Skip to 0 minutes and 10 secondsIn this practical activity we use the enzyme amylase – this one is bacterial amylase that you can buy from a school science supplier. The starch we are using has been made up into a clear colourless solution. In the reaction starch is broken down to simple sugars. The enzyme amylase speeds up the reaction. We are going to use iodine in potassium iodide solution to detect whether starch is present. When you add iodine solution to starch you can see it changes from golden brown to this blue black colour. Start by setting up the iodine solution in the dimple tile - ready to test the enzyme and starch mixture. Put one drop of the iodine solution in each dimple.
Skip to 0 minutes and 59 secondsIf you don’t have dimple tiles you could use a row of test tubes with a drop of iodine solution in each one. If you are looking at the effect of temperature have your water baths set to a range of suitable temperatures either side of the optimum temperature for your enzyme. Label 2 test tubes with the temperature you are testing and add 2 cm3 of starch to one and 1 cm3 of amylase to the other. Put the tubes in the water bath for about 5 minutes to reach temperature. After they have got to the correct temperature mix the contents of the two tubes together and start the stop clock straight away.
Skip to 1 minute and 45 secondsImmediately take a drop from the well mixed tube and put it into a dimple of iodine solution. You can see that the starch is still there, because the iodine solution goes blue black. The amylase hasn’t broken it down yet. After 60 seconds take another sample. Repeat until the iodine solution stays golden brown – now all the starch has been broken down into the reaction. It has changed into simple sugars that don’t make the iodine solution change colour. This can often take up to 20 minutes, so it is important to consider this when you are planning your lesson. If your independent variable is pH, substrate concentration or concentration of enzyme, you will only have one water bath.
The breakdown of starch by amylase
Here is the final video of three to watch as part of the learning objectives activity.
This enzyme practical can be quite long (and boring) if the concentration of amylase is too low. So to avoid students sitting for twenty minutes doing nothing but testing a sample every minute, it is essential to use a low concentration of starch and to practice the experiment first. Aim to find a concentration of enzyme which will break down the starch within five minutes.
The protocol for this practical is available below.