Biology 11–12 · Year 11

Conditions needed for photosynthesis: starch tests on treated leaves

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

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

A leaf stores the product of photosynthesis as starch only in the cells that had chlorophyll, light and carbon dioxide together, so an iodine test maps exactly where photosynthesis happened.

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

Ethanol is highly flammable
Near-boiling water scalds
Soda lime is corrosive
Iodine solution irritates eyes and stains skin

Controls

No naked flames in the room while ethanol is out; heat ethanol only in a water bath with the hotplate off
Use forceps and eye protection at the water bath
Staff alone handle soda lime and set up the bell jars
Rinse splashes of iodine with water

Note

Ethanol, soda lime and hydrochloric acid 0.1 mol/L: 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) and RiskAssess (https://www.riskassess.com.au/).

What you need

Pot geraniums (Pelargonium), one variegated (green and white leaves) and two fully green, de-starched by keeping them in the dark for 48 hours (the source time for a pelargonium)
Strips of black paper or aluminium foil 2 to 3 cm wide, long enough to wrap both faces of a leaf; paperclips, 2 per strip
Two bell jars set up by staff on sealed trays: one over a green plant with a 100 mL beaker holding soda lime about 1 cm deep (removes carbon dioxide); one over a green plant with 10 to 20 marble chips in a 250 mL beaker to which about 50 mL of 0.1 mol/L hydrochloric acid has been added (adds carbon dioxide)
Beaker of water kept near boiling on a hotplate (water bath); boiling tubes; ethanol (industrial denatured alcohol), about 10 mL per leaf; forceps; white tile
Iodine solution in a dropper bottle; paper and pencil for tracing each leaf

How to do it

Test one leaf from a de-starched plant before starting (steps 5 to 7) to confirm that it holds no starch.
Trace a variegated leaf and shade its green areas, then place the variegated plant in bright light for 24 hours.
On a green plant, fold a foil or paper strip over part of a leaf so it covers both faces, clip it firmly in contact with the leaf, and place the plant in bright light for 24 hours.
Place both bell jars in the same bright position for 24 hours.
Remove each test leaf, dip it in the near-boiling water for about 30 seconds to kill and soften the cells, then turn the hotplate off.
Place the leaf in a boiling tube of ethanol and stand the tube in the hot water until the leaf is pale (ethanol boils at about 78 degrees Celsius and extracts the chlorophyll); never heat ethanol over a flame.
Rinse the leaf in the warm water, spread it on the white tile, cover it with iodine solution for a minute and record the pattern of blue-black (starch present) and orange-brown (no starch) areas; compare it with your tracing and with the position of the strip.

What you should see

The de-starched control leaf stays orange-brown throughout. On the variegated leaf the blue-black stain matches the green areas of the tracing and the white areas stay orange-brown. On the covered leaf the area under the strip stays orange-brown while the uncovered area turns blue-black, reproducing the strip's outline. The leaf from the soda-lime jar gives little or no starch while the leaf from the jar with added carbon dioxide stains blue-black. The learner knows it worked when the de-starched control is negative and each stained pattern matches its own treatment.

What changes

What you change: presence of chlorophyll, light or carbon dioxide in part of a leaf
What you measure: presence of starch shown by the iodine colour pattern
What you keep the same: plant species
de-starching time (48 hours)
time in light (24 hours)
starch-test procedure

Common misconceptions

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

Plants take their food from the soil; the starch is made in the leaf from carbon dioxide and water using light.
Every part of a green plant makes starch in light; only cells with chlorophyll do.
Light alone is enough for photosynthesis; without carbon dioxide no starch forms even in bright light.

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.

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