A potometer is a piece of apparatus used to measure the rate at which a leafy shoot takes up water. A student set up the potometer shown in Fig. 3.1 and use...

This question tests the potometer and the factors that affect transpiration and therefore water uptake. Faster evaporation from the leaves keeps the diffusion gradient of water vapour steep, so water is pulled up faster.

(a) [1] The highest rate is moving air at 30 °C (38 mm per minute) [1].

(b) [3] Moving air blows away the water vapour that collects around the stomata [1]; this keeps the water-vapour concentration (water potential) gradient between the leaf and the air steep [1]; so transpiration, and therefore water uptake, is faster [1].

(c) [2] A higher temperature gives the water molecules more kinetic energy, so evaporation from the leaf is faster [1]; this speeds up transpiration and so water uptake [1].

(d) [2] As the shoot takes up water, the air bubble moves along the scale [1]; measuring the distance the bubble moves in a known time gives the rate of uptake (e.g. mm per minute) [1].

(e) [1] Uptake slightly exceeds loss because some of the water taken up is used by the cells (for photosynthesis) or retained in the shoot rather than being lost as vapour [1].

(f) [2] Name one variable and say why it matters, e.g. light intensity must be kept constant [1] because it changes how far the stomata open and so alters transpiration, which would make the comparison unfair [1] (accept: the same shoot/leaf area; humidity).