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Enzyme Activity


To investigate enzyme activity by testing for changes in a starch solution when the enzyme amylase is added and how this reaction is affected by heat, pH, ratios etc.

Background of Enzymes

Enzymes are biological catalysts produced by living cells. Like all catalysts, they greatly speed up all the processes that go on inside living organisms. A single cell may contain 100,000 different enzymes, which are needed to take part in its 1,000 to 2,000 chemical reactions. For example digestive enzymes break down food into simpler substances. Many enzymes work in the opposite way, linking simple substances together to form larger, more complex ones needed to build up tissues. Enzymes themselves are made of protein and each one has a differently shaped active site, where the reaction takes place.

It is this shape which dedicates it to a particular chemical reaction.

This can be described as a lock and key system, whereby the two substances to be linked, the keys, fit into the enzyme, the lock. Only the correctly shaped substances can fit into the lock.

Enzymes work best at different temperatures, which is one of the reasons our bodies are kept at a constant temperature. If there is a change in temperature the enzymes are less, if at all, effective. A fall in temperature results in a lack of energy slowing the reaction down, a rise, and the active site is warped out of shape by the heat.

Diagram of enzyme activity:

1.One substance and 2.Substance fits into enzyme 3.Two separate an enzyme and is broken down substances are released

The bonding of substances works in the opposite direction, two substances and an enzyme, the substances fit into the active site and are joined together, one new substance is released.


In an experiment such as this there are many variables which will effect the results.

To identify the effect of a specific variable on the reaction I will firstly need to select a variable to change, which consequently, will show me which variables I need to keep constant.

These variables are: Ratio of enzyme to substance/substances

Temperature of enzyme/substance

pH of enzyme/substance

Shape of container in which reaction takes place

I have chosen to investigate the relation between pH and enzyme activity. I have made this choice as the effects of heat and ratios of substances are already known to me and a range of suitable containers are not available to me, leaving pH.

Fair Testing

To ensure all testing is fair I will keep all of these variables constant with the exception of the pH of the enzyme which will changed to specific levels relevant to the test.

The testing will be done at room temperature in test tubes of the same size at an amylase to starch ratio of 1:5

I will repeat the experiment 5 times to obtain an average (mean) result.

I will remove the fastest and slowest times of the experiment to eliminate any chance of the mean result being effected by a very different result caused by a change in room temperature or an error in measuring.


My prediction is that the rate of enzyme activity will be most effective at a more acidic pH level although an extremely high level of acidity could damage the active site of the enzymes. I think this because the digestive juices within the body are acidic, this would mean the enzymes would be working in an environment less different to that of the stomach/intestines.

This is how I expect results to turn out, in the form of a graph

This shows the rate of reaction increasing as the enzymes are less alkaline and more acidic, except at high acidity when the rate slows down due to damage caused to the active site.


Equipment: Test tubes

Testing wells


Measuring cylinders

Stop clock

1) Put 25ml of starch into each of 5 different test tubes.

2) To the first, using a pipette, add hydrochloric acid, vinegar to the second, nothing to the third, sodium bicarbonate to the fourth and to the fifth add caustic soda, until the test tubes are of pH 1, 4, 7, 10, 14(test with litmus paper).

3) Add iodine to each of the test tubes followed by 5ml of amylase and start the stop clock.

4) Record the times at which the iodine turns black, indicating that the starch solution has been fully broken down.


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