Biology 11–12 · Year 11

Gas exchange of plants and animals in light and dark with hydrogencarbonate indicator

Module 1: Cells as the Basis of Life (Cell Function); also Module 2 (Transport)

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Living things change the carbon dioxide concentration around them: animals always add it, plants remove it in light and add it in the dark.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

Soda lime is corrosive and handled only by staff
Pond water may carry microorganisms
Glassware must be free of acid or alkali residue

Controls

Staff prepare the soda-lime bag
Cover cuts, wash hands after handling organisms
Rinse glassware with dilute hydrogencarbonate as the source directs
Animals are held no longer than the run needs, recovered in fresh pond water and returned to the site or tank; any that have not recovered are not returned

Note

Soda lime: NSW Department of Education Chemical Safety in Schools package (https://education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/1._Section_1_-_General_information_for_all_staff.pdf). The Department's Animals in Schools framework sets out a school's animal welfare obligations (https://education.nsw.gov.au/teaching-and-learning/animals-in-schools); every animal here is held only for as long as the activity needs and is returned to where it came from. The source notes that the animals spend up to 24 hours in an unnatural solution and that most recover fully; a class that would rather not enclose animals can run the plant-only and empty tubes and read the animal result from the source.

What you need

Hydrogencarbonate indicator, 200 mL, equilibrated with air (red-orange)
Elodea sprigs or filamentous alga; water snails or Gammarus from an educational supplier or a school pond
Eight test tubes with bungs; foil; lamp; soda lime in a muslin bag for the no-carbon-dioxide standard
Straw or tubing for the high-carbon-dioxide standard; potassium hydrogencarbonate 0.01 mol/L for rinsing glassware

How to do it

Prepare three colour standards: breathe through a straw into one tube of indicator (high carbon dioxide, yellow), leave one open to air (normal, red-orange), and hang a soda-lime bag in a sealed third (no carbon dioxide, purple).
Fill eight tubes with indicator: A and E animal only, B and F plant only, C and G plant and animal, D and H nothing; bung each.
Place A to D in continuous light and E to H in the dark (foil) for at least 12 hours.
Compare each tube against the standards and record the colour, then stand the animals and plant material in fresh pond water for a few hours before returning them, and return only living material, as the source directs.
Explain each result in terms of respiration and photosynthesis.

What you should see

Animal-only tubes turn yellow in light and dark; plant-only in light turns purple and in the dark turns yellow; plant with animal in light stays near red-orange or shifts purple depending on the balance, and in the dark turns yellow; the empty controls stay red-orange. The learner knows it worked when both controls are unchanged and the plant-only tubes differ between light and dark.

What changes

What you change: presence of plant, animal or both, and light or dark
What you measure: indicator colour after 12 hours
What you keep the same: indicator volume
temperature
time
size of organisms

Common misconceptions

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

Plants photosynthesise instead of respiring; they do both, and in the dark they only respire.
Plants in the dark produce oxygen; in the dark they produce carbon dioxide.
The indicator turns yellow because it is warmer; it responds to dissolved carbon dioxide.

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 BIO11-9 BIO11/12-3
Biology 11–12 Syllabus (2025), Year 11 focus area Cells as the basis of life; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-01
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-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities