Biology 11–12 · Year 11

Floating leaf discs: photosynthesis measured by the time discs rise

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

PracticalLow risk

The idea

Leaf discs whose air spaces have been replaced by solution sink, then float again as oxygen from photosynthesis refills the spaces, so the time to float measures the rate.

What you need

Fresh spinach leaves or brassica cotyledons (the source names both); hole punch
Sodium hydrogencarbonate solution 0.2 percent, 100 mL, with one drop of detergent to wet the discs
Plastic syringes 10 or 20 mL without needles, 2 per group
Clear cups or beakers; lamp; metre rule; stop clock; thermometer

How to do it

Punch 10 discs from leaf tissue avoiding large veins and place them in a syringe with about 5 mL hydrogencarbonate solution.
Replace the plunger, expel the air, seal the tip with a finger and pull the plunger to create a partial vacuum; release, and repeat until the discs sink.
Pour the discs and solution into a cup under the lamp at a set distance and start the clock.
Record the number of discs floating every 30 seconds until all float; note the time at which half the discs (5 of 10) are floating.
Repeat in a cup wrapped in foil (dark) and at two other lamp distances.
Compare sun and shade leaves if available (the source suggests pelargonium and aspidistra).

What you should see

Under a bright lamp the discs begin to rise after a few minutes and all float within the lesson, giving a clear half-float time; in the dark none rise, and at greater lamp distance the half-float time lengthens. The learner knows it worked when the dark control stays sunk while the illuminated set floats, and replicate cups agree to within about a minute.

What changes

What you change: light intensity (lamp distance) or leaf type
What you measure: time for half the discs to float
What you keep the same: hydrogencarbonate concentration
disc size and number
temperature
leaf source
solution volume

Common misconceptions

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

Discs float because they are heated by the lamp; they float because oxygen gas refills the air spaces.
Plants only photosynthesise in sunlight; any light of suitable intensity and wavelength works.
The discs in the dark are dead; they are respiring and would float when returned to light.

Safety card

Low riskLearners carry it out

Hazards

Hole punch and syringe plunger pinching
Hot lamp

Controls

Keep fingers clear of the punch
Keep the lamp away from water

Note

Sodium hydrogencarbonate is low hazard: RiskAssess (https://www.riskassess.com.au/).

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 2027BIO11-8 BIO11/12-3
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-02 BI-11WS-03
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities