Photosynthesis in immobilised algae measured by carbon dioxide uptake · Concept Studio

The idea

Photosynthesising cells remove dissolved carbon dioxide, raising the pH of the surrounding solution, and the colour of a pH indicator tracks the rate.

What you need

Concentrated culture of Scenedesmus quadricauda (the alga named in the source) or another green alga from an educational supplier, 20 mL
Sodium alginate solution 2 to 3 percent, 20 mL; calcium chloride solution 2 percent, 100 mL
Hydrogencarbonate indicator, 100 mL, equilibrated with air
Syringe 10 mL; small screw-cap bottles or tubes, 6; foil; lamp; a large flat-sided glass vase or container of water to stand between the lamp and the bottles as a heat filter, as the source directs; thermometer; metre rule; colorimeter with a 550 nm filter if available

How to do it

Mix equal volumes of algal culture and sodium alginate, draw into the syringe and drip into calcium chloride solution so each drop sets into a bead; leave 5 minutes then rinse the beads with water.
Rinse each bottle with indicator, add 10 beads and a fixed volume of indicator, and cap.
Wrap one bottle in foil (dark) and place the others at 10, 20, 40 and 80 cm from the lamp, with the water-filled heat filter between the lamp and the bottles; record the temperature beside the nearest and farthest bottles at the start and the end.
After at least 60 minutes (the source notes the colours will probably take more than 60 minutes to spread) compare the indicator colours with standards, or read absorbance at 550 nm.
Plot colour rank or absorbance against light intensity (1 over distance squared).
Return the beads to indicator and repeat the next day to show the beads remain alive.

What you should see

In the dark the indicator drifts toward yellow (carbon dioxide released by respiration); at the closest lamp positions it turns purple (carbon dioxide taken up faster than it is produced); at middle distances it stays near its equilibrium red-orange. Absorbance at 550 nm changes monotonically with light intensity. The learner knows it worked when the dark bottle and the brightest bottle bracket the starting colour on opposite sides.

What changes

What you change: light intensity (lamp distance)
What you measure: indicator colour or absorbance at 550 nm after a fixed time
What you keep the same: number and size of beads
indicator volume
time
temperature

Common misconceptions

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

Algae in the dark do nothing; they respire and release carbon dioxide.
The indicator measures oxygen; it responds to dissolved carbon dioxide through pH.
The beads are dead once immobilised; they photosynthesise for days.

Safety card

Low riskLearners carry it out

Hazards

Calcium chloride solution is a mild irritant
Hydrogencarbonate indicator contains dyes
The lamp gets hot

Controls

Wear eye protection
Rinse splashes
Do not touch the lamp while it is hot and do not leave it switched on unattended, as the source warns

Note

Low-hazard reagents; see 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 BIO11/12-4
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-03 BI-11WS-04
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.