6 - Photosynthesis

• What gas is given off by plants during photosynthesis?

  Oxygen.
• What is the method for this practical (part 1)?

  1. Place a test tube rack containing a boiling tube 10 cm away from the light source, measured using the ruler.
  2. Fill the boiling tube with a fixed volume of sodium hydrogen carbonate solution.
  3. Place the cut pondweed into the boiling tube with the cut end at the top. Gently push the pondweed down with the glass rod.
  4. Leave the boiling tube to rest for 5 minutes.
  5. Start the stopwatch and count the number of bubbles produced in one minute.
• What is the method for this practical (part 2)?

  6. For each light intensity/distance, repeat the count twice more and take a mean.

  7. Record in a table as seen below.

  8.Repeat steps 1-7 for 3 more distances (20, 30, 40 cm) of the boiling tube from the light source.

  9.Plot a graph of the rate of photosynthesis (given by the no. of bubbles) against light intensity

  (using the inverse square law, light intensity = 1/distance? between pondweed and light source).
• What are potential hazards of this practical?

  • Allergy risk - involves pondweed
  • Lamp may get hot
  • Water - keep away from outlets and wiring
• What is the independent variable in this practical?

  Light intensity.
• What is the dependent variable in this practical?

  Number of bubbles.
• Why is it best to use an LED light source rather than a lamp?


  A lamp will emit more heat energy than an LED - therefore it may change the temperature of the experiment and affect the rate of photosynthesis.
• How can the measurement of oxygen production be made more accurate?

  If the pondweed is placed under a filter funnel with an inverted measuring cylinder over the spout, the volume of oxygen produced can be determined.
• Why is sodium hydrogen carbonate solution used during this experiment?

  It releases CO2, which is used by the pondweed during photosynthesis.
• Why can the number of bubbles produced be used to calculate the rate of photosynthesis?

  Oxygen bubbles are formed as a product during photosynthesis. The number of bubbles produced in a given time is proportional to the rate of photosynthesis.