AQA GCSE Testing a leaf for starch (Biology)

Testing a leaf for starch

This is a link to the food tests page where there is a general test for starch.

We can test a leaf for starch using iodine solution. If the leaf tests positive for starch it will prove that the leaf has carried out photosynthesis.

Using iodine solution to test for starch is an example of a qualitative testing. Qualitative means that a substance is either present or absent.

Method

Step 1

The leaf is boiled in water for several minutes.

There are two reasons for this:

Boiling the leaf in water kills it. This stops all enzyme activity, including enzymes that could break down starch. Without this step, starch might be broken down before you test it, giving a false negative.

Boiling also softens the leaf tissue, making it easier for ethanol to penetrate the leaf in the next step.

Step 2

After boiling in water, the leaf is placed in a test tube containing ethanol. This test tube is then placed in hot water to gently heat the ethanol, which removes the green chlorophyll from the leaf.

It is important at this stage that the bunsen burner is switched off. This is because the ethanol is highly flammable. So, no naked flames are allowed near the highly flammable ethanol.

Step 3

After decolourising the leaf in ethanol, the leaf is rinsed in warm water to soften it and remove any remaining ethanol before adding iodine solution.

Finally, iodine solution is added to the leaf. Areas containing starch turn blue-black, showing where photosynthesis has occurred, while areas with no starch remain brown.

The leaf on the left does not contains starch, but the leaf on the right did contain starch.