The potometer is the transpiration practical that IGCSE Biology Paper 5 and Paper 6 keep coming back to. By hand, hold the reservoir tap open until the air bubble sits on the 0 mark, start the stopwatch (its speed lever saves you the real minutes) and read the front of the bubble on the millimetre scale every minute for five minutes. Carry the fan in front of the shoot, switch it on and do it again. Lay one spare leaf on centimetre-squared paper and count the squares. Then plot both sets of readings, fit a line to each, turn the gradients into rates and work out the rate of water uptake per square centimetre of leaf. Every attempt uses a different shoot. Allow about 30 minutes.
This workout needs JavaScript.
Headings with quantity / unit, every reading the position of the bubble when the stopwatch showed that whole minute, whole millimetres throughout, the bubble sent back to the 0 mark before each run with the tap closed, and the fan off for the still air run but in place and on for the whole of the moving air run. Examiner reports on 0610 practical papers regularly note headings without units and readings not taken at the stated times.
Two straight lines of best fit, each through the middle of its own points, and each rate taken as the gradient of its line (two points far apart), with the unit mm / min and 2 or 3 significant figures.
The leaves on the shoot counted, the squares under the leaf counted with the more-than-half rule (within 2 of a careful count), total area = leaves × area of one leaf, and the rate per cm² from your own moving-air rate to 2 significant figures.
Moving air carries water vapour away from the leaf surface, so the diffusion gradient is steeper and water leaves the stomata faster; the potometer measures uptake, which is slightly more than transpiration because some water is used in photosynthesis and for turgor.
Two control variables (temperature, light intensity, the same shoot), a real source of error (reading a moving bubble, the start not exactly at 0), an improvement that adds reliability (repeats and a mean rate), and why the apparatus is set up under water (no air in the xylem, a continuous water column).
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.