IB Biology SLTopic 2 — Osmosis & Water PotentialPaper 1 & 2Practical skill~10 min read
Osmosis: The Potato Practical
This is the classic osmosis experiment, and it does something clever: by measuring how much mass potato cylinders gain or lose in different sugar solutions, you can work out the concentration inside cells you never looked at.
📘 What you need to know
Tissue in a hypotonic solution gains mass; in a hypertonic solution it loses mass; in an isotonic solution the mass is unchanged.
Always calculate percentage change in mass, because the cylinders do not all start at exactly the same mass.
Plot percentage change against solution concentration. Where the line of best fit crosses the x-axis is the concentration inside the tissue.
Preliminary research and trials come before the real experiment — they identify variables and sensible quantities.
Standard deviation measures the spread of repeat values around the mean.
Standard error is shown as error bars: overlapping bars suggest no significant difference, non-overlapping bars suggest a significant one.
Designing it properly first
Planning is not a formality. An experiment run without it usually produces data you cannot draw a valid conclusion from, because something you never thought about was changing as well.
Preliminary research (the word just means “coming before”) should settle:
What results you will collect. Quantitative data — numbers — allows more valid conclusions; qualitative data such as how firm the cylinders feel can support them.
How measurements will be made so they are as precise and accurate as possible, and which apparatus and techniques suit the science being investigated.
How many repeats you will do, so the data is reliable.
Which variables you are testing and which must be controlled.
Then run preliminary trials. These reveal additional variables you had not spotted, show you how to control them, and tell you what quantities you need so you do not run out halfway through.
The method
🧩 Potato cylinders in sucrose solutions
Use a cork borer to cut cylinders of potato of the same diameter, working on a white tile. Cut at least five for each solution you are testing.
Trim them all to the same length with a scalpel and ruler.
Blot them dry with paper towel, then measure and record the initial mass of each on a balance reading to 0.01 g.
Measure equal volumes of each sucrose solution into labelled test tubes. Use a range of at least five concentrations, and include distilled water as one of them.
Add one cylinder to each tube and leave for a set time (30 minutes is typical), at a controlled temperature.
Remove the cylinders, blot them dry again, and record the final mass and length of each.
Blotting matters more than it sounds. A film of solution clinging to the outside of a cylinder is liquid you did not intend to weigh, and it can easily be bigger than the change you are trying to measure.
Everything except the sucrose concentration is a control variable. If the tubes sat for different lengths of time, the results would mean nothing.
Analysing the results
Raw mass changes are not comparable, because no two cylinders start at exactly the same mass. Convert everything to a percentage change:
Percentage change in mass
% change = (final mass − initial mass) ÷ initial mass × 100
Here is a set of results worked through. A positive value means the cylinder gained water; a negative value means it lost water.
Sucrose / mol dm−3
Initial mass / g
Final mass / g
Change / g
% change
0.0 (distilled water)
5.00
5.40
+0.40
+8.0
0.2
5.00
5.15
+0.15
+3.0
0.4
5.00
4.95
−0.05
−1.0
0.6
5.00
4.75
−0.25
−5.0
0.8
5.00
4.60
−0.40
−8.0
1.0
5.00
4.45
−0.55
−11.0
The crossing point is the useful bit. At that concentration the solution is isotonic with the potato cells, so it tells you what is inside them.
Reading the graph
A positive percentage change means the potato gained water, so that solution had a lower osmotic concentration than the potato. The cells become turgid and the cylinders feel hard.
A negative percentage change means the solution had a higher osmotic concentration. Water left the cells, which become flaccid, and the cylinders feel floppy. Under a microscope, cells from the strongest solution may be plasmolysed.
The strongest sucrose solution gives the biggest loss, because it creates the steepest concentration gradient between the cells and the solution.
A cylinder with no change in mass was in a solution with the same osmotic concentration as its cells, so there was no gradient and no net movement.
