Lab: Examining the Water Potential of Potato Cells
Introduction:
A potato cell is composed of water and numerous cellular components. When the potato cell is placed into water or a sucrose solution, water will move across the selectively permeable cell membrane. Passive movement of water across a selectively permeable membrane is called osmosis. How much water will move and the direction of water movement will depend on the relationship of the cellular concentration to the sucrose solution concentration.
Botanists use the term water potential when predicting the movement of water into or out of plant cells. Water will always move from an area of higher water potential (higher free energy, more water molecules) to an area of lower water potential (lower free energy, fewer water molecules). Water potential, then, measures the tendency of water to leave one place in favor of another place. You can picture the water diffusing down a water potential gradient. Water potential is influenced by the relative solute concentration inside and outside of the cell and physical pressure inside and outside of the cell.
Research Question: What is the water potential of a potato cell?
Determine your independent and dependent variables. Determine what other variables might affect your experiment and as a result should be controlled.
Procedure:
1. Place 100 mL of each of the following solutions into 6 separate beakers (dist. Water, 0.2M, 0.4M, 0.6M, 0.8M, 1.0M sucrose).
2. Slice a potato into discs that are approximately 3 cm thick.
3. Use a 5mm diameter cork borer to cut six potato cylinders for each beaker. Do not include the skin.
4. Keep your potato cylinders in a covered Petri dish.
5. Determine the mass of the six cylinders and record the mass in a data table. Place the cylinders into their designated beaker. Cover the beaker with foil.
6. Let the beakers stand overnight.
7. Remove the potato cores. Gently blot with a paper towel and mass.
8. Record the mass and calculate the percentage change in mass.
Calculating percentage change in mass:
Percentage change in mass = Final Mass – Initial Mass x 100
Initial Mass
Data Collection and Processing:
You need to design a data table to record the raw data for multiple trials of your experiment. Use the data table to determine the percent change in mass and the averages and standard deviation for the percent change in mass for all of your trials. Be sure to include units and uncertainties in your tables. Consult your IA guide for further information.
Include sample calculations of how you determined percentage change in mass and class averages.
Prepare a graph using the averages with the independent variable on the x-axis and the dependent variable on the y-axis. Graph must be prepared by computer. Consult your IA guide for further information. The x-intercept represents the molar concentration of sucrose with a water potential that is equal to the potato tissue water potential.
Conclusion and Evaluation:
Draw conclusions and evaluate your procedure. Consult your IA guide for further information.