Testing a Leaf for Starch

During photosynthesis, a plant absorbs light energy using chlorophyll, allowing it to convert CO2 and water into glucose. This glucose is:

• transported to the growing parts of the plant for use in respiration
• transformed into cellulose, proteins and oils.
• turned into STARCH for storage

You can see whether a plant is photosynthesising by testing whether starch is present.

Test for starch

• heat a leaf in boiling water (to stop any chemical reactions occurring)
• add the leaf to BOILING ETHANOL in a water bath (to remove the chlorophyll so it turns white and it easy to see a colour change)
• wash with water to rehydrate and soften the leaf
• spread the leaf onto a white tile
• add IODINE solution with a pipette

After a few minutes, the parts of the leaf that contain starch turn the iodine from brown to blue/black

Variegated Leaves

They have green parts (where the cells contain chlorophyll) and white parts (where they don’t).

Only the parts that contain chlorophyll turn blue/black with iodine solution.

De-starching

A plant can be de-starched by leaving it in the dark for a few hours. The lack of access to sunlight means the plant cannot get energy through photosynthesis and so resorts to using up its storage of starch.