Cambridge O Level Biology · Syllabus 5090 · Transport in Flowering Plants
Potometer
What is Potometer?
An apparatus used to measure the rate at which a cut leafy shoot takes up water. The shoot is sealed into a water-filled tube connected to a horizontal capillary tube containing a single air bubble, and the distance the bubble moves along a scale in a measured time gives the rate of water uptake. Because most of the water taken up is lost from the leaves as water vapour, this uptake is used as an estimate of the rate of transpiration; it is not a direct measurement of it, since some water is used in photosynthesis, retained in growth and held in the cells to maintain turgor.
This definition is part of the Transport in Flowering Plants chapter in Cambridge O Level Biology.
Common mistakes with Potometer
- 11. “A potometer measures transpiration.” Why wrongIt records the water drawn into the shoot. Water loss is not measured at all; it is inferred. WriteA potometer measures the rate of water uptake, which is used as an estimate of the rate of transpiration.
Questions students ask about Potometer
Does a potometer measure transpiration?
No. A potometer measures the rate at which a cut shoot takes up water, which is used as an estimate of the rate of transpiration. The two differ because a small amount of the water absorbed is used as a raw material in photosynthesis, retained in new tissue during growth, and held in cells to maintain turgor.
Why must a potometer shoot be cut under water?
To stop air entering the xylem vessels. An air bubble inside the xylem would break the continuous column of water, so the transpiration pull could not be transmitted and water uptake would slow or stop, giving a falsely low reading.