OCR GCSE Biology B (J257) — Ecosystems and Photosynthesis
Have you ever wondered how scientists measure a plant "drinking" when the water moves invisibly inside the stem? A potometer is a piece of apparatus used to measure the rate of water uptake by a leafy shoot.
This water uptake is used as an estimate for the rate of transpiration (the evaporation and diffusion of water vapour from leaves). It is vital to remember that a potometer does NOT measure the exact rate of water loss, because approximately 1% of the water taken up is used by the plant for photosynthesis or to keep cells turgid.
To set up a simple potometer, you must follow specific steps to ensure the continuous flow of water is not broken.
First, you must cut the plant shoot underwater and at a slant. Cutting underwater prevents an air embolism (a trapped air blockage) from entering the xylem vessels, which would break the continuous water column and stop water uptake. Cutting at a slant increases the surface area for water absorption.
Next, assemble the apparatus underwater and seal all joints (like the bung and glass tube) using petroleum jelly. This creates an airtight and watertight seal, ensuring that only water evaporating from the leaves causes the water to move up the tube. Any leaks will lead to inaccurate results or a stationary bubble.
Finally, gently dab the leaves dry. Wet leaves artificially increase humidity around the stomata to 100%, which slows the rate of transpiration. Leave the plant to acclimatise to its environmental conditions for 5–10 minutes before testing.
Once the setup is acclimatised, you can begin your investigation.
First, introduce a single air bubble into the capillary tube (a tube with a very small internal diameter). This bubble acts as a highly visible marker.
Then, use a stopwatch to measure a set time (e.g., 10 minutes) and use a ruler to measure the distance the bubble moves. To ensure a fair test, control variables like temperature or the specific shoot used (to maintain the same leaf surface area) must be kept constant unless they are the variable you are actively investigating.
To repeat the experiment or test a new variable, use the integrated water reservoir with a tap to gently push the air bubble back to the start (zero) of the capillary tube.
The simplest way to estimate the transpiration rate is to calculate the basic rate of water uptake using the distance moved.
For higher tier analysis, you may need to calculate the actual volume of water taken up using the formula for the volume of a cylinder: (where is the internal radius of the capillary tube and is the distance the bubble moved).
An air bubble in a potometer moves in . The capillary tube has an internal radius of . Calculate the rate of water uptake in .
Step 1: Write down the known values.
Step 2: Calculate the total volume of water taken up using .
Step 3: Calculate the rate.
Students often state that a potometer measures the exact rate of transpiration (water loss) — it actually measures water uptake, because roughly 1% of the water is kept by the plant for photosynthesis or turgidity.
In 6-mark procedure questions, examiners expect you to explicitly state WHY you cut the shoot underwater (to prevent air bubbles breaking the continuous water column in the xylem).
To score marks for improving the reliability of the experiment, always state that you would use the reservoir tap to reset the bubble, repeat the readings, and calculate a mean distance moved.
Do not forget to mention allowing the plant to acclimatise for 5–10 minutes before starting the stopwatch; this is a common marking point for experimental validity.
Potometer
A piece of apparatus used to measure the rate of water uptake by a leafy shoot, which acts as an estimate for the rate of transpiration.
Transpiration
The loss of water vapour from plant leaves or aerial parts, mainly through the stomata, driven by evaporation and diffusion.
Xylem
The plant vascular tissue that transports water and dissolved minerals from the roots up to the leaves in a continuous column.
Air embolism
A blockage in the xylem caused by trapped air that breaks the continuous transpiration stream and prevents water uptake.
Watertight seal
A secure, leak-proof connection (often made using petroleum jelly) that ensures only water loss from the plant's leaves causes the potometer bubble to move.
Capillary tube
A glass tube with a very small internal diameter that makes tiny volumes of water uptake easily visible as large movements of an air bubble.
Transpiration rate
The speed at which water is lost from a plant, estimated in a potometer by measuring the distance an air bubble moves over a set time.
Put your knowledge into practice — try past paper questions for Biology B
Potometer
A piece of apparatus used to measure the rate of water uptake by a leafy shoot, which acts as an estimate for the rate of transpiration.
Transpiration
The loss of water vapour from plant leaves or aerial parts, mainly through the stomata, driven by evaporation and diffusion.
Xylem
The plant vascular tissue that transports water and dissolved minerals from the roots up to the leaves in a continuous column.
Air embolism
A blockage in the xylem caused by trapped air that breaks the continuous transpiration stream and prevents water uptake.
Watertight seal
A secure, leak-proof connection (often made using petroleum jelly) that ensures only water loss from the plant's leaves causes the potometer bubble to move.
Capillary tube
A glass tube with a very small internal diameter that makes tiny volumes of water uptake easily visible as large movements of an air bubble.
Transpiration rate
The speed at which water is lost from a plant, estimated in a potometer by measuring the distance an air bubble moves over a set time.