The whole point of the graph: the point where the line of best fit crosses the x-axis is the concentration of sucrose that would be isotonic with the potato cells — an estimate of the osmotic concentration inside the tissue.
Standard deviation and standard error
Repeats let you calculate a mean for each concentration, but a mean on its own hides how consistent the readings were. Two sets of results can have the same mean and look completely different.
Standard deviation measures the spread of the data around the mean. A small standard deviation means the repeats were close together; a large one means they were scattered.
Standard error indicates how close your sample mean is likely to be to the true population mean. It is the standard deviation divided by the square root of the sample size, so a larger sample gives a smaller standard error.
On a graph, standard error is drawn as error bars extending above and below each plotted mean.
What the error bars do
What it suggests
Error bars overlap
The difference between the means is not significant
Error bars do not overlap
There is a significant difference between the means
Error bars are short
The repeats were consistent, so the mean is more trustworthy
You are not asked to memorise these formulae. What you are asked to do is use the values — look at a graph with error bars and say whether a difference is meaningful or could just be scatter.
Worked examples
WORKED EXAMPLE
Calculate a percentage change
A potato cylinder had an initial mass of 5.00 g. After 30 minutes in 0.2 mol dm−3 sucrose its mass was 5.15 g. Calculate the percentage change in mass and state what it shows.
Step 1: Find the change in mass5.15 − 5.00 = +0.15 gStep 2: Divide by the initial mass0.15 ÷ 5.00 = 0.03Step 3: Multiply by 1000.03 × 100 = 3.0+3.0% — the cylinder gained waterKeep the plus sign. It is doing real work in the answer, not decoration.
WORKED EXAMPLE
Use the graph to find what is inside the cells
Using the graph above, estimate the osmotic concentration of the potato tissue and explain how you got it.
Step 1: Find where the line crosses the x-axisBetween 0.2 (+3.0%) and 0.4 (−1.0%), at about 0.35 mol dm−3Step 2: Say what that point means
At that concentration there was no net movement of water, so the solution was isotonic with the cells.
Step 3: State the conclusion
The tissue has the same osmotic concentration as that solution.
About 0.35 mol dm−3You never measured inside a cell. The crossing point did it for you.
WORKED EXAMPLE
Interpret error bars
On a graph of mean percentage change, the error bars for 0.0 and 0.2 mol dm−3 do not overlap, but those for 0.8 and 1.0 mol dm−3 do. What can you conclude?
For 0.0 and 0.2
The bars do not overlap, so there is a significant difference between those two means.
For 0.8 and 1.0
The bars overlap, so the difference between those means is not significant — it could be down to variation between cylinders.
Significant at the low end, not at the high endUse the word “suggests”. Error bars indicate significance; they do not prove it.
💡 Exam tip
Always use percentage change, not change in grams, and say why: the cylinders started at different masses.
Show your working in three steps. Method marks survive an arithmetic slip.
Keep the sign on your answer. A negative percentage change is a different conclusion from a positive one.
Name the control variables when asked: temperature, time, volume of solution, size of cylinder, potato variety.
For a conclusion, quote the x-axis intercept and say what it represents.
Link the numbers back to cells: gained mass means turgid, lost mass means flaccid or plasmolysed.
⚠ Common mix-up
Dividing by the final mass instead of the initial mass. The initial mass is always the denominator.
Forgetting to blot the cylinders. Surface liquid is weighed along with the potato.
Saying the potato “absorbed sucrose”. Water moves; the sucrose stays in the solution.
Reading the y-intercept instead of the x-intercept. The useful point is where the line crosses zero on the horizontal axis.
Treating a large standard deviation as an error. It describes spread; it does not mean you did something wrong.
Claiming overlapping error bars prove no difference. They suggest the difference is not significant.
That completes Osmosis & Water Potential. One idea runs through all three pages: water moves down a water potential gradient, and everything a cell does about it is damage control.
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