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Biology 11–12 · Year 11

Rate of photosynthesis in pondweed against light distance and carbon dioxide

Module 1: Cells as the Basis of Life (Cell Function)

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The idea

Photosynthesis releases oxygen at a rate set by light and carbon dioxide supply, and light intensity falls with the square of the distance from a lamp.

What you need

  • Elodea shoot about 8 cm, freshly cut, weighted with a paper clip (not Cabomba, which the source also names: NSW prohibits its import, sale or exchange under the Biosecurity Act 2015)
  • Beaker 250 mL of water; large beaker or tank of water as a heat shield between lamp and plant
  • Lamp with an LED or halogen bulb; metre rule; stop clock; thermometer
  • Potassium hydrogencarbonate or sodium hydrogencarbonate, 0.05 g portions (the source adds 0.05 g at a time)
  • Optional: gas syringe or inverted graduated tube to collect gas instead of counting bubbles

How to do it

  1. Set the shoot cut-end-up in the beaker of water with the heat shield between it and the lamp, in a darkened room.
  2. Place the lamp 10 cm from the plant, wait 2 minutes, then count bubbles from the cut end for 1 minute; repeat twice and take the mean.
  3. Move the lamp to 20, 30, 40 and 50 cm, allowing 2 minutes at each before counting.
  4. Record the water temperature at each distance to confirm the heat shield is working.
  5. With the lamp back at a fixed distance, add 0.05 g hydrogencarbonate portions to the water, mixing gently, and count bubbles after each addition.
  6. Plot bubbles per minute against 1 divided by distance squared, and against hydrogencarbonate added.

What you should see

Bubble rate falls as the lamp is moved away and rises again when hydrogencarbonate is added; at 20 cm the light reaching the plant is one quarter of that at 10 cm (computed from the inverse square relation), so the count falls sharply over the first few positions and levels off in dim light. When rate is plotted against 1 over distance squared the points rise roughly in a straight line while light is limiting and level off once another factor limits. The learner knows it worked when the temperature stays within about 1 degree Celsius across the run and the count at the closest distance is reproducible to within a few bubbles.

What changes

What you change
distance from the lamp (cm), then hydrogencarbonate added (g)
What you measure
bubbles of gas per minute from the cut stem
What you keep the same
  • plant shoot
  • temperature (heat shield)
  • lamp
  • counting time
  • water volume

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Doubling the distance halves the light; it quarters it.
  • The bubbles are carbon dioxide; they are mostly oxygen released by photosynthesis.
  • Adding more hydrogencarbonate always increases the rate; once light is limiting the rate stays flat.

Safety card

Low riskLearners carry it out

Hazards

  • Hot lamp
  • Water near mains electricity
  • Pond water may carry microorganisms

Controls

  • Keep the lamp away from water and switch off before moving
  • Cover cuts and wash hands after handling the plant
  • Return any aquatic invertebrates found on the shoot to a suitable tank
  • Never tip plants or water into drains or waterways; dry used plant material and put it in the general waste

Note

Hydrogencarbonate salts are low hazard: RiskAssess (https://www.riskassess.com.au/). Elodea is a water weed in NSW: it is not prohibited matter, but the general biosecurity duty under the Biosecurity Act 2015 applies, so no plant or tank water may reach a drain or waterway (https://weeds.dpi.nsw.gov.au/Weed/Elodea).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO11-8BIO11/12-3BIO11/12-4
  • Biology 11–12 Syllabus (2025), Year 11 focus area Cells to systems; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-02
  • Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-03BI-11WS-04BI-11WS-05
  • Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/biology-stage-6-2017
  2. www.nsw.gov.au/sites/default/files/noindex/2025-03/biology-stage-6-syllabus-2017.docx
  3. curriculum.nsw.edu.au/learning-areas/science/biology-11-12-2025/outcomes
  4. curriculum.nsw.edu.au/learning-areas/science/biology-11-12-2025/content/year-11/fa79a477bc
  5. practicalbiology.org/energy/photosynthesis/investigating-factors-affecting-the-rate-of-photosynthesis.html
  6. weeds.dpi.nsw.gov.au/Weeds/Cabomba
  7. weeds.dpi.nsw.gov.au/Weed/Elodea

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