant tissue by creating a calibration curve using solutions of known concentration. Water potential describes the ability of water molecules to move from one location to another, with movement occurring from regions of high water potential towards areas of low water potential through osmosis.
Understanding Calibration Curves
Calibration curves serve as analytical tools that allow determination of unknown concentrations by comparing sample responses against a series of standards with known values. These are sometimes referred to as standard curves. A dilution series provides the range of known concentrations needed to construct this calibration relationship.
The underlying principle involves placing potato tissue in solutions of varying solute concentration and measuring mass changes. Water movement across cell membranes depends on the relative concentration of solutes on either side of the membrane.
Apparatus and Materials
The following equipment is required for this investigation:
- Potato tuber
- Cork borer
- Scalpel
- Ruler
- Distilled water
- Sucrose solution (1M stock)
- Boiling tubes
- Boiling tube rack
- Timer
- Digital balance
- Paper towels
Ensure all glassware is clean and the digital balance is calibrated before beginning the experiment. The potato should be fresh and firm for consistent results.
Method
- Prepare dilution series: Create solutions containing 0.0M, 0.2M, 0.4M, 0.6M, 0.8M and 1.0M sucrose by diluting the 1M stock solution with distilled water.
- Distribute solutions: Measure 5cm³ of each dilution into separate test tubes.
- Prepare potato samples: Use a cork borer to extract six potato cylinders. Cut these into identically sized chips using the scalpel. Gently dry each piece with paper towel to remove surface moisture without applying pressure.
- Record initial masses: Weigh each potato chip individually before beginning the experiment.
- Incubate samples: Place one potato chip into each test tube containing different sucrose concentrations. Leave undisturbed for 20 minutes.
- Final measurements: Remove each potato chip and gently dry using paper towel before weighing again.
- Calculate changes: Determine the percentage change in mass for each sucrose concentration.
Critical Technique Points
- Ensure potato chips are identical in size to maintain validity
- Pat dry gently without squeezing to avoid damaging cell structure
- Time the 20-minute incubation precisely for all samples
- Handle chips carefully during transfer to prevent damage
Data Collection and Processing
Record initial and final masses in a suitable table format, ensuring measurements are taken to appropriate precision using the digital balance. Calculate percentage mass change using:
Record masses to at least 2 decimal places for precision. Ensure consistent units throughout all measurements and calculations.
Analysis and Interpretation
Plot a graph showing change in mass (y-axis) against concentration of sucrose solution (x-axis). The line of best fit should pass through all data points.
The critical point occurs where this line crosses the x-axis at zero change in mass. This intersection indicates the isotonic point - the concentration at which the sucrose solution has identical water potential to the potato tissue. At this point, there is no net movement of water into or out of the potato cells.
Worked Example: Identifying the Isotonic Point
Step 1: Plot all data points on a graph with concentration (M) on x-axis and percentage change in mass on y-axis
Step 2: Draw the line of best fit through the points
Step 3: Find where the line crosses the x-axis (y = 0)
Step 4: Read the concentration value at this point - this is the isotonic concentration of the potato tissue
Graph Interpretation Key Points
- Points above the x-axis indicate mass gain (water moved into potato)
- Points below the x-axis indicate mass loss (water moved out of potato)
- The isotonic point is where the line crosses x-axis at zero change
Expected Results and Biological Explanation
Potato chips placed in lower concentrations of sucrose will show increased mass. This occurs because these dilute solutions have higher water potential than the potato tissue, causing water to move passively via osmosis into the potato cells.
Understanding Water Movement Direction
Water always moves from areas of high water potential to areas of low water potential. Think of it as water "flowing downhill" along the water potential gradient.
Conversely, chips in higher concentrations of sucrose will decrease in mass. The concentrated solutions have lower water potential than the potato, so water moves out of the potato tissue into the surrounding solution.
The isotonic concentration represents the point where water potential inside potato cells equals that of the external solution, resulting in no net water movement and therefore no mass change.
Risk Assessment
Hazard: Scalpel
- Risk: Cuts from the sharp blade
- Precautions: Cut away from fingers using controlled movements and keep the blade away from desk edges
- Emergency action: If cuts occur, elevate the wound, apply pressure, and seek medical assistance
- Risk level: Low
Hazard: Broken Glass
- Risk: Cuts from damaged glassware
- Precautions: Handle all glass equipment carefully and store away from desk edges
- Emergency action: If breakage occurs, do not attempt to remove glass fragments from wounds - elevate cuts, apply pressure, and seek medical assistance
- Risk level: Low
Links to Theory
This practical directly demonstrates osmosis - the passive movement of water molecules across partially permeable membranes down a water potential gradient. The results illustrate how solute concentration affects water potential and consequently the direction and extent of water movement in biological systems.
The calibration curve method shows how unknown water potentials can be determined experimentally, linking to broader concepts of water relations in plants and cell membrane transport.
Key Points to Remember:
- Water always moves from high to low water potential via osmosis
- The isotonic point occurs where mass change equals zero
- Dilute solutions cause potato cells to gain water and increase mass
- Concentrated solutions cause potato cells to lose water and decrease mass
- Calibration curves allow determination of unknown concentrations by comparison with known standards
- Precise technique and timing are essential for valid results