Transpiration rates: distance divided by time
Definition
Potometer
A piece of apparatus that estimates the rate of transpiration by measuring the water a leafy shoot takes up.
- Potometer means apparatus that estimates transpiration rate by measuring water uptake by a leafy shoot.
- The air bubble moves as the shoot takes up water, so its distance travelled can be measured against time.
- A potometer does not measure transpiration directly because some absorbed water is used in photosynthesis, growth and maintaining cell turgor.
Common Mistake
State precisely: the apparatus measures water uptake and uses it to estimate transpiration.
Calculate distance per unit time
- Use rate=distance moved by bubbletime taken\text{rate}=\frac{\text{distance moved by bubble}}{\text{time taken}}rate=time takendistance moved by bubble.
- If distance is measured in millimetres and time in minutes, the unit is mm min−1\mathrm{mm\,min^{-1}}mmmin−1.
- Convert quantities before calculating when the question gives different time or distance units.
Example
- Question: a bubble moves 54 mm54\,\mathrm{mm}54mm in 9 min9\,\mathrm{min}9min.
- Calculation: rate=549=6.0 mm min−1\text{rate}=\frac{54}{9}=6.0\,\mathrm{mm\,min^{-1}}rate=954=6.0mmmin−1.
Calculate water volume per unit time
- For a cylindrical capillary tube, its cross-sectional area is A=πr2A=\pi r^{2}A=πr2.
- The volume of water taken up is V=Al=πr2lV=Al=\pi r^{2}lV=Al=πr2l, where lll is the bubble distance.
- The volume uptake rate is rate=πr2lt\text{rate}=\frac{\pi r^{2}l}{t}rate=tπr2l.
- Use matching length units throughout so the volume unit is correct, such as mm3\mathrm{mm^{3}}mm3.
Example
- Question: a tube has radius 0.50 mm0.50\,\mathrm{mm}0.50mm, and the bubble moves 40 mm40\,\mathrm{mm}40mm in 5.0 min5.0\,\mathrm{min}5.0min.
- Cross-sectional area: A=π(0.50)2=0.785 mm2A=\pi(0.50)^{2}=0.785\,\mathrm{mm^{2}}A=π(0.50)2=0.785mm2.
- Volume: V=0.785×40=31.4 mm3V=0.785\times40=31.4\,\mathrm{mm^{3}}V=0.785×40=31.4mm3.
- Rate: rate=31.45.0=6.28 mm3 min−1\text{rate}=\frac{31.4}{5.0}=6.28\,\mathrm{mm^{3}\,min^{-1}}rate=5.031.4=6.28mm3min−1.
Calculate means and percentage change
- For repeats, calculate mean=sum of valid readingsnumber of valid readings\text{mean}=\frac{\text{sum of valid readings}}{\text{number of valid readings}}mean=number of valid readingssum of valid readings.
- Exclude an anomalous reading only when there is evidence that it does not fit the repeat pattern.
- Calculate percentage change using percentage change=new rate−original rateoriginal rate×100\text{percentage change}=\frac{\text{new rate}-\text{original rate}}{\text{original rate}}\times100percentage change=original ratenew rate−original rate×100.
Example
- Question: water uptake rises from 4.0 mm min−14.0\,\mathrm{mm\,min^{-1}}4.0mmmin−1 to 5.6 mm min−15.6\,\mathrm{mm\,min^{-1}}5.6mmmin−1.
- Calculation: percentage increase=5.6−4.04.0×100=40%\text{percentage increase}=\frac{5.6-4.0}{4.0}\times100=40\%percentage increase=4.05.6−4.0×100=40%.
Use graphs and tables correctly
- Put the independent variable on the horizontal axis and transpiration or water-uptake rate on the vertical axis.
- Include units in each axis label and use a suitable linear scale that fills most of the graph.
- A straight or smooth line of best fit should represent the overall pattern rather than joining every point automatically.
Exam technique
- Method mark: write the substituted equation before calculating so your process is clear.
- Unit mark: attach the correct compound unit, such as mm min−1\mathrm{mm\,min^{-1}}mmmin−1 or mm3 min−1\mathrm{mm^{3}\,min^{-1}}mm3min−1.
- Calculator display: keep extra digits during working and round only the final answer to a sensible precision.
- Confirm that the unit matches the quantity calculated and that a faster bubble movement gives a larger positive rate.
Self review
- What does a potometer measure directly?
- How do you calculate bubble distance per unit time?
- How is water volume calculated from tube radius and bubble distance?
- How do you calculate percentage change in rate?
- Which variables belong on each graph axis?
