Biology - 8 Photosynthesis - Required Practical 6: The Effect of Light Intensity on the Rate of Photosynthesis in Pondweed

Aims:

  • to investigate how light intensity affects the rate of photosynthesis

Risk Assessment:

  • sodium hydrogen carbonate is an irritant

  • light can damage eyes

  • bench lamp bulb can become hot

Safety:

  • wear safety goggles and clean up any spills

  • do not look directly at the bulb for extended periods of time

  • do not touch the bench lamp bulb

Equipment:

  • bench lamp (preferably LED)

  • boiling tube

  • beaker

  • thermometer

  • water

  • tripod

  • metre rule

  • 0.5% sodium hydrogen carbonate solution

  • pondweed

  • stopwatch

Control variables:

  • concentration of CO₂

  • temperature

  • volume of hydrogen carbonate solution

  • length of pondweed

Independent variable:

  • light intensity

Dependent variable:

  • number of oxygen bubbles per minute

Method:

  1. Fill a boiling tube with 35cm² of 0.5% sodium hydrogen carbonate solution

  2. Select a piece of pondweed approximately 5cm long

  3. Insert the pondweed into the tube and cut the end of the submerged stem at an angle

  4. Fill the beaker with water and stand the boiling tube inside. Insert the thermometer in the beaker. Place both on a tripod

  5. Set up a bench lamp as close as possible to the pondweed. Set the metre rule at the pondweed.

  6. Switch on the bench lamp and wait for 5 minutes for equilibriation. Constantly check the thermometer and keep a constant temperature

  7. (Take three readings of this for each distance) Use the stopwatch to time 30 seconds and record the number of bubbles emitted from the stem in that time

  8. Repeat steps 5-7 as many more times as possible, moving the bench lamp 5cm further away each time

Calculations/graphing:

  • Calculate light intensity using ‘light intensity = 1/d²’

  • Calculate the rate of photosynthesis in bubbles/minute

  • Plot a graph of light intensity against rate of photosynthesis

Alternatively, you can place a measuring cylinder and funnel on top of the pondweed and collect your data not in bubbles/minute (which leaves room for human error) but using the volume of oxygen produced.