A potometer measures how far an air bubble moves along a narrow tube as a cut shoot takes up water. You divide distance by time to get a rate, compare conditions, and then say what the apparatus cannot show. Questions on this appear with a table, a graph or a labelled diagram of the apparatus.
It applies the ideas from environmental conditions and transpiration and belongs to the plant transport module.
How do you read the dataset?
A potometer is set up so that water uptake by the shoot pulls the bubble along the tube. The faster the shoot takes up water, the faster the bubble moves. Follow the same four steps every time.
- Identify what was changed and what was kept the same. The changed factor is the independent variable. The same shoot, time and temperature are controlled.
- Calculate a mean from the repeats.
- Calculate a rate: mean distance divided by time.
- Compare the rates and explain the difference using transpiration.
Worked example
The data below are invented for teaching. A shoot was set up in a potometer with a capillary tube of diameter 1.0 mm. The bubble distance was recorded over 5 minutes, three times for each condition.
| Condition | Trial 1 (mm) | Trial 2 (mm) | Trial 3 (mm) |
|---|---|---|---|
| Still air | 24 | 26 | 25 |
| Fan on | 58 | 62 | 60 |
Step 1, means: still air is (24 + 26 + 25) ÷ 3 = 75 ÷ 3 = 25 mm. Fan is (58 + 62 + 60) ÷ 3 = 180 ÷ 3 = 60 mm.
Step 2, rates: still air is 25 ÷ 5 = 5 mm per minute. Fan is 60 ÷ 5 = 12 mm per minute.
Step 3, compare: the fan rate is 12 ÷ 5 = 2.4 times the still air rate.
Step 4, explain: moving air removes water vapour from beside the leaf, so the gradient is steeper and the shoot transpires, and therefore takes up, water faster.
Step 5, volume for the fan: the radius is 0.5 mm, so the area is π × 0.5² = 0.785 mm² (3 s.f.). Volume in 5 minutes is 0.785 × 60 = 47.1 mm³ (3 s.f.), and the volume rate is 47.1 ÷ 5 = 9.42 mm³ per minute.
Check: 9.42 ÷ 0.785 = 12 mm per minute, which matches step 2.
What are the limitations?
The potometer measures uptake, and uptake is only an estimate of transpiration. Learn these as reasons, not a list.
- Some water is used in photosynthesis and some is kept in cells to keep them firm, so uptake is slightly more than the water lost.
- The shoot is cut, so its xylem may be damaged or blocked, and it has no roots.
- Leaks or air in the joints can move the bubble or stop it from moving.
- Conditions such as temperature or light can drift during the experiment.
- One shoot is one sample. A different shoot may behave differently.
The mistake to watch for
The usual mistake is to write “the potometer measures transpiration”.
Mistaken answer: The bubble moved 60 mm, so the shoot transpired 60 mm of water.
The bubble shows uptake, not loss, and a distance is not a volume.
The correction is to say “water uptake, used as an estimate of transpiration”, and to convert distance into volume by using the tube’s cross-sectional area if the question asks for it.
Check yourself
Try these first, then open each answer.
1. A bubble moved 18 mm, 21 mm and 21 mm in three 2-minute trials. Calculate the mean distance and the mean speed.
Show answer
Mean distance = (18 + 21 + 21) ÷ 3 = 60 ÷ 3 = 20 mm. Speed = 20 ÷ 2 = 10 mm per minute.
2. Give two ways to make the experiment a fairer comparison between “still” and “fan”.
Show answer
Use the same shoot or shoots of similar leaf area, and keep the temperature and light the same for both conditions. Also allow the shoot to settle before timing, and use the same time period.
3. A student says the larger bubble movement proves the plant lost more water. Give one reason this conclusion needs care.
Show answer
The bubble shows water uptake, and some of that water is used in photosynthesis or kept in the cells. Uptake is therefore an estimate, not an exact measure of water lost by transpiration.
Where this leads next
With transpiration measured, it helps to be clear about the two tissues involved, so move on to distinguishing xylem and phloem transport. Mixed questions are in the plant transport practice set, and the mistake log and retest queue is a good place to record any calculation slip.
Evaluation sentences such as limitations and improvements are the part of data questions that many students find hardest to phrase. Online one-to-one Biology tuition lets a teacher rehearse them with you on new datasets until they come out in your own words.