Determine the Concentration of Cell Sap of a Plant Tissue
The concentration of a potato's cell sap can be found by immersing potato cylinders in a range of sucrose solutions, measuring the percentage change in mass of each, and reading off the concentration at which mass does not change.
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Aim
This experiment determines the concentration (molarity of sucrose solution) of the cell sap of a plant tissue, using potato as the tissue and the change-in-mass method.
Variables
- Manipulated variable, the concentration of sucrose solution, for example 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol/dm³.
- Responding variable, the percentage change in mass of the potato cylinders after immersion.
- Controlled variables, the size (diameter and length) and initial mass of the potato cylinders, the immersion time, and the temperature of the solutions.
Materials and Apparatus
- a potato
- a cork borer
- a scalpel
- a ruler
- sucrose solutions of a range of known concentrations
- test tubes or beakers
- an electronic balance
- blotting paper
- forceps
- a stopwatch
- graph paper
Procedure
- Use a cork borer to cut several potato cylinders of equal diameter, then trim each one with a scalpel and ruler to the same length.
- Blot each cylinder dry with blotting paper and weigh it on the electronic balance to record its initial mass.
- Place one potato cylinder into each sucrose concentration, held in a separate test tube or beaker, for a fixed time such as 30 to 40 minutes.
- Remove each cylinder with forceps, blot it dry to remove surface solution, and weigh it again to record its final mass.
- Calculate the percentage change in mass for each concentration using the formula: percentage change in mass = ((final mass - initial mass) ÷ initial mass) × 100.
- Plot a graph of percentage change in mass (y-axis) against concentration of sucrose solution (x-axis).
- Read off the concentration at which the graph crosses zero percentage change (the x-intercept).
Expected Results
The mass of the potato cylinders is expected to increase in dilute sucrose solutions and decrease in concentrated sucrose solutions, with the percentage change becoming more negative as concentration increases. On the graph of percentage change in mass against concentration, the line crosses zero change at a particular concentration, in this example, at about 0.4 mol/dm³, based on the table below.
| Concentration (mol/dm³) | Initial mass (g) | Final mass (g) | % change in mass |
|---|---|---|---|
| 0.0 | 5.00 | 5.60 | +12.0 |
| 0.2 | 5.00 | 5.30 | +6.0 |
| 0.4 | 5.00 | 5.00 | 0.0 |
| 0.6 | 5.00 | 4.75 | -5.0 |
| 0.8 | 5.00 | 4.50 | -10.0 |
| 1.0 | 5.00 | 4.25 | -15.0 |
Conclusion
The concentration of sucrose solution at which the potato cylinder shows no net change in mass, found from the x-intercept of the graph, is equal to the concentration of the potato's cell sap. At this concentration there is no net osmotic movement of water into or out of the potato tissue, because the external solution and the cell sap have the same water potential.
Safety
- Handle the scalpel and cork borer with care, cutting on a tile or board and always away from your fingers, to avoid cuts.
- Wipe up any spilt solution at once so the bench and floor do not become slippery.
- Wash your hands after handling the potato tissue and solutions, and do not taste any material in the laboratory.
Common Mistakes
Paper 3-style questions
Question 1 (hypothesis). A student wants to find the concentration of cell sap in a sample of carrot tissue using the change-in-mass method. State a suitable hypothesis for this investigation.
Model answer. The higher the concentration of the external sucrose solution, the greater the decrease in mass of the carrot cylinders, because more water leaves the cells by osmosis when the surrounding solution has a lower water potential than the cell sap.
Question 2 (variables). For the same investigation, state the manipulated, responding and one controlled variable, and give an operational definition of the responding variable.
Model answer. The manipulated variable is the concentration of sucrose solution; the responding variable is the percentage change in mass of the carrot cylinders; a controlled variable is the length (or initial mass) of each cylinder, or the immersion time or temperature. Operationally, the responding variable is defined as the difference between final and initial mass, divided by initial mass, multiplied by one hundred.
Question 3 (tabulation and graph). The student soaked carrot cylinders in sucrose solutions of 0.0, 0.2, 0.4, 0.6 and 0.8 mol/dm³ and recorded a percentage change in mass of +8, +4, 0, -5 and -11 respectively. Describe how the student should present these results and how the concentration of the cell sap is obtained.
Model answer. The results should be tabulated with column headings and units, then plotted as a graph of percentage change in mass on the y-axis against sucrose concentration on the x-axis, with a line of best fit. The concentration of the cell sap is read from the point where the line crosses zero change (the x-intercept), here about 0.4 mol/dm³, because at that concentration there is no net osmosis and the external solution is isotonic with the cell sap.
Source:SRC-DSKP-EN
Frequently asked questions
Why is percentage change in mass used instead of the actual change in mass?
What does it mean if a potato cylinder shows zero percentage change in mass?
Why must the potato cylinders be blotted dry before each weighing?
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