#19. Following the course of an enzyme-catalysed reaction

Measurement of the rate of formation of the product or the rate of disappearance of the substrate.

1. Measurement of the rate of formation of O₂ in the reaction:

• Mash up some biological material like potato tuber or celery stalks, mix them with water and filter the mixture to obtain a solution containing catalases. 
• Add the mixture to H₂O₂ (hydrogen peroxide) in a test tube. Use small tubes --> not too much gas in the tube above the liquid. 
• Collect the gas in a gas syringe and recording the volume every minute until the reaction stops.

Note - You can replace the gas syringe by an inverted measuring cylinder over water.

2. Measurement of the rate of disappearance of starch in the reaction:

• Add amylase solution to starch suspension in a test-tube. 
• Take samples of the reacting mixture at regular time intervals, and test for the presence of starch using iodine in KI solution. 
• When starch is present, iodine is dark blue.
• If the blue colour lightens, starch is breaking down.
• When there is no starch, the iodine solution will remain orange-brown.

To obtain quantitative results, use a colorimeter.

• Put some of the iodine solution into one of the colorimeter tubes, place it in the colorimeter and adjust the dial to give a reading of 0. This is your standard, with no starch. 
• Every minute, take a sample of the liquid from the starch-amylase mixture and add it to a clean colorimeter tube containing iodine solution. Mix thoroughly, then measure the light absorbance. 
• The darker the blue-black colour, the greater the absorbance, and the greater the concentration of starch.

Syllabus 2015 (c) [PA] follow the progress of an enzyme-catalysed reaction by measuring rates of formation of products (for example, using catalase) or rates of disappearance of substrate (for example, using amylase);

Syllabus 2016  - 2018 d)   investigate the progress of an enzyme-catalysed reaction by measuring rates of formation of products (for example, using catalase) or rates of disappearance of substrate (for example, using amylase)