Science 7–10 · Year 8

Testing a variegated leaf for starch: where photosynthesis happens

Science understanding: Biological sciences

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

Starch is made only in the parts of a leaf that hold chloroplasts and have had light, so the iodine test maps where photosynthesis occurred.

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 flammable
Near-boiling water
Iodine stains and irritates

Controls

No naked flames in the room; ethanol heated only by standing in hot water from a kettle or a thermostatic bath
Tongs and safety glasses
Small ethanol volumes; tubes capped when not in use

Note

Chemicals and heat: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package (section 1, general information for all staff) and complete a RiskAssess risk assessment before the lesson.

What you need

A variegated geranium (Pelargonium) or similar plant kept in the dark for 24 h to remove the starch already in its leaves and then in bright light for 24 to 48 h, which the SAPS technical and teaching notes require, with one leaf on the same plant wrapped in foil throughout the light period as the dark control
Beaker of near-boiling water from a kettle, an electric water bath or the hot water beaker as the ethanol bath (no naked flame), boiling tube of ethanol (about 20 mL), forceps
Iodine in potassium iodide solution, white tile, safety glasses

How to do it

Draw the leaf, marking the green and white areas.
Drop the leaf into near-boiling water for one minute to stop enzyme action and soften it.
Move the leaf into the tube of ethanol and stand the tube in the hot water (heat source switched off) for five to ten minutes until the ethanol is green and the leaf pale.
Rinse the leaf in the warm water to soften it again and spread it on the tile.
Cover it with iodine solution, wait one minute, and draw the result over the first drawing.
Repeat with the leaf that was wrapped in foil through the light period.

What you should see

The formerly green areas turn blue-black; the white areas stay the yellow-brown of iodine; the leaf wrapped in foil through the light period shows no blue-black anywhere. The dark pre-treatment is what makes the result readable: the starch present before the light period was removed, so blue-black in the green areas was made during the light period. The learner knows it worked when the ethanol turned green and the leaf pale before the iodine went on, and the blue-black pattern then matches the green pattern drawn before the test. SAPS warns that the practical does not work if the plant is not photosynthesising, so a lit leaf with no blue-black anywhere is a failed run to report rather than evidence that the green areas make no starch.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

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

Leaves are green because they contain starch.
A white part of a leaf is dead.
Plants get their food from the soil.

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-LIV-01 SC4-WS-04
Australian Curriculum v9AC9S8U01AC9S8U02AC9S8I03

Sources

The pages the author read to write this activity.

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