Required Practical 3: Water Potential
How to make a dilution series, use potato cylinders, calculate percentage change in mass and read the water potential from a calibration curve. Includes the 4-mark exam question, a worked model answer and the examiner trap on using mass change in grams.
Chapters (9)
Exam question [4 marks]
A student wants to find the water potential of potato tissue. Describe how a dilution series and a calibration curve could be used to do this.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Make a dilution series (range of concentrations) of a solute such as sucrose from a stock solution, using distilled water | "serial dilution", "different concentrations of sucrose", "diluting a stock with water" | "different solutions" with no range of concentrations; "salt water" with no range |
| 2 | Put same-sized potato cylinders in each solution for the same time; measure mass before and after, blotting dry first | "dry with paper towel", "same length / surface area", "weigh before and after" | Mass measured only once; no control of size or time (blotting omitted but size, time and before/after mass given still scores) |
| 3 | Calculate the percentage change in mass for each concentration | "change in mass / start mass x 100" | "change in mass" in grams only |
| 4 | Plot percentage change in mass against concentration; the concentration where the line crosses zero (x-intercept) has the same water potential as the tissue | "no net osmosis at the intercept", "read the x axis where change is zero", "convert to water potential using a table" | Reading the y-intercept; "where the potato is biggest" |
Each mark is a separate stage of the method (range, measure, calculate, interpret). Using absolute mass change instead of percentage change loses mark 3 and weakens mark 4. Plotting concentration or water potential of the solution on the x axis are both acceptable. A named concentration range is not required. Mark 2 needs same-sized cylinders, same time and mass before and after; blotting is credited wording but its omission alone does not lose the mark. Mark 3: accept mass change if start masses were identical (spec wording is gain/loss of mass).
Model answer (4/4)
Make a dilution series of sucrose from a stock solution, using distilled water, to give a range of concentrations. Cut potato cylinders of the same size, measure the mass of each, and put one in each solution for the same length of time. Remove them, blot dry with a paper towel, and measure the mass again. Calculate the percentage change in mass for each cylinder. Plot percentage change in mass against sucrose concentration and draw a line of best fit. Where the line crosses the x axis there is no net movement of water, so the solution has the same water potential as the potato tissue.
Examiner trap
- Using absolute mass change instead of percentage change
- Reading water potential from the wrong axis point
- Not blotting the tissue dry
Key terms
- dilution series
- calibration curve
- water potential
- mass change
- intercept
- potato cylinders
- percentage change in mass
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:31Making the dilution series
The aim is to find the water potential of plant tissue, here potato. You cannot measure that directly, so you find a solution that has the same water potential as the tissue. First you need a range of solutions. Start with a concentrated stock solution of sucrose. Then make a dilution series, by mixing measured volumes of the stock with distilled water.
You get several different concentrations, from pure water up to the strong stock. Use a measuring cylinder or pipette so the volumes are accurate.
1:04Cutting the potato cylinders
Next, the potato. Use a cork borer to cut cylinders, and cut them all to the same length. Same size means the same surface area, so surface area is a controlled variable in every tube. Dry each cylinder with a paper towel, then measure its mass. Write that down. It is the start mass.
1:25Setting up and timing
Put one cylinder into each concentration, and leave them all for the same length of time. Then take them out. Blot each one dry with a paper towel, to remove the solution stuck on the surface. Blotting removes excess solution from the surface. If you skip that, you weigh the liquid as well, and the result is wrong. Now measure the mass again. That is the end mass.
1:49What happens in each tube
Why does the mass change? Water moves by osmosis, down the water potential gradient. In a dilute solution, the solution has a higher water potential than the potato. Water moves in, and the cylinder gains mass. In a concentrated solution, the potato has the higher water potential. Water moves out, and the cylinder loses mass. In between, there is a concentration where the water potentials are equal, so there is no net movement, and no change in mass.
2:21Calculating percentage change
Now the step examiners love. Do not use the plain change in mass. Your cylinders will not start at exactly the same mass. So work out the percentage change. Take the change in mass, divide by the start mass, and multiply by one hundred. A gain is positive, and a loss is negative.
Percentage change lets you compare cylinders fairly, whatever they weighed at the start.
2:47Drawing the calibration curve
Now plot the graph. Concentration goes on the x axis. Percentage change in mass goes on the y axis. Plot the points and draw a line of best fit. This is your calibration curve. The line crosses the x axis where the percentage change is zero. That is the x intercept. At that point there is no net movement of water. So the solution has the same water potential as the potato tissue.
Read down to the concentration on the x axis. If the axis is concentration, you can use a table of concentration against water potential to convert it.
4:03Examiner trap and recap
The trap. Using the plain change in mass instead of percentage change loses the mark. And remember to read the answer where the line crosses the x axis, not the y axis.
Recap. Make a dilution series, weigh the potato before and after, use percentage change, and the x intercept gives the water potential.
Next time, required practical four. How do we show that a membrane has been damaged, using beetroot?
Watch first
This lesson builds on:
- Osmosis and Water Potential Explained · AQA 7402 · 3.2.3
Keep going
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- Simple vs Facilitated Diffusion · AQA 7402 · 3.2.3
- Osmosis and Water Potential Explained · AQA 7402 · 3.2.3
- Active Transport and Co-transport (Ileum) · AQA 7402 · 3.2.3
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-039.