This question is about photosynthesis - AQA - GCSE Biology Combined Science - Question 3 - 2018 - Paper 1

Photosynthesis is classified as an endothermic reaction. This means it requires energy input from sunlight to transform carbon dioxide and water into glucose.

1. Measure the volume of gas released: Instead of counting bubbles, use a measuring cylinder or a gas syringe to accurately quantify the volume of oxygen produced.
2. Increase the time of observation: Conduct the experiment over a longer period to obtain a more representative average rate of photosynthesis.

Maintaining a constant temperature is crucial because temperature can affect the rate of enzymatic reactions involved in photosynthesis. If the temperature fluctuates, it may result in variable rates of photosynthesis and thereby skew the results.

Light intensity follows the inverse square law, which states: $I ext{ (intensity)} ext{ is inversely proportional to } d^2 ext{ (distance)}.$ When the distance (d) is doubled, the light intensity (I) is reduced to one-quarter of its original value. For example, if the pondweed is initially at 5 cm (I), moving it to 10 cm results in: I_2 = rac{I_1}{d^2} = rac{I}{(2d)^2} = rac{I}{4}

Based on the trend observed in the given data, we could predict approximately 2 bubbles for 40 cm, as the rate of bubble production decreases significantly with increased distance from the light source.

To investigate the effect of carbon dioxide concentration, the student should:

1. Change the independent variable: Use different concentrations of sodium hydrogen carbonate solution to alter CO2 availability.
2. Control variables: Keep the light intensity constant, use the same type of pondweed, and allow time for the plant to equilibrate before measuring.