Science 7–10 · Year 8

Pondweed and light: oxygen bubbles against lamp distance

Science understanding: Biological sciences (no ACARA v9 content description covers the rate of photosynthesis, and the only one that names photosynthesis at all is AC9S9U03, a Year 9 Earth and space sciences description of the carbon cycle, so the ACARA side is coded to Year 8 science inquiry only; the NSW side carries the concept in Stage 4 Cells and classification, whose content point asks learners to identify photosynthesis via chloroplasts as a process that takes place in a specialised organelle)

Practical, model not builtLow risk

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The idea

Photosynthesis in the chloroplasts of a water plant releases oxygen, and the rate of release falls as the lamp moves further away and less light reaches the plant.

Safety card

Low riskAn adult supervises

Hazards

Electrical lamp near water

Controls

Lamp on a stand away from the beaker; dry hands on the switch
Beaker stood in a tray to catch spills and kept away from sockets, as the SAPS notes advise

Note

No hazardous chemicals; record a RiskAssess risk assessment for the activity as school procedure requires.

What you need

A 7 to 10 cm sprig of hornwort (Ceratophyllum demersum), which PlantNET lists as native to NSW and SAPS lists as suitable, bought from an aquarium supplier and cut so there is one cut end. Do not use Cabomba caroliniana, which NSW WeedWise says must not be sold anywhere in NSW, and never release aquarium plants into a waterway.
1 % sodium hydrogencarbonate solution (10 g/L), 300 mL, in a 400 mL beaker
A desk lamp with an LED globe of 1200 lumens or more (SAPS recommends LED globes because they add little heat), and a metre rule
Paper clip to weight the sprig, thermometer, stopwatch

How to do it

Recut the stem under water. Weight the sprig so the cut end points up in the hydrogencarbonate solution; place it 5 cm from the lamp until bubbles stream steadily from the cut end (if none appear within two minutes, SAPS advises snipping the end again).
Count bubbles for one minute, three times, at 5 cm; record the mean.
Repeat at 10, 15, 20, 25 and 30 cm, allowing five minutes at each new distance before counting, and record a count of zero where bubbling has stopped. SAPS advises keeping the steps short over a 1 to 30 cm range because light falls quickly with distance, and reports that bubbling may stop completely beyond about 20 cm.
Check the water temperature at each distance; it should not change by more than one degree.
Plot bubbles per minute against distance, then against 1 divided by distance squared.
Cover the beaker with foil and count again as a dark control.

What you should see

Bubble rate falls as the lamp moves away, and SAPS reports that bubbling may stop completely beyond about 20 cm from the lamp. Plotted against 1 / d^2, the relative light from a small lamp, the rate rises with light but the points do not lie on a line through the origin: the count reaches zero while light still reaches the sprig (at 25 cm, 1 / d^2 is 0.16 of its 10 cm value), because at low light the oxygen the plant makes is used in its own respiration or dissolves in the water before it forms bubbles, and very close to the lamp the rate may stop rising. Under the foil the bubbling slows and stops. The learner knows it worked when the rate falls with each step away from the lamp and the three counts at each distance are similar.

What changes

What you change: Lamp distance (cm), which sets light intensity
What you measure: Bubbles per minute
What you keep the same: Same sprig
Hydrogencarbonate concentration
Temperature (LED lamp, checked with a thermometer)
Counting period
Room light kept low

Common misconceptions

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

Plants photosynthesise only in sunlight, not lamplight.
The bubbles are air escaping from the stem; the dark control shows they need light.
Doubling distance halves the light; it quarters it.

Curriculum references

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

Science 7–10 Syllabus (2023)SC4-CLS-01 SC4-WS-03 SC4-WS-05 SC4-WS-06
Australian Curriculum v9AC9S8I02AC9S8I04AC9S8I05

Sources

The pages the author read to write this activity.

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