In the leaf, excess glucose is rapidly converted to starch, so we test leaves for starch to show that photosynthesis has happened, rather than testing for glucose.

We often indicate that light and chlorophyll are required for the process by adding them to the equation near the arrow.

You could demonstrate the effect of iodine solution on starch by adding a few drops of solution to a starch powder or starch suspension in a boiling tube. You can then discuss the fact that this colour change might be hard to see in a dark green leaf until the chlorophyll is removed.

Depending on student’s prior knowledge, you could discuss the structure of plant cells and explain that the starch is within the cells, the cells are surrounded by cell membranes and tough cellulose cell walls and that some leaves also have a protective waxy cuticle. The hot water treatment softens up the protective structures, and disrupts the cell membranes to let the chlorophyll out and the iodine solution in.