The Photosynthesis Data Puzzle
A student investigates the effect of light intensity on the rate of photosynthesis in an aquatic plant. She places the plant in a beaker of water with sodium bicarbonate and counts the number of oxygen bubbles produced per minute at different distances from a lamp: 10 cm, 20 cm, 30 cm, and 40 cm. The results are: 10 cm = 42 bubbles/min, 20 cm = 28 bubbles/min, 30 cm = 18 bubbles/min, 40 cm = 12 bubbles/min. She notices that at 10 cm the water temperature rises by 2°C during the experiment. She also observes that the bubbles become smaller at 40 cm. She must decide whether the results support the hypothesis that light intensity is the only limiting factor.
1. Analyse the data and explain whether the hypothesis is supported.
Read the worked answer
The data show that as distance increases (light intensity decreases), the bubble count decreases: 42, 28, 18, 12 bubbles/min. This supports the idea that light intensity affects the rate of photosynthesis. However, the hypothesis that light intensity is the only limiting factor is not fully supported. At 10 cm, the water temperature rises by 2°C, which could also increase the rate of photosynthesis, acting as a confounding variable. Additionally, the bubble size changes at 40 cm, suggesting that the volume of oxygen may not be proportional to bubble count. Therefore, other factors such as temperature and bubble size must be controlled or accounted for.
2. Suggest improvements to the experimental design to test the hypothesis more reliably.
Read the worked answer
Improvements: (1) Use a heat filter or place a transparent barrier between the lamp and the beaker to prevent temperature changes. (2) Measure the volume of oxygen collected using a capillary tube or gas syringe instead of counting bubbles, since bubble size varies. (3) Keep the sodium bicarbonate concentration constant and record it. (4) Allow the plant to acclimatise at each distance before counting. (5) Repeat each measurement to calculate a mean and reduce random error. (6) Control other variables such as pH and plant species.