2.2.9Scaling Up

Affecting Water Uptake by a Plant — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

5 min read 9 parts Lesson 16 of 16

📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision

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Part 1 of 7What is being measured

The volume of water taken up by a cut shoot is almost equal to the water lost by transpiration, because only a small fraction is used in photosynthesis or kept in the cells.

Measuring water uptake therefore gives an estimate of the transpiration rate.

Potometer

The apparatus used is a potometer: a capillary tube containing an air bubble, connected to a cut leafy shoot sealed into a water-filled tube.

As the shoot takes up water, the air bubble moves along the scale.

The distance the bubble moves in a fixed time shows the rate.

Part 2 of 7Setting up a potometer

Cut a leafy shoot under water, at a slant, to prevent air from entering the xylem and breaking the water column.

Fit the shoot into the potometer under water, with all joints sealed with petroleum jelly to make them airtight.

Dry the leaves and let the plant acclimatise.

Introduce a bubble of air by lifting the capillary tip out of the water and putting it back.

Record the position of the bubble at the start, wait a fixed time, and record again. Repeat several times and calculate a mean.

Reset the bubble using the reservoir tap to repeat the run.

Rate = distance moved by the bubble / time (mm per minute). If the radius of the capillary is known, volume = pi x r² x distance.

Part 3 of 7Investigating one factor

Change only one independent variable and keep the others constant (control variables):

Light

Light move a lamp to different distances, using a heat shield or a water tank to prevent temperature changing.

Temperature

Temperature use rooms or water baths at different temperatures, or the same shoot at different times.

Humidity

Humidity place a clear polythene bag over the plant, with a damp paper towel.

Air movement

Air movement use a fan at different distances.

Use the same plant, or same sized shoots, since the number of leaves and stomata would otherwise alter the result.

Part 4 of 7Interpreting results

A typical data table shows a rate that rises with light intensity, up to a point.

Interpreting results

You should describe the trend (for example, increases steadily from 0.5 to 2.0 mm per minute as the lamp moves from 100 cm to 20 cm),

then explain (stomata open wider in brighter light,
allowing faster diffusion of water vapour out of the leaf).

A flat region might mean another factor (such as water supply or stomata fully open) has become limiting.

Compare mean values, check for anomalies and note that the potometer measures uptake, which is only an estimate of the water lost.

Part 5 of 7Evaluating the method

Sources of error include air leaks that make bubbles move irregularly, the shoot being different from the one used earlier, humidity changes in the room, and the bubble sticking in the tube.

Evaluating the method

Improvements: use more repeats, use a datalogger or a camera to record the bubble position, measure the leaf area and calculate the rate per unit area,

and allow the shoot to acclimatise before each run.

Using the same shoot for every condition reduces variation between plants, but the leaves may become damaged or stressed.

Part 6 of 7Presenting and analysing data

Record results in a table with the independent variable in the first column, with units in the headings, and repeated readings next to each other.

Calculate a mean for each condition, excluding any anomalies, and plot the mean rate on the y-axis against the independent variable on the x-axis.

Draw a line of best fit or join the points, depending on the pattern.

For a statement such as "rate doubled" calculate from the graph.

Percentage change in rate between two conditions = (new rate - old rate) / old rate x 100.

A rate that increases with temperature might level off or fall if the plant starts to lose too much water and closes its stomata.

When asked to conclude, refer to the data: "the higher the light intensity, the faster the rate of water uptake, because stomata are more open".

Presenting and analysing data

When asked about validity, say that only one variable should be changed.

When asked about reliability, say that repeating and getting similar results makes the data more reliable.

Safety points include keeping electrical equipment (lamps) away from water and cutting shoots carefully with the blade moving away from the body.

Part 7 of 7Exam-style questions

Describe how you would find the effect of wind speed on the rate of water uptake of a shoot. [4]4 marks
A bubble moves 24 mm in 6 minutes. Calculate the rate and explain what would happen if the experiment was repeated with the leaves smeared with petroleum jelly on the lower surface. [3]3 marks
Suggest why the shoot is cut under water. [2]2 marks

Key terms

Term	Meaning
Potometer	Apparatus used to measure the rate of water uptake by a shoot.
Independent variable	The factor that is deliberately changed.
Control variable	A factor kept the same so that it cannot affect the result.
Reliability	How consistent results are when repeated.
Rate	A change per unit time.
Acclimatise	Allow a plant time to adjust to new conditions.
Anomaly	A result that does not fit the pattern.

Avoid these mistakes

Changing more than one variable at a time, such as moving a lamp but not controlling the temperature rise.

Giving rate without units or time.

Forgetting to seal the joints so that the bubble moves due to air leaks.

Saying that the potometer measures photosynthesis.

Failing to repeat and calculate a mean.

Last updated: 20 October 2026