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3.3.5Unit 3 — Life on Earth

Practical: Limiting Factors

5 min read 5 parts Lesson 19 of 31

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Section 1 of 5Planning the Investigation

3.3.5 Practical: Limiting Factors
Plant used
  • Elodea or Cabomba pondweed is used because it gives off bubbles of oxygen that can be counted or collected.
Independent variable
  • The distance of the lamp from the plant is changed, which changes the light intensity.
Dependent variable
  • The rate of photosynthesis is measured as bubbles per minute or the volume of oxygen collected in cm³ per minute.
Controlled variables
  • Temperature, carbon dioxide supply, the same piece of plant and the time of counting are all kept constant.

Section 2 of 5Method

1
Cut the stem at an angle, place the pondweed cut end up in water and add sodium hydrogencarbonate to supply carbon dioxide.
2
Cover the pondweed with an upturned funnel and collect the gas in a test tube or gas syringe.
3
Place a tank of water between the lamp and the beaker to absorb heat so the temperature stays constant.
4
Set the lamp 10 cm away, allow 5 minutes for the plant to settle, then count the bubbles for 1 minute.
5
Repeat three times, find the mean, and then repeat at 20, 30, 40 and 50 cm.

Section 3 of 5Results and Conclusion

Results and Conclusion
Expected trend
  • As the lamp moves further away, light intensity falls and the number of bubbles per minute decreases.
Graph
  • Plot distance on the x axis and bubbles per minute on the y axis, then describe the trend.
Explanation
  • Less light means less energy for the light reactions, so less ATP and hydrogen reach carbon fixation.
Limit of the trend
  • At short distances the rate may level off because carbon dioxide or temperature becomes the limiting factor.

Section 4 of 5Limitations and Improvements

Heat from lamp
  • Using an LED lamp or a water tank stops the water warming up and affecting enzyme activity.
Bubble size varies
  • Collecting the oxygen in a gas syringe and measuring its total volume is more accurate than counting bubbles.
Carbon dioxide runs out
  • Sodium hydrogencarbonate solution keeps carbon dioxide concentration constant and not limiting.
Plant not settled
  • Waiting several minutes before timing makes sure the rate has become steady.

Section 5 of 5Exam Checklist & Tips

3.3.5 – Key Points for the Exam

  • Elodea A pondweed that releases oxygen bubbles, used to measure photosynthesis.
  • Independent variable The variable the investigator changes.
  • Dependent variable The variable that is measured.
  • Sodium hydrogencarbonate A chemical added to water to supply dissolved carbon dioxide.
  • Reliability Getting similar results when the investigation is repeated.

Common Mistakes

  • Calling bubbles per minute the independent variable; it is the dependent variable.
  • Forgetting to control temperature, so the lamp heat changes enzyme activity as well as light.
  • Taking only one reading at each distance, so no mean can be found and results are less reliable.
  • Writing that the plant makes carbon dioxide bubbles; the gas collected is oxygen.
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