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OCR GCSE Biology B · J257
OCR J257 · Transpiration and Translocation Check the specification (PDF) (opens in a new tab)
Water evaporates from leaves and diffuses out through open stomata. The shoot draws up water through its xylem to replace much of this loss. A simple potometer follows the water taken up by a leafy shoot: it does not collect or directly measure the water vapour leaving the leaves.
Track the air bubble along the capillary tube to measure the shoot’s rate of water uptake.
Fill the tubing and capillary tube with water. Cut the shoot under water and fit its stem into the tubing, keeping the cut end under water so that air does not enter the xylem. Seal the joint around the stem to make the apparatus airtight. Introduce one small air bubble into the capillary tube. As the shoot takes up water, the bubble moves towards the shoot.
Mark the bubble’s starting position. Use a ruler to measure how far it moves and a stopwatch to measure the time taken. Calculate the rate of water uptake as
For example, if distance is recorded in millimetres and time in minutes, report the rate in mm/min. Repeat the measurement while keeping the conditions the same if you want a more reliable estimate. Check that the apparatus remains filled with water and that no air can leak in at the joint.
The bubble’s movement gives a measure of water uptake. This is used to estimate the rate of transpiration because much of the water taken up replaces water lost from the leaves. The two rates need not be exactly equal: some water is used or retained by the shoot.
A water-filled potometer holds a leafy shoot with an airtight seal. Water uptake draws an air bubble along a capillary tube. Measure the bubble’s distance moved and the time taken:
Use matching units, such as mm/min. Bubble movement measures water uptake, which is an estimate of transpiration, not a direct measurement of water loss.
State that a potometer measures water uptake by a shoot, not transpiration directly. Most of the water taken up replaces water lost by transpiration, but some is used by the plant.
When describing its use, include the air bubble, a measured distance, a measured time and the rate calculation.
An air leak makes the reading unreliable because the shoot may draw in air instead of water through the capillary tube.
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Potometer
An apparatus that measures how quickly a plant shoot takes up water, usually by tracking an air bubble in a water-filled capillary tube.
Transpiration
The loss of water vapour from a plant, mainly through open stomata in its leaves.
Capillary tube
A narrow tube in which a small movement of water can be tracked by following an air bubble.
Put your knowledge into practice — try past paper questions for Biology B
Potometer
An apparatus that measures how quickly a plant shoot takes up water, usually by tracking an air bubble in a water-filled capillary tube.
Transpiration
The loss of water vapour from a plant, mainly through open stomata in its leaves.
Capillary tube
A narrow tube in which a small movement of water can be tracked by following an air bubble.