RP 3 - Investigating Water Potential

• AQA Biology A-Level Required Practical 3 involves the production of a dilution series of a solute to produce a calibration curve with which to identify the water potential of plant tissue.
• Calibration curves are graphs used to determine an unknown concentration of a sample by comparing the unknown to a set of standard samples with known concentrations, also known as standard curves.
• A dilution series can be used to create a set of samples with known concentrations.
• A calibration curve can be used to determine an unknown water potential in a potato sample.
• Water potential is the tendency of water to diffuse from one area to another.
• Water molecules move from areas of high water potential to areas of low water potential by osmosis.
• The water potential is determined by the concentration of solutes.
• The movement of water in and out of cells is related to the relative concentration of solutes either side of the cell membrane.
• Equipment for AQA Biology A-Level Required Practical 3 includes a potato tuber, cork borer, scalpel, ruler, distilled water, sucrose solution (1M), boiling tubes, boiling tube rack, timer, digital balance, and paper towels.
• In the first step of AQA Biology A-Level Required Practical 3, a series of dilutions of 1M sucrose solution are made using distilled water.
• The dilutions should be at 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0M sucrose.
• The potato chips are cut using a cork borer and placed in test tubes, each containing a different concentration of sucrose solution, and left for 20 minutes.
• The change in mass of the potato chips is calculated after the experiment.
• In the second step of AQA Biology A-Level Required Practical 3, a calibration curve is plotted using the change in mass of the potato chips and the concentration of sucrose solution.
• The point at which the line of best fit crosses the x axis (zero change in mass) indicates the point at which the solution is isotonic, when the water potential of sucrose solution is the same as the water potential of the potato tissue, so there is no net movement of water in or out of the potato.
• Potato chips in lower concentrations of glucose solution will increase in mass, whilst those in the higher concentrations of glucose solution will decrease in mass.
• In dilute glucose solutions, the solution has a higher water potential than the potato, so water passively moves via osmosis to the area of lower water potential (the potato), causing the potato to increase in mass.
• In concentrated glucose solutions, water will move out of the potato, thus the potato will decrease in mass.