Required Practical: Photosynthesis Rate (AQA GCSE Combined Science: Trilogy: Biology): Revision Note

Investigating photosynthesis

Aim:

• Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pond weed

Method

1. Place a piece of pond weed (Elodea or Cabomba are often used) into a beaker of water

2. Use a light a set distance from the plant

   • To prevent heating the plant, there are two options:

     • placing a beaker or glass tank of water between the lamp and the plant as a heat shield, or standing the pondweed in a water bath

     • using an LED bulb that releases very little heat energy

3. Record the number of bubbles produced in one minute

4. Repeat the experiment at least twice for each distance and calculate the mean number of bubbles, leaving out any anomalous results

5. Repeat steps 2–4 for different distances

6. Calculate a rate of photosynthesis at each distance as follows:

rate = mean number of bubbles ÷ time (unit e.g. bubbles per minute)

• If you instead time how long it takes to release a set number of bubbles, rate = number of bubbles ÷ time taken, so a shorter time means a faster rate

Improvements

• The volume of gas collected can be measured using a gas syringe to collect the gas produced

• This allows rate of photosynthesis to be calculated as follows:

rate = mean volume of oxygen ÷ time

Variables

• Different variables can be investigated using this method - the diagram above shows the setup for light as the independent variable

  • To investigate carbon dioxide: use different concentrations of sodium hydrogencarbonate in the beaker

  • To investigate temperature: change the temperature of the beaker containing the pondweed using a water bath

• Control variables: keep the temperature, carbon dioxide concentration, and the size/mass and type of pondweed the same. The variable you change (independent variable) can never be a control variable

Results

• A graph of the independent variable against the number of bubbles produced per minute can be drawn to see the pattern or trend

• We would expect the number of bubbles produced per minute to decrease with distance from the lamp