Eduqas Biology for A Level Year 2: Study and Rev Guide

Measuring the rate of photosynthesis Aquatic plants are a good subject to use when investigating how different factors affect photosynthesis. Temperature and carbon dioxide concentrations are more easily controlled than with terrestrial plants, by using a water bath and controlling hydrogen carbonate concentration. It is also easy to collect and accurately measure the oxygen produced in a capillary tube. The volume of the bubble collected is calculated by the formula: Volume = πr 2 × length of bubble Where π = 3.14 and r = radius or diameter 2 syringe scale capillary tube water bath funnel pond weed sodium hydrogen carbonate solution Photosynthometer Worked example A photosynthometer with a capillary tube diameter of 0.1 cm was used to measure the volume of oxygen produced by a piece of Canadian pond weed in ve minutes at 20 ° C. The results are shown in the table below: Temperature / o C Length of bubble in capillary tube /mm Trial 1 Trial 2 Trial 3 Mean 20 25 20 30 25 Calculate the mean volume of oxygen produced in mm 3 . Volume = π r 2 × length of bubble diameter = 0.1 cm = 1 mm, so radius = 0.5 mm. volume = 3.14 × 0.5 2 × 25 = 3.14 × 0.25 × 25 = 19.6 mm 3 (1 dp) Grade boost Remember to perform the experiment three times so a mean can be calculated, which is more reliable than a single result. Pointer Remember to work in the same units. In our example, convert 0.1 cm to 1 mm, as the bubble length was also measured in mm, so that the answer is in mm 3 . 20 A Level Biology: Study and Revision Guide