Transpiration in plants can be investigated using a potometer, which measures water uptake by plants. Fig. 1.1 shows a potometer that was used by a student. [Figure 1.1] As water is lost by the leaves through transpiration, the air bubble moves along the graduated tube. The student used this apparatus to investigate the effect of light intensity on the rate of transpiration in plants.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

1. [2] independent light intensity; dependent distance moved by bubble (in a set time) or time taken for the bubble to move (a set distance);
2. [3] any three variables for one mark two correct methods for one mark each 1 temperature - temperature-controlled room / heat shield / environmental chamber/incubator / cold light source / LED light; 2 background light – dark room (with fixed light) / closed blinds / AW ; 3 same wavelength / type / colour of light – use the same bulb / lamp / AW ; 4 idea of air flow / wind – e.g. windows closed / doors closed / fans off or on throughout / limit movement / AW ; 5 plant/ (leafy) shoot use same, type / species/plant/shoot/age / number of leaves / area of leaves ; 6 humidity - any valid way it might be controlled ; 7 CO2 levels - any valid way it might be controlled ; 8 time measure for same time / time over same distance ;
3. [6] any six from 1 ref. to cutting/ inserting, stem under water; 2 use, petroleum jelly / silicone gel / silicone tape / AW (to make joints air tight); 3 idea of removing tube from water to introduce an air bubble ; 4 ref. to method of obtaining a minimum of 5 different light intensities; 5 ref. to method of controlling one variable ; 6 allow (apparatus / plant) to, equilibrate / AW, before starting measurements ; 7 idea of setting / resetting / returning, air bubble (to start position / between measurements); 8 ref. to measuring distance moved by bubble over a set time or ref. to measuring time for bubble to move a set distance; 9 take a minimum of 2 repeats at each light intensity + take a mean / identify anomalies ; 10 low/medium, risk investigation or cutting stem + cut away from your hand (with scalpel) / cut with secateurs or scissors;
4. [2] 30.0 (%); (3.03 -2.12)/3.03 or (1 – 2.12/3.03) × 100
5. [2] any two from 1 idea of only two, concentrations of CO2/ light intensities, measured ; 2 idea of only 1, type / species, of plant tested; 3 not replicated / repeated; 4 idea that laboratory conditions may not be replicated in the field; 5 no statistical analysis; 6 idea of other (stated) variable(s), not controlled / can affect transpiration ;