January: Testing leaves for starch, and conditions needed for photosynthesis

If we want to tell whether a plant is photosynthesising, we might check whether it is giving off oxygen (as with the pondweed in a tube of water), or we can check whether starch is being produced in its leaves.

Plants produce glucose as a product of photosynthesis.  Just like animals, plants need glucose for respiration and growth.  Glucose is soluble in water and is a small molecule, so it moves easily across cell walls by diffusion.  But for transport and storage, plants need to store glucose in the form of long chains of molecules, known as starch.  Starch is made in the leaves from any surplus glucose not immediately needed.

If a plant is kept in the dark so that it cannot produce new glucose by photosynthesis, it will break down starch stored in the leaves, and in the roots, to release the glucose needed.

Destarching a plant

When we are investigating the conditions needed for photosynthesis, we want to make sure that the plant doesn't have any starch stored in its leaves before the experiment begins, otherwise we won't know whether it has added to its starch stores by photosynthesis.  This is why we 'destarch' the plant first, by placing it in darkness for a period so that it is forced to use up starch stored in its leaves.  For an ordinary geranium houseplant this takes 48 hours.

Testing for Starch

We use the standard food test for starch; we add iodine solution, and if starch is present, a blue-black colour is seen.  A helpful student demonstrated this by dropping iodine solution on some known starchy substances such as cornflour and wheat flour.  He also wrote his name in saliva on some paper soaked in starch solution, and added iodine, to see his letters magically appear white where his spit had digested the starch.

Testing leaves for starch

1. Put the leaf in boiling water for one minute.  This is to kill the leaf, so that it won't be affected by being in the light while we are working on it.
2. Place the leaf in a beaker of alcohol and put this in a hot water bath.  Alcohols boil at relatively low temperatures, so we could see beaker bubbling at 70 - 80C.  The alcohol acts as a solvent and removes the chlorophyll from the leaf, so that it is pale.  The reason for removing the chlorophyll is that its dark colour would make it impossible to see the results of the starch test.
3. Rinse the leaf , pat it dry, and add iodine solution to test for starch.

A freshly picked leaf, and one that has been heated in alcohol.

 This photo shows the alcohol boiling at fairly low temperatures.  It was bubbling slightly at this point, and had just finished bubbling vigorously.

 Our helpful demonstrator is holding a hand boiler next to the beaker.  This is a physics toy - a glass vessel, sealed with low pressure inside, and a volatile liquid which boils at very low temperatures under those conditions.  The heat from your hand makes the liquid boil. We enjoyed watching things bubble away for a while.

 The results of our test were a reflection of the time of year.  The plant which had been kept in the dark certainly didn't have any starch in its leaves.  However, neither did the geranium which had been kept in the light!  We thought that the temperature and light levels were probably just too low for pelargoniums to cooperate.  However, on the tile above you can also see a rectangular strip of aquarium plant, and some of the Cabomba fronds which had been used in another photosynthesis activity, showing traces of blue-black where starch is present.

Links

BBC Bitesize: storage and use of glucose in plants

 Nuffield Practical Science: Testing Leaves for Starch

Nuffield Practical Science: Identifying the conditions needed for photosynthesis (we didn't have time to do this, but it would be a good activity to do on your own).